Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
AROMATIC COMPOUND
Aliphatic and aromatic compounds
Chemists have found it useful to divide all organic compounds into two broad
classes: aliphatic compounds and aromatic compounds .the original meanings of
the words aliphatic (fatty) and aromatic (fragrant ) no longer have any
significance.
aliphatic compounds are open –chain compounds .the families we have studies
so far alkanes, alkenes and alkynes are all members of the aliphatic class
Aromatic compounds
Benzene and the aromatic character
The organic compounds were divided into two classes:1-aliphatic compounds
2- aromatic compounds.
The aliphatic compounds are studies previously. Aromatic compounds are
benzene and compounds that resemble benzene in chemical behaviour. Aromatic
compounds are characterized by a tendency to undergo ionic substitution
(electrophilic aromatic substitution)
Structure of benzene
Benzene has been known since 1825, but its structure not known until about
1931 .The understanding of the structure of benzene is important in our study of
aromatic compounds . we shall examine the fact upon which this structure of
benzene to build .
Molecular formula ,kekule structure
1-Benzene has the Molecular formula C6H6 . Benzene is consisting of six carbon
atoms and six hydrogen atoms. Now, how are these atoms arranged? Kekule
had proposed that carbon atoms join to each other to from a chains then these
carbon chains can sometimes be closed to form rings .There are many
structures consistent with the Molecular formula C6H6 ,
1
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
H
H
H
H
H
H
H
H
H
H
H
H
H
CH2
H
H
H
III
I
II
Kekule structure
CH3
C
C
C
C
Dewar formula
CH3
H2C
C
H
C
C
C
H
CH2
From all these structures, kekule structure (1) was expected as the most nearly
satisfactory
1- Benzene yields only mono substitution product , C6H5y like C6H5Br .by
replacing the hydrogen atom by another atom such as bromine , chloride
,this mean that all the hydrogen atoms must be exactly equivalent i .e
.replacement of any one of them yields the same product therefore , I , II
and v must be rejected because they would yield two isomeric mono
substituted derivatives .
2- Benzene yields three isomeric disibstituted products , C6H4y2 or C6H4yz
such as C6H4Br2 therefore(1v)must be rejected
( give two isomers ) and
only the structure(1)seems to be consistent with this fact ,the three isomeric
dibrom derivatives 1,2- 1,3- and 1,4- dibromobezene .
Br
Br
Br
1
1
1
Br
2
2
2
3
3
Br
1,3-dibromobenzene
1,2-dibromobenzene
4
Br
1,4-dibromobenzene
Examination of structure (I) ,shows that two 1,2-dibromo isomers (II ) should
be possible
Br
1
Br
1
Br
2
Br
2
(II)
(I)
There are two tautomeric structure for benzene (I) ,(II) There is
tautomeric between the two structures
2
two
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
H
H
1
H
2
H
tautomerism
H
H
H
1
H
H
H
2
H
H
(I)
(II)
3-Stablityof benzene ring
The Kekule structure is consisted with the previous fact (1, 2,and 3) but there
are a number of facts that are not accounted for by this structure . this fact are
related to the un usual stability of the benzene ring. This stability is found in
the chemical reaction of benzene
4-benzene undergoes substitution rather than addition reaction ;Kekule structure of benzene can be called (1,3,5-cyclohexatriene) which is
expected to undergo readily the addition reaction similar to cyclohexane and
cyclohexadiene . but actually . benzene did not react at all or react very slowly
under the similar condition of addition of alkenes .
for example
Reagent
KMNO4 (cold,dil. aqu)
Br2 / CCl4 (in dark)
cyclohexane gives
Rapid addition
Rapid addition
Rapid addition
HI
Rapid hydrogenation
H2 , Ni
At 25 ,20
benzene gives
No reaction
No reaction
No reaction
slow hydrogenation
at 100- 200 , 1500
In place of addition benzene undergo substituted reaction benzene undergoes
nitration ,sulfonation ,halogenation alkylation and acylation .In each of these
reactions on atoms or group of atoms have been substituted for one of the
hydrogen atom of benzene . in all these reaction the benzene has retain its
characteristic structure . It would be appear that benzene resists addition , in
which the benzene ring structure would be destroyed ,whereas it readily
undergoes substitution ,in which the ring system is preserved.
3
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
5-Heat of hydrogenation and combustion
The of hydrogenation and combustion of benzene are lower than expected the
benzene is more stable than cyclohexatriene , How much it more stable the heat
of hydrogenation is the quantity of heat evolved when one mole of an
unsaturated compound is hydrogenated . The of hydrogenation of one double
bond is about 28.6 K.Cal., cyclohexadiene has = 55.4 K.Cal.,( about twice ).we
might expected cyclohexatriene to have heat of hydrogenation about three
times as large as cyclohexene , that is about 85.8 K.Cal., K.Cal., ,but actually
the value for benzene 49.8 K.Cal., ,is 36 k cal less than the expected value
benzene evolved 36 K cal less energy that predicted ,i .e .benzene contain36 K
cal less than predicted , .benzene is more stable cyclohexatriene by 36 K .cal
1- Cabon –carbon double bond length in benzene :
All Cabon –carbon double bond in benzene are equal and are intermediate in
length between single bond and double bonds .Bond length of C=C
=
1.34 A
Bond length of C- C
=1.53A
Bond length of
C- C
= 1.50 A
Bond length of
C- C
1.48 A
(IN ethane )
(IN propylene )
( in butadiene )
if benzene actually possessed three single and three double bonds as in
kekule structure ,we expct to to find three short bonds (1.34 a) and three long
bonds ( 1.48A) as in butadiene ) Actually , x-ray diffraction studies show that
six C- C bonds in benzene are equal and have a length of 1.39A ,and are
this intermediate between single and double bonds.
Resonance structure in benzene :The benzene have the Kekule structure according to the prevision facts and
present in two Resonance structures (I ,II )
i.e . Benzene is a hybrid of I and II .
Resonance structures:
4
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
H
H
H
1
H
H
1
H
Structures that differ
H
H
2
2
H
H
H
H
Only in the arrangement of electrons .I ,and II are exactly equivalent , and
hence of exactly the same stability ,and they make equal contribution to the
hybrid .The unusual expected due to the hybrid of equivalent structure . The 36
Kcal . of energy of benzene does not contain ( compared to cyclohexatriene ) is
resonance energy , which is responsible for the new set of properties (a
aromatic properties) .Addition reactions convert an alkene into a more stable
saturated compounds but addition would convert benzene into a less stable
product by destroying the resonance stabilized benzene ring system (aromaticity
) .While substitution retain the resonance –stabilized benzene ring hence it place
easier than addition reaction.
Orbital picture of benzene :Each carbon in benzene is bonded to three other atoms , it uses Sp2 orbitals . The
orbitals lie in the same plane that of carbon nucleus and are directed toward the
corners of equilateral triangle as in (a)
Representation of benzene ring :It represented as
Which means a hybrid of
and
Aromatic character :- The Hukle 4n+2 rule .
5
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
There are many substances that are called aromatic but not possess. Benzene
ring or resemble benzene .
Now ,what properties do all aromatic compounds have in common ? Aromatic
compounds are compounds of high degree of unsaturation and which resist
addition reactions (that is the characteristic of unsaturatedcompounds have
unsual stability (low haets of hydrogenation and combustion ) aromatic
compounds are cyclic ( generally containing 5-7 membered rings ) flat (or
nearly flat ) molecules .
The aromatic compounds must have a molecule that contains cyclic clouds of
declocalization π-electrons above and below the plane of molecule , and the πclouds must contain a total of (4n +2) π-electrons ,i.e ,2,4,6,10that is to say , the
high stability and the delocalization of π-electrons is not enough to give the
aromatic charcter to the molecules but must contain 4n +2 π-electrons (which is
called Hukel rule).
6e
aromatic
10e
aromatic
H
H
5e
non aromatic
H
6e
aromatic
7e
non-aromatic
H
4e
non aromatic
H
6e
non aromatic
H
8e
non-aromatic
Each molecule is a hydride of either five of seven equivalent structures, which
the charge or add electron on each carbon , but only two of them ( iii, IV ) have
unusual high stability.
6
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Nomenclature of benzene derivatives:Naming the benzene derivatives by prefix the name of substituent group the
word –benzene .for example chlorobenzene , flourobenzene , bromobenzene
,nitrobenzene
F
NO2
fluorobenzene
Br
Cl
nitrobenzene
chlorobenzene
bromobenzene
There are other derivatives which have a special name such as :
CH3
NH2
OH
OH
O S O
COOH
toluene
aniline
methyl benzene
phenol
Aminobenzene
hydroxybenzene
benzoic acid
benzene sulfonic acid
If several groups are attached to the benzene ring , we must indicate the position
and the name of the groups . Only for disibstituted benzene we can use ortho ,
meta and para to indicate the relative position 1,2 ,1,3 and 1,4- di substituted
Br
Br
Br
Br
1,2-dibromobenzene
benzene .
Br
1,3-dibromobenzene
m-dibromobenzene
o-dibromobenzene
Br
1,4-dibromobenzene
p-dibromobenzene
If there is different group , we simply naming the two groups with their position
and ending the word with benzene .
I
NO2
Cl
Br
m-bromo iodobenzene
O-chloro nitrobenzene
For di substituted benzene with groups of special name : for example
7
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
SO3H
CH3
OH
NO2
I
P-iodobenzenesulfonic acid
COOH
Cl
Br
P-methyl nitrobenzene
m-bromophenol
O-chlorobenzoic acid
p-nitro toluene
If there are more than two groups attached to the benzene ring , number mustbe
used to indicate their relative position
NH2
CH3
NO2
NO2
O2N
Br
Cl
2,6-di nitro toluene
Br
Br
1-bromo-3-chloro-5-nitrobenzene
Br
2,4,6-tribromobenzenamine
OH
Br
Cl
Br
NO2
Br
1,2,4-tribromobenzene
2-chloro-4-nitrophenol
Nomenclature of Disubstituted
and Polysubstituted Benzenes
Disubstituted Benzenes
The relative positions of two substituents on a benzene ring can be indicated
either by numbers or by the prefixes ortho, meta, and para. Adjacent
substituents are called ortho, substituents separated by one carbon are called
meta, and substituents located opposite one another are designated para. Often,
only their abbreviations (o, m, p) are used in naming compounds.
8
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
If the two substituents are different, they are listed in alphabetical order. The
firststated substituent is given the 1-position, and the ring is numbered in the
direction that gives the second substituent the lowest possible number.
If one of the substituents can be incorporated into a name that name is used and
the incorporated substituent is given the 1-position.
A few disubstituted benzenes have names that incorporate both substituents.
Polysubstituted Benzenes
9
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
If the benzene ring has more than two substituents, the substituents are
numbered so that the lowest possible numbers are used. The substituents are
listed in alphabetical order with their appropriate numbers.
10
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
As with disubstituted benzenes, if one of the substituents can be incorporated
into a name, that name is used and the incorporated substituent is given the 1position. The ring is numbered in the direction that results in the lowest possible
numbers in the name of the compound.
11
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
PROBLEM 1
Name the following compounds:
PROBLEM 2_
Draw structures of the following compounds:
a. para-toluidine
c. para-xylene
b. meta-cresol
d. ortho-chlorobenzenesulfonic
Draw the structure of each of the following compounds:
a. m-chlorotoluene
e. 2-bromo-4-iodophenol
b. p-bromophenol
f. m-dichlorobenzene
c. o-nitroaniline
g. 2,5-dinitrobenzaldehyde
d. m-chlorobenzonitrile
h. o-xylene
PROBLEM 2
Correct the following incorrect names:
a. 2,4,6-tribromobenzene
b. 3-hydroxynitrobenzene
c. para-methylbromobenzene
d. 1,6-dichlorobenzene
Electrophilic Aromatic Substitution
The characteristic reactions of benzene is substitution reactions in which the
resonance stabilized ring system a preserved . In benzene molecule there is a
cloud.of π electron above and below the plane of the carbons there π electron
are loosely held and available to a reagent that is seeking electrons
(electrophilic reagents or acids) .so which the benzene rings serves as a source
of electrons. Electrophilic Aromatic Substitution includes ; 1-nitration -2halogenation -3-sulfonation and friedel –crafts -4- alkylation and-5- acylation.
12
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
NO2
H
conc. HNO3
H2O
conc. H2SO4
nitrobenzene
H
SO3H
H2SO4 \ SO3
H2O
H
X
AlCl3
X2
HCl
X =Cl ,Br ,
R
H
AlCl3
RCl
HCl
O
H
R
R
O
C
AlCl3
HCl
Cl
Effect of substitution groups:Any group attached to benzene ring affects the reactivity of the ring and
determines the orientation. i.e . the group already attached determines how
readily the attack occurs and there it occurs A group that makes the ring more
reactive than benzene is called an activating group .
A group that cause attack to occurs chiefly at ortho and para position to it is
called ortho , para director . A group that cause attack to occur chiefly at .A
group that cause attack to occur chiefly position meta to it is called meta
director for example ;Toluene undergoes electrophilic aromatic substitution like benzene .such as
sulfonation ,to from three monosulfonation products of different percentage .the
reaction actually yields a chief products of o- and p- isomers.
CH3
CH3
CH3
o
H2SO4 ,SO3 ,35 C
SO3H
and
and 6% isomers
SO3H
62%
32%
Benzene reacts with fuming sulphuric acid completely within 20 to 30 minutes
whereas toluene is found to read within only a minute or two . This mean that
toluene is more reactive than benzene .i.e .methyl group makes the ring more
13
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
reactive than unsubstituted benzene and directs the attacking reagent to the ortho
and para position of the ring On the other hand ,nitrobenzene ,has been found to
undergo substitution more slowly than benzene and to yield chiefly the meta
isomer.
Determination of orientation:From analyzing the percentage of three products of the reaction we can
determine the orientation of the group that present on the benzene ring any
group can be put into one of two classes : ortho , para directors or meta
director the following table summerized the nitration of number of substituted
benzene .table
Determination of relative reactivity
A group is classified as activating if the ring attached to it is more reactive than
benzene and classified as deachvating if the ring attached to it is less reactive
than benzene. The comparison between the reactivity of benzene and substituted
benzene is in the following ways.
1-The time required for the reaction to occur under identical condition ex;
toluene is found to react with fuming sulphuric acid is about 1 / 10 or 1 /20 the
time required by benzene i.e . toluene is more reactive than benzene and
therefore –CH3 is an activating group .
2- The severity of conditions required for comparable reaction to occur
within the same period of time ex .benzene is nitrated in less than one hour
at 60 oC by a mixture of conc.H2SO4 and conc.HNO3 ;conyabable nitration
14
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
of nitro benzene requires treatment at90oC with fuming HNO3 and
conc.H2SO4 therefore nitrobenzene is less reactive than benzene and group
is deactivating group
3- Competitive reaction quantitative comparison under identical reaction
conditions for example if equimolar of benzene and toluene aretreated
with a small amount of nitric acid in a suitable solvent ,the nitrobenzene
formed is about 25 times as nitrobenzene formed ,i.e .toluene is more
reactive than benzene by about 25 times . also benzene and chlorobenzene
give 30 /1 nitrobenzene to chloronitrobenzene .this mean that chloro
benzene is less reactive than benzene by 30 times .therefore CH3group is
activating group while Cl group is deactivating group .
Classification of substituent groups:the groups that attached to benzene ring have a great effect on the
electrophilic substitution ,and nearly all groups fall into one of two classes .
1- Activating and ortho ,para directly
2- Deactivating and meta directly
3- Halogens are deactivating but orthob,para, directly.
The following table shows the effects of groups on the electrophilic aromatic
substitution.
15
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
The table is very important for determining the electrophilic aromatic
substitution products and to predict the rate of reaction and the reaction
conditions , required for the reaction to proceed.
This table is necessary for the student to solve rapidly the synthetic
problems involving compound.
Orientation of di substituted benzenes
the Orientation in this case more complication ,there are many cases the
two groups may be located so that directive influence of one reinforce that of
the other ,for example.
CH3
NHCOCH3
CN
NO2
16
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
2-when the directive effect of one group opposes that of the other , a
complicated mixtures of several products are obtained , but it is possible to
product according to
a- Strongly activating group generally overcome deactivating or weakly
activating groups .The difference in directive power in the sequence
NH2 , OH > -OCH3 , -NHCOCH3 > -C6H5 , -CH3 > meta
Directors .for example;OH
OH
NO2
conc. HNO3
conc. H2SO4
CH3
CH3
NHCOCH3
NHCOCH3
Br
Br \ FeBr 3
CH3
CH3
CHO
Br \ FeBr 3
CHO
Br
OH
OH
There must be large difference in activity effects of the two groups for
clear –cut result ,otherwise many products can be obtained
CH3
CH3
conc. HNO3
CH3
NO2
conc. H2SO4
Cl
Cl
Cl
58%
NO2
42%
If there two groups in meta position to each other , the substitution
between then is very little ,due to the crowding ( or steric hindrance)
CH3
59%
1%
59%
CH3
9%
Cl
Cl
62%
32%
17
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
ORIENTATION AND SYNTHESIS
Th e aim of synthesis to obtained a single pure product, and we avoid a
reactions that give a mixture of product because it lower the yield of the wanted
product and cases difficult problems of purification ,therefore we must apply
our knowledge of orientation to synthesis of pure aromatic compounds.
1- In the preparation of aromatic compounds.:-first of all ,we must consider
the order in which we introduce this various Substituent into the ring.
For example:- preparation of bromonitro benzene, if the nitration is before
bromination ,we will obtained the m-isomer ,whereas if the bromination is
before nitration, we will obtained amaxture of the ortho and para –isomers this
is depend on which isomer we want
NO2
NO2
Br, Fe
HNO3 / H2SO4
Br
H
meta bromonitrobenzene
Br, Fe
NO2
NO2
Br
Br
HNO3 / H2SO4
ortho
38%
Br
para
62%
2- Next ,if the synthesis involves conversion of one group into another ,we
must consider the proper time for this conversion . in the preparation of
nitrobenzoic acid from toluene the particular product obtained depends
upon where oxidation or nitration is carry out firt.
18
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
COOH
NO2
HNO3 / H2SO4
KMNO4
NO2
CH3
NO2
CH3
HNO3 / H2SO4
O2N
ortho
K2CrO7
CH3
para
K2CrO7
COOH
COOH
NO2
NO2
The otho and para nitrotoluene can be separated by fractional distillation .
General Mechanism for Electrophilic
Aromatic Substitution Reactions
Because electrophilic substitution of benzene involves the reaction of an
electrophile with an aromatic compound, it is more precisely called an
electrophilic aromatic substitution reaction. In an electrophilic aromatic
substitution reaction, an electrophile
Substitutes for hydrogen of an aromatic compound. The following are the five
most common electrophilic aromatic substitution reactions:
Y
H
Y
H
1. Halogenation: A bromine (Br), a chlorine (Cl), or an iodine (I) substitutes for
a hydrogen.
2. Nitration: A nitro group(NO2) substitutes for a hydrogen.
3. Sulfonation: A sulfonic acid group(SO3) substitutes for a hydrogen.
4. Friedel–Crafts acylation: An acyl group (RC=O ) substitutes for a hydrogen.
5. Friedel–Crafts alkylation: an alkyl (R) group substitutes for a hydrogen.
All of these electrophilic aromatic substitution reactions take place by the same
two-step mechanism. In the first step, benzene reacts with an electrophile
forming a carbonium ion intermediate. The structure of the carbocation
19
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
intermediate can be approximated by three resonance contributors. In the second
step of the reaction, a base in the reaction mixture pulls off a proton from the
carbonium ion intermediate, and the electrons that held the proton move into the
ring to re-establish its aromaticity. Notice
that the proton is always removed from the carbon that has formed the new bond
with the electrophile.
General mechanism for electrophilic aromatic substitution
Mechanism of nitration
The first step is relatively slow and endergonic because an aromatic compound
is being converted into a much less stable non aromatic intermediate . The
second step is fast and strongly exergonic because this step restores the stability
enhancing
aromaticity.
We will look at each of these five electrophilic aromatic substitution reactions
individually. As you study them, notice that they differ only in how the
electrophile needed to start the reaction is generated. Once the electrophile is
formed, all five reactions follow the same two-step mechanism for electrophilic
aromatic substitution.
Halogenation of Benzene
The bromination or chlorination of benzene requires a Lewis
acid such as ferric bromide (FeBr3) or ferric chloride(FeCl3) . Recall that a
Lewis acid is a compound that accepts a share in a pair of electrons .
H
bromination
FeBr3
Br
Br2
HBr
bromobenzene
H
chlorination
Cl2
FeCl3
Cl
HCl
chlorobenzene
20
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
In the first step of the bromination reaction, bromine donates a lone pair to the
Lewis acid. This weakens the Br - Br bond, thereby providing the electrophile
necessary for electrophilic aromatic substitution.
mechanism for bromination:-
To make the mechanisms easier to understand, only one of the three resonance
contributors of the carbocation intermediate is shown in this and subsequent
illustrations. Bear in mind
however, that each carbocation intermediate actually has the three resonance
contributors,. In the last step of the reaction, a base (: B) from the reaction
mixture removes a proton from the carbocation intermediate. The following
equation shows that the catalyst is regenerated:
Chlorination of benzene occurs by the same mechanism as bromination
mechanism for chlorination
To know the effect of orientation and the reactivity of the substituted on the
electrophilic aromatic substitution ,we must know the mechanism of
electrophilic aromatic substitution we will start with nitration :21
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
The commonly accepted the mechanism for nitration with ortho mixture of
nitric acid and sulphuric acid involves the following sequence of reactions
Ferric bromide and ferric chloride react readily with moisture in the air during
handling, which inactivates them as catalysts. Therefore, instead of using the
actual salt, ferric bromide or ferric chloride is generated in situ (in the reaction
mixture) by adding iron filings and bromine or chlorine to the reaction mixture.
Therefore, the halogen in the Lewis acid is the same as the reagent halogen.
Unlike the reaction of benzene with Br2 or Cl2 the reaction of an alkene with Br2
or Cl2 or does not require a Lewis acid. An alkene is more reactive than benzene
because an alkene has a smaller activation energy, since carbocation formation
is not accompanied by a loss of aromaticity. As a result, the or bond does not
have to be weakened to form a better electrophile
ectrophilic iodine (I+) is obtained by treating (I2) with an oxidizing agent such
as nitric acid.
Iodination
Once the electrophile is formed, iodination of benzene occurs by the same
mechanism as bromination and chlorination.
mechanism for iodination
Nitration of Benzene:-
Nitration of benzene with nitric acid requires sulfuric acid as a catalyst.
22
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
To generate the necessary electrophile, sulfuric acid protonates nitric acid. Loss
of water from protonated nitric acid forms a nitronium ion, the electrophile
required for nitration. Remember that any base present in the reaction mixture (
solvent) can remove the proton in the second step of the aromatic substitution
reaction.
mechanism for nitration:-
Sulfonation of Benzene
Fuming sulfuric acid (a solution of SO3 in sulfuric acid) or concentrated sulfuric
acid is used to sulfonate aromatic rings.
As the following mechanism shows, a substantial amount of electrophilic sulfur
trioxide is generated when concentrated sulfuric acid is heated, as a result of the
23
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
electrophile losing a proton. Take a minute to note the similarities in the
mechanisms for forming the electrophile for sulfonation and the electrophil for
nitration.
mechanism for sulfonation
Friedel–Crafts Acylation of Benzene
Two electrophilic substitution reactions bear the names of chemists Charles
Friedel and James Crafts. Friedel–Crafts acylation places an acyl group on a
benzene ring,and Friedel-Crafts alkylation places an alkyl group on a benzene
ring.
24
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
An acylium ion is the electrophile required for a Friedel–Crafts acylation
reaction.This ion is formed by the reaction of an acyl chloride or an acid
anhydride with a Lewis acid.
mechanism for Friedel–Crafts acylation
Because the product of a Friedel–Crafts acylation reaction contains a carbonyl
group that can complex with Friedel–Crafts acylation reactions must be carried
out with more than one equivalent of When the reaction is over, water is added
to the reaction mixture to liberate the product from the complex.
Friedel–Crafts Alkylation of Benzene
The Friedel–Crafts alkylation reaction substitutes an alkyl group for a hydrogen.
25
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
In the first step of the reaction, a carbocation is formed from the reaction of an
alkyl halide with Alkyl fluorides, alkyl chlorides, alkyl bromides, and alkyl
iodides can all be used. Vinyl halides and aryl halides cannot be used because
their carbocations are too unstable to be formed
we will see that an alkyl-substituted benzene is more reactive than benzene.
Therefore, to prevent further alkylation of the alkyl-substituted benzene, alarge
excess of benzene is used in Friedel–Crafts alkylation reactions. This approach
ensures that the electrophile is more likely to encounter a molecule of benzene
than a molecule of alkyl-substituted benzene.
Recall that a carbocation will rearrange if rearrangement leads to a more stable
carbocation When the carbocation can rearrange in a Friedel–Crafts alkylation
reaction, the major product will be the product with the rearranged alkyl group
on
the benzene ring. The relative amounts of rearranged and unrearranged product
depend on the increase in carbocation stability achieved as a result of the
rearrangement.
For example, when benzene reacts with 1-chlorobutane, a primary carbocation
rearranges to a secondary carbocation, and 60–80% of the product (the actual
percentage depends on the reaction conditions) is the rearranged product
26
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
When benzene reacts with 1-chloro-2,2-dimethylpropane, a primary carbocation
rearranges to a tertiary carbocation. Thus, there is a greater increase in
carbocation stability and, therefore, a greater amount of rearranged product—
100% of the product
(under all reaction conditions) has the rearranged alkyl substituent.
Alkylation of Benzene by Acylation–Reduction
It is not possible to obtain a good yield of an alkylbenzene containing a straightchain alkyl group via a Friedel–Crafts alkylation reaction, because the incipient
primary carbocation
will rearrange to a more stable carbocation
Acylium ions, however, do not rearrange. Consequently, a straight-chain alkyl
group can be placed on a benzene ring by means of a Friedel–Crafts acylation
reaction, followed by reduction of the carbonyl group to a methylene group. It is
called a reduction reaction because the two bonds are replaced by two bonds
.Only a ketone carbonyl group that is adjacent to a benzene ring can be reduced
to a methylene group by catalytic hydrogenation (H2/Pd) ..
27
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
The Effect of Substituents on Reactivity
Like benzene, substituted benzenes undergo the five electrophilic aromatic
substitution reactions halogenation, nitration,sulfonation, alkylation, and
acylation. Now we need to find out whether a substituted benzene is more
reactive or less reactive than benzene itself. The answer depends on the
substituent. Some substituents make the ring more reactive and some make it
less reactive than benzene toward electrophilic aromatic substitution. The slow
step of an electrophilic aromatic substitution reaction is the addition of an
electrophile to the nucleophilic aromatic ring to form a carbocation intermediate
Substituents that are capable of donating electrons into the benzene ring will
stabilize both the carbocation intermediate and the transition state leading to
its formation thereby increasing the rate of electrophilic aromatic substitution.
In contrast, substituents that withdraw electrons from the benzene ring will
destabilize the carbocation intermediate and the transition state leading to its
formation, thereby decreasing the rate of electrophilic aromatic substitution
.Before we see how the carbocation intermediate is stabilized by electron
donation and destabilized by electron withdrawal, we will look at the ways in
which a substituent can donate or withdraw electrons.
There are two ways substituent can donate electrons into a benzene ring:
inductive donation and electron donation by resonance. There are also two ways
substituent can withdraw electrons from a benzene ring: inductive electron
withdrawal
and electron withdrawal by resonance.
28
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Why the _CF3 group is meta directing
All meta-directing groups have either a partial positive charge or a full positive
charge on the atom directly linked to the benzene ring. In the trifluoromethyl
group (CF3), there are three electronegative fluorine atoms, which make this
group strongly electron withdrawing. As a result _CF3 deactivates the benzene
Why the _CH3 group is ortho–para directing
The stability of the carbocation intermediate formed in the rate-determining
step is actually the underlying factor for a substituent to direct an incoming
29
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
electrophile to a particular position, ortho, meta or para. The methyl group
(_CH3) donates electrons inductively, and in the presence of this
electrondonating
group the resonance contributors formed from ortho, meta and
para attacks are shown below. In the most stable contributors, arising from
ortho and para attacks, the methyl group is attached directly to the
positively charged carbon, which can be stabilized by donation of electrons
through the inductive effect. From meta attack no such stable contributor is
formed. Thus, the substitutions in ortho and para positions are favoured.
Therefore, the methyl group is an ortho and para directing group
30
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Electrons from the ring through the inductive effect more strongly than they
donate electrons by resonance. It is the resonance-aided electron-donating
effect that causes halogens to be ortho–para-directing groups. Halogens can
stabilize the transition states leading to reaction at the ortho and para
positions. On the other hand, the electron-withdrawing inductive effect of
halogens influences the reactivity of halobenzenes. A halogen atom, e.g. Cl,
donates an unshared pair of electrons, which give rise to relatively stable
resonance structures contributing to the hybrids for the ortho- and
parasubstituted
31
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
arenium ions. Thus, despite being deactivators, halogens are
ortho- and para-directors. The resonance contributors formed from ortho,
meta and para attacks on the chlorobenzene are shown below.
Why halogens are ortho–para directing
Halogens are the only deactivating substituents that are ortho–para directors.
However, they are the weakest of the deactivators. Halogens withdraw electrons
from the ring through the inductive effect more strongly than they
Donate electrons by resonance. It is the resonance-aided electron-donating
Effect that causes halogens to be ortho–para-directing groups. Halogens can
stabilize the transition states leading to reaction at the ortho and para
positions. On the other hand, the electron-withdrawing inductive effect of
halogens influences the reactivity of halobenzenes. A halogen atom, e.g. Cl,
donates an unshared pair of electrons, which give rise to relatively stable
resonance structures contributing to the hybrids for the ortho- and para
substituted
arenium ions. Thus, despite being deactivators, halogens are
ortho- and para-directors. The resonance contributors formed from ortho,
meta and para attacks on the chlorobenzene are shown below.
32
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Arenes “alkyl benzene “
These compounds contain both aliphatic and aromatic units , and have the
chemical behaviour of both . the ring undergoes electrophilic aromatic
substituted and the side chain (alkyl group) undergoes free radical substitution
and each proton of the molecule should effect the chemical properties of the
other (the reactivity) and determine the orientation of attack alkyl groups
activate the ring towared electrophilic aromatic substitution and ortho ,meta
,para directors , the bromination of the side chain (R- group ) takes place more
readily than ethane , because the presence of benzene ring , and occurs
exclusively on the carbon nearer the ring ( benzyl position )
Physical properties:
They are insoluble in water but soluble in non polar solvate, less dense
than water, b-p increased with increasing the M.Wt. and rise about 20.30C for
additional carbon atom.
The m.p depend on molecular weight and molecular shape (structure) pisomer has higher b.p and m.p than the other isomer because P- is p-isomer
generally less soluble in so a given went than the other symmetrical.
Preparation of alkyl benzene:
1- Friedel Croft`s alkylation:
If small amount of anhydrous aluminium chloride is added to a mixture of
benzene and methyl chloride, a vigorous reaction occurs.
33
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
R
Lewis acid
R- X
Lewis acid
HX
= AlCl3 , BF3, HF ------etc
Any alkyl halide can be used, also substitution alkyl halide such as benzyl
chloride can be used (PhCH2Cl) but aryl halide cannot be used because the band
between benzene ring and halide is very strong. (Ar-X).The catalyst is Lewis
and such as BF3, HF, phosphoric acid, Fecl3.The resulting products is more
reactive than the starting material, thus further attack may be take place (poly
alkylation), from poly halogenated alkanes it is possible to prepare compounds
containing more than one aromatic ring.
CH3
CH3Cl
AlCl3
HCl
The F-C alkylation may be accompanied by rearrangement of carbonium ion.
12-
R-Cl
AlCl3
R-C l4
C6H6
R
C6H5
R
R
H
3-
R
C6H5
R-C l4
H
C6H5 R
RCl3
HCl
Sometime rearrangement not occurs because the benzene ring very
reactive and react faster than rearrangement.
Limitation of F.C. alkylation:
1- The possibility of poly alkylation.
2- Possibility of rearrangement of carbonium ion.
3- Aryl halide cannot use for alkylation.
34
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
4- Aromatic rings containing – NH2 – NHR or NR2 do not undergo F.C.
alkyl because these groups are strongly basic which read with the Lewis
acid.
NH2AlCl3
NH2
AlCl3
2- The Second Method:
Conversion of side chain.
Reaction of alkyl benzene
1- Hydrogenation by hydrogen in presence of Ni or Pt or Pd.
CH2CH3
Ni , Pd , Pt
H2
2- Oxidation: alkane and benzene are quite uncreative toward oxidation
but alkyl benzene the side chain is oxidized to carboxylic group KMnO 4
is generally used.
CH2CH3
COOH
KMnO4
Or HNO3
Or K2Cr2O7
This reaction is used for
a- Preparation of carboxylic acid.
b- For identification of alkyl benzene
3- Electrophilic aromatic substitution:
35
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
NO2
conc. HNO3
CH3
NO2
CH3
conc. H2SO4
and
nitrobenzene
H2SO4 \ SO3
CH3
CH3
SO3H
SO3H
CH3
and
CH3
X
X
CH3
AlCl3
X2
CH3
and
CH3
R
CH3
R
AlCl3
RCl
CH3
and
CH3
CH3
R
O
C
R
AlCl3
Cl
R
O
CH3
O
and
CH3
X =Cl ,Br ,
The alkyl group activate benzene ring and directs or than and Para
as discussed previously.
Halogenations of alkyl benzene:
There are two positions for the reaction either the benzene ring or the side
chain depending on the reaction condition.
Halogenations of the side chain by using halogen in presence of light or
high temp. The mechanism for reaction is free radical mechanism as in
halogenation of alkane.
The benzyl position is more probable for attach because the free radical
formed is more stable.
Halogenation of benzene ring involves electrophilic aromatic substitution in
presence of Lewis acid as catalyst.
36
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Halogenation of side chain may produce a poly halogenated Compounds.
CH3
CH3
Cl
Cl2
FeCl3
For mono halogenation of alkyl benzene it produces mixture of isomers.
Side Chain Halogenation of alkyl benzenes:
Bromination and chlorination of side chain give a mixture of isomers for
example bromination of Ethyl benzene, there are two position of attack but
actually only one product is formed.
CH2Cl
CH3
CHCl2
Cl2
Heat
Cl2
Heat
CCl3
Cl2
Heat
An abstraction of benzylic hydrogen is more favourable because the
benzylic radical fund through free radical mechanism is more stable than radical.
The order of ease of abstraction of hydrogen atoms
The mechanism of halogenation of the side chain is the same mechanism
of halogenation of alkane i.e. free radical mechanism.
The benzyl radicals are usually easy to form because it is more stable.
Ease of formation of free radicals.
Ease of formation of free radical mean that the more stable the radical the
more rapidly it is formed.
Stability of free radicals
Chlorination is less selective because chlorine is more reactive than
bromine.
37
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Resonance Stabilization of the Benzyl Radical:
8Toluene contain only two resonance structures I & II (kekule structures).
CH3
CH2
H
Also benzyl radical is a hybrid of two kekule structures III & IV.This resonance
stabilizes both toluene and benzyl radical to the same extent. But for benzyl
radical we can draw three additional resonance structure (V & VII).
CH3
CH3
I
II
CH3
CH2
V
III
The add electron located on the carbon atom ortho and Para to the side chain.
This mean that the add electron is delocalized. The contribution of these three
structures stabilized the benzyl free radical 19 Kcal/ml.
Triphenyl methyl: a stable free radical:
C
Cl
38
Cl
C
Zn
C
C
ZnCl2
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
The triphenyl radicals are usually stable because of resonance of the
benzyl radical there are large number of resonance structures i.e. the add
electron is highly delocalized and distributed over three aromatic ring.
The crowding among the large aromatic rings tend to stretch and weaken
the C-C band joining the triphenyl groups in the di mer (I) also the presence of
three bulky groups in the triphenyl radical make difficult for the carbon atoms to
approach each other closely enough for band formation. So I is not form at all.
Therefore one triphenyl methyl radical attack the other of the Para position.
Preparation of alkenyl benzenes:
Elimination with ring industrial of styrene by dehydrogenation.
CH2CH3
CH2= CH2
Al2O3
H3PO4
H2
CH2= CH2
Styrene
ETHYL BENZENE
In laboratory, by dehydration or Dehydrohalogenation.
H
C
CH3
KOH
H
alcohol
C
CH2
Cl
H
C
H
heat
CH3
ZnCl2
C
CH2
OH
Dehydrohalogenation of 1- phenyl – 2 chloro propane or dehydration of
1- phenyl – 2 propanol
H
H2
C C CH3
Cl
39
H
H
C C CH3
KOH
alcohol
heat
ZnCl2
H2 H
C C CH2
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
H
H2
C C CH3
OH
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
I is more stable than II, then the preferred product is I.
II can be converted rapidly in to 1- phenyl propane by treatment with hot
alkali.
H2 H
C C CH2
H
H
C C CH3
KOH
heat
The stability of the caudated system effect the orientation and the easy of
elimination.
C
H
C
H
Reaction of alkenylbenzenes:
1- substitution is the ring.
2- Addition to the double band.
The ring and the double bond are good sources of electrons.
The double band show higher reactivity than resonance stabilized benzene
ring toward electrophilic reagents.
1- Hydrogenation: The double band hydrogenated first under mild condition
while benzene ring hydrogenated under sever condition.
CH2CH3
CH2= CH2
H2
CH2CH3
20OC
H2
75min
125oc
100 min
Styrene
2- Oxidation: double band oxidized under mild condition to glycol and more
vigorous oxidation give carboxylic acid.
CH2= CH2
COOH
H2O2
Styrene
40
H
C CH2
KMnO4
OH OH
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Halogenation: halogen consumed by the double band first then after that
halogenation of the ring occurs. Since for preparation of ring halogenated
alkylbenzene. The halogenation of the ring before formation of double
band.
CH2CH3
CHClCH3
CH2CH3
Cl2 , FeCl3
CH2= CH2
Cl2 , heat
Cl
KOH
Cl
Cl
Addition to conjugated alkanylbenzene: orientation stability of benzyl cat ion:
This reaction is similar to the addition reaction to the double band of
alkene of unsymmetrical reagent. The addition is either electrophile or free
radical addition. The first step takes place in the way that yields the more stable
particle (carbonium ion is electrophilic addition or free radical in free radical
addition). This is consistent with the rule that the more stable particle is formed
faster.
The effect of benzene is to produce benzylic free radical or carbonium ion.
Stability of carbonium ions 3 benzyl > 2 alcohol > 1> CH3+
the benzyl cation is more stable due to the resonance that involving the benzene
ring
the same resonance structure warmed for benzylic free radical and this radical is
more stable .
benzyl
3
> 2
allyl
> 1>
CH3
therefore benzylic cation or radical are formed faster than that form from single
alkene . this resonance stabilization of the conjugated cation and free radical is
more important than the resonance stabilization of reaction.
41
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Analysis of alkyl benzene
alkyl benzene is identified by the treatment the compound with chloroform and
AlCl3 to give orange to red color .
if the compound give orange to red color on treatment it with chloroform and
AlCl3 , it mean that the compound is alkyl benzene, if other funci group analysis
is –ve .
the color is due to triaryl methyl cations Ar3C+ .
42
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Alkyl halide
H2
F C
CH3
CH3CH2 Cl
Ethyl Chloride
CH3CH2CH2CH2Cl
Butyl Chloride
CH3CH2 Cl
CH3CH2CH2CH2Cl
chloroethane
1-chlorobutane
C
H
CH3
Isopropyl
H
C
CH3
CH3
1-fluoro -2-methyl propane
CH3
Cl CH
CH3
2-chloropropane
Physical properties:
Alkyl halides have higher boiling point than alkane of the number of carbon
because the greater M.wt., Also iodide compound have higher b.p than Br > Cl
>F
They are insoluble in water in spite of their polarity soluble in most organic
solvent.
Iodo , bromo, poly chloro compound are more dense than water .
Industrial source:-
43
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Certain important halides are prepared industrially.
Preparation:
1- from alcohol by reaction of alcohol with hydrogen halide some time the
reaction require catalyst for least reactive hydrogen halide with 1 and 2
alcohol
In this method rearrangement be occur, this can be minimized by using
phosphoro halide.
44
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
2- halogenation of contain hydrocarbon
3- Addition of hydrogen halide to alkenes.
C
HX
C
C
C
H
X
+
HX= HCl, H Br, HI
peroxides
CH3CHBrCH3
CH3
H
H
C
C
H
Markonikov`s addition
2- bromo propane
or isopropylbromide
HBr
CH3CH2CH2Br
no peroxides
1- bromo propane
or n-propylbromide
anti Markonikov`s addition
4- Addition of halogens to alkenes and alkynes :
5- Halide exchange
45
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Alkyl iodide is often prepared from the corresponding bromide or chloride by
treatment with a solution iodide in acetone.
C
C
H
benzylic hydrogen
C
C
X
C
benzyl halide
C
C
H
C
X
Allylic hydrogen
Allylic halide
Reactions:
1- Nucleophilic substitution
Halide ion is weak base, so the hydrogen can be readily displaced as halide ion
by other stronger bases. These bases possess an un shared pair of electron and
seeking the positive site (nucleus) . Electron rich reagents are called nucleophilic
reagents. The typical reactions of alkyl halides are the nucleophilic substitution.
R:X+:Z
R:Z+X
Nucleophilic substitution
Nucleophilic reagent (strong base) leaving group ,The nucleophilic reagents are
either bearing negative charge like OH , CN , RO or neutral bases like ammonia
and water which contain unshared pair of electrons .
46
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
1- Elimination Reaction : Dehydrogenation previously discussed .
HX
C
47
C
C
C
X
H
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
2- Preparation of Grignard reagents
4- Reduction
Nucleophilic aliphatic substitution: Second order and first order kinetics.
For example reaction of methyl bromide with sodium hydroxide to produce
Methanol.
The reaction is carried out in aqueous ethanol is order to soluble all compound if
the reaction result from collision between hydroxide ion and methyl bromide
molecule , then the rate will depend on the conc. of both of those two reactant
.i.e.
Rate α conc of CH3 Br x conc . of OH as the conc increase the rate will be
increase
Rate = K[CH3 Br][ OH-]
K is the rate constant . for a particular reaction K value is constant at a given
temperature and a given solvent
48
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Therefore this reaction is second order reaction because the rate depends on
conc of two substances. Now the reaction of tetra – butyl bromide with
hydroxide ion .The rate of reaction depend only on the conc . of alkyl halide and
if we change the conc . Of OH the rate always the same.
Rate α cone of tetra butyl bromide
H3C
H3C
H3C
C
OH-
Br
H3C
H3C
C
OH
Br
H3C
Rate = K [Rx] .
This reaction is first order reaction because it depend on the conc of only one
substant.
Now why reaction
1- depend on conc . of OH while reaction 2- independence on conc . of OH
From the order of reaction we can say that the two nucleophilic substitutions
proceed by two different mechanisms.
The SN2 reaction Mechanism and kinetics:
For the reaction
CH3Br
CH3OH
OH
Br
Rate = K[CH3 Br][ OH-]
The rate depends upon the conc of both reactions i.e. the reaction occur by the
collision between a hydroxide ion and methyl bromide molecule.
The OH attack CH3Br from the side back side for away from bromide i.e.
attacks the molecule from the rear.
49
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
C2H5
C2H5
H
C
Br
HO
H
C
CH3
Br
HO
C
H
Br
C2H5
H3C
CH3
OH
SN2
The HO- C bond form simultaneously with breaking of C- Br bond and the CBr bond in the T.S is not completely and the C- OH is not completely formed
and each group have partially negative charge. the – OH and – Br are located is
for apart as possible .
1- This mechanism is called bimolecular nucleophilic substitution (SN 2) which
is follows second order kinetics particles (SN2 substitution nucleophilic
bimolecular).
Bimolecular is used because the r.d.s. involves collision of two particles. This
mechanism is consistent with the kinetics of reaction.
2- Another evidence for SN2 is the stereochemistry. In which the reaction yield a
product whose configuration is opposite to that of reactant.
3- i.e. the SN2 reaction proceeds with complete inversion of configuration.
Evidence
The rate of SN2 reaction depends on size of groups attached by the reaction
center. The increase of the group’s size will decrease bulk of substitution the
rate of SN2 reaction. i.e. the differences in rate between two SN2 reactions seem
to be due chiefly to steric factors and not to electronic factors .
In SN2 reactions the order of reactivity of Rx is CH3X > 1º > 2º > 3º
The SN1 reaction Mechanism, kinetic, relative reactivity.
50
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
The reaction between tert – butyl bromide and hydroxide ion to yield tert – butyl
alcohol follows first order kinetic , i.e. the rate depends upon the conc.of only
one reactant , tert – butyl bromide .
C2H5
C2H5
H
OH
C
Br
H
C
H3C
H3C
The rate is consistent with the following mechanism:
The first step is the slow dissociation of t- butyl bromide to bromide ions and
tert– butyl alcohol. then the carbonium ions combine rapidly (syep2) with
hydroxide ions to yield tert- butyl alcohol .The rate of reaction is determined by
the slow step (breaking of C – Br bond) r.d.s : A single step whose rate
determines the overall rate of a stepwise reactions . The energy require for
breaking of C- Br bonds coming from the formation of ion – dipole bonds
between the two kinds of ions and the solvent.
This mechanism called SN1 substitutes unimoleculer nucleophilic substitution.
Evidence
1- The mechanism is consistent with the first order kinetics. i.e. SN1 reaction
follows first order kinetics. Therefore the rate of reaction determines by the rate
of ionization of alkyl halides, that is , depend only on the conc of alkyl halide .
3-The SNI reaction Stereo chemistry:
51
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
H2O
a
b
C6H13
H
CH
x
CH3
a
b
C6H13
C6H13
H
HO
OH
C
H
C
CH3
H3C
Enantiomers
SNI reaction proceeds with racemization. The carbonium ion has a flat structure
, then the nucleophilic reagent OH – attached itself to the carbonium ion either
face of this flat ion .
3- The reactivity, In SN1 mechanism is face carbonium ion 'is formed then any
group stabilize the carbonium ion will increase the rate of reaction . therefore the
order of reactivity of alkyl halides in SN1 reaction is the same order of stability
of carbonium ions .In SN1 reactions the order of reactivity of R x is alkyl, benzyl
> 3º > 2º> 1º
The rate of SN1 reaction is affected largely by electric factors, i.e. the ability of
substituent’s to release or with draw electrons.
4- Rearrangement:
If the SN mechanism involves formation of carbonium ion , then we expect
occurrence of rearrangement to more stable carbonium ion .
52
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
CH3
C2H5O-
H3C C
SN1
CH2OC2H5
NO rearranagment
CH3
Neo pentyl ethyl ether
CH3
H3C
C
CH2Br
CH3
CH3
H3C C
CH2CH3
OC2H5
Tert-pentyl ethyl ether
C2H5OH
Rearranagment
CH3
SN1
H3C C
CHCH3
2-methyl -2-butene
Also SN1 reaction is accompanied by elimination reaction.
2- Elimination reaction:
Also there are two mechanisms for elimination. E2 (bimolecular elimination)
and E1 (unimoleculer elimination).
E2 involve single step , base pulls a hydrogen ion a way from carbon , and
simultaneously a halide ion separates .
E2 (bimolecular elimination
X
C
X
C
H
C
C
H:B
B
An this mechanism the r.d.s. involves two molecules . Some time alkyl halide
particularly tert – alkyl halide can carried out by two , dissociation step of alkyl
halides into halide ions and carbonium ions in slow step , than the carbonium
ion loss proton to base to form alkene in the second step .
53
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
E elimination
X
C
C
C
X
Carbanion
H
H
C
slow
C
C
C
H
fast
H:B
C
B
The r.d.s (slow step) involve one molecule which the first step .
The order of reactivity of alkyl halide toward E2 or E1 is the same. 3º > 2º > 1º .
Evidence of E1 mechanism
1- Follow first – order kinetics.
2- The reaction accompanied by rearrangement.
Evidence for E2 mechanism
1- Follow second – order kinetics.
2- Are not accompanied by rearrangement.
X
C
X
C
C
H
C
H:B
Carbanion
B
X
C
C
X
C
C
orientation of elimination :The formation of more stable alkene is called saytzelf orientation according to
substituted alkenes.
54
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Elimination as substitution
There is competition between substitution and elimination both reactions result
from attack by the same nucleophilic reagent : attack at carbon causes
substitution , attack at hydrogen causes elimination .
X
C
C
E2 , SN2
H
Z:
The speed of elimination of alkyl halides depend chiefly upon the stability of the
alkene formed . text – alkyl halides which yield highest branched (more stable)
alkenes , undergo elimination fastest and substitution slowest primary halides ,
undergo substitution fastest and elimination slowest .
Reaction of others ethers are un reaction compounds. They are star towards
bases oxidizing and reducing agents ethers undergo just one k of reaction, which
is cleavage by acids>
55
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Alcohol:
Structure of alcohols :The general formula for alcohol is R-oH when R is any alkyl or substituted
alkyl group. And it any primary, secondary or tertiary and it may be open
chain or cyclic, also it may contain double or triple bond, halogen or
romantic ring.
The function group is the hydroxyl group (-OH)
Classification of alcohol:- primary, secondary or tertiary
We classify a carbon atom as primary , secondary or tertiary according to the
number of other a carbon atoms attached to it . An alcohols is classified
according to thekind of carbon that bears the - hydroxyl group (-OH):
H
H
R C
OH
R
C OH
R
R
H
primary
R
O
secondary
1
O
2
C OH
R
tertiary
O
3
These classes may be different in rate or mechanism of reaction.
Naming of alcohol
Common name such as ethyl alcohol, isopropyl alcohol. Naming the alkyl
group then the word alcohol.
56
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Another classes is by using carbinol i.e. alcohol is derived from methyl alcohol.
CH2(CH3)2
CH3
H
H3C CH2
OH
CH3
OH
CH2(CH3)2
Di ethyl carbinol
ethyl di isopropyl carbinol
The IUPAC Name.
By selecting the longest chain which contain OH group. Then replacing –e
from alkane by – OH indicating the position of OH .
Physical properties:
1- The present of H attached with highly electronegative group, make
permits the formation of hydrogen bonding.
2- This is responsible for the high boiling point of alcohol much higher than
hydrocarbon.
3- Alcohol show increase in b.p with increasing carbon number and decrease
in b.p. with branching.
4- Lower alcohol miscible with water and the solubility decease on the no. of
carbon increase.
5- Poly hydroxyl alcohol such ethylene glycol miscible with water with high
b.p (anti-freeze).
57
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Hydrogen bonding
Industrial source:
Alcohol os important because nearly every other kind of aliphatic compounds
alkene, alkyl halide , aldehydes, ketones, acids, ester, also used as a solvent (for
carrying out the reaction and for recentralization:
1- By hydration of alkene: which obtained from petroleum.
2- Fermentation of carbohydrate to product ethanol.
Preparation of alcohol:
1- oxymercurtion – demecuration of alkene.
Involve addition of HgOAc and –OH to C = C then, replacement of HgOAc by
C –H.
Advantages: the reaction is fast, convenient, tack place under mild condition
give excellent yield (90%). Tack place at room temp. within minutes, then
reduction in situ with NaBH4. Addition according to make.
-hydroboration- oxidation and hydrogen attached to other carbon C = C. then
by oxidation the boron is replaced by – OH. C2& C3: the reaction produce is
simple and convenient to give high yield, and the produce are ones difficult to
obtain from alkene in any other way. It involve to other carbon of C = C.
58
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
For example:-
Grignard synthesis:
Grignard regents react with carbonyl compounds (aldehydes or ketones) to
yield alcohol.
X = Cl ,Br ,I , R= Alkyl ,benzyl ,aryl , (Phenyl and subs. Phenyl).
C- Mg bond to foully polar, carbon, is negative relative to Mg, than the
carbon of carbonyl and mg because attached to oxygen.
59
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Reaction of Grignard reagent with epoxides give alcohols Contusing two
more carbons than the Grignard reagent.
And with ester to give 3aldehydes
Useful of Grignard regent in alcohol synthesis:
We must look to the structure of the alcohol we went of the groups attached
to the carbon bearing the OH group, one must come from Grignard reagent,
the other two must come from the carbonyl compound.
The routes uses depend on the starting material, which one is available.
60
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Limitation of the Gigrand synthesis:
1- any acidic hydrogen decompose Grignard Regent any hydrogen attached
to electron negative element (oxygen, nitrogen, sulfur, or even triply
bonded carbon).
2- Grignard regent reach rapidly with atmospheric humidity, oxygen and
carbon dioxide.
3- Grignard reagent reacts with every organic compounds containing
carbon- oxygen or carbon- nitrogen multiple bond.
4- Grignard regent cannot prepared from compounds contain in additional to
halogen. Contain certain groups such as carboxyl COOH, hydroxyl (-OH)
amino (NH2) and SO3H (All contain acidic hydrogen which decompose
Grignard reagent . Another groups such as ( C = O ), COOR, and C=N,
which Grignard reagent added to it. NO2 group oxidize Grignard reagent .
Also the aldehyde or ketene must not contain groups that react toward
Grignard reagent in which the Grignard reagent decomposed before it could
add to the carbonyl group ( G may not , NO2, COOH, NH2)
Reactions of alcohols :The functional group of alcohol is –OH group, reaction of an alcohol can
involve the breaking of either , C….OH bond, (removal of OH group), or
O….H bond (removal of ……H bond halide).
A- C-- OH bond cleavage
1- reaction with hydrogen halide:
Alcohols react readily with hydrogen halide to yield alkyl halide and water.
61
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
This reaction is:
1- Catalyzed by acid. (such as H2SO4 )
2- Rearrangement of alkyl group occurs. Except with most, alcohols
3- the order of reactivity of alcohol toward HX is , alkyl, benzyl, > 3º, > 2º,
>1º >CH3.
Mechanism: (nucleophilic substitution)
1- protonation of alcohol (catalysis by acid)
2- formation of carbon ion ( occurrence of rearrangement)
the order reactivity follows the stability of carbonium ion.
3- carbonium of halide ion the carbonium ion to from alkyl halide.
62
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Primary alcohols do not undergo rearrangement because they do not read by
this mechanism.
2 reaction with phosphor tri halide:
The reaction is preferred because to avoid arrangement do not take place. If we
want to prepare
IF we start with dehydration of alcohol we get
Therefore we cannot to alkyl halide by using PB3 then dehdrohalogen by
alcohol by alcoholic KOH.
63
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
3 Dehydration, discussed previously:
Reactivity of ROH,: 3º< 2º<,1º.
E2 not occur.
Reactivity: depend on the easy of formation of carbonium which inturn depend
on the stability carbonium ion benzyl: 3º, 2º,1º.
B. O…..H bond cleavage:
4. reaction as acids, reaction with metal:
Alcohol is a base, it accept proton to form proton alcohol , ROH2, alcohol loss
proton to form alkoxide ion RO- , because the hydrogen is banded to
electronegative atom therefore alcohol is considered to have appreciable
activity. This is shown by the reaction of alcohol with active metal.
RO-H
M
CH3CH2OH
Na
RO M
1/2
CH3CH2OHNa
H2
M= Na . K , Mg ,Al etc
1/2
H2
Alcohol is water acid, then the water but stronger acid than acetylene :
4 Ester formation:
64
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
ArSO2OR
:Z
ASrO2O
ArSHO3
R:Z
:B
C
C
ArSO3
H:B
C C
O
S
H3C
Elimination
O
O
S
Br
O
O
tosyl or Ts
brosyl or Bs
H3C S
O
Mesyl or M s
Or formation sulfonation Easter
Base
H3C
CH3CH2 OH
SO2Cl
H3C
SO2CH2CH3
5 - oxidation of alcohol:
The product of oxidation of alcohols Depends on the structure of alcohols
Whether the alcohol is 1º, 2º, or 3ºtertiary .
OH
(O2)
O
H
H
Primary alcohol oxidize to aldehydes by use K2Cr2O7, but aldehydes oxidize
easier than alcohol to carboxylic acid, therefore the aldehyde must be remove
from the reaction mixture.
RCH2OH
1º
KMNO4
purple
RCOOK
Soluble in H2O
RCOOH
RCH2OH
Cr2O7
R
H
C
O
MNO2
Brown
KOH
Carboxlic acid in soluble in water
Cr+3
H
1º
K2Cr2O7
RCOOH
Secondary alcohols are oxidized to ketone by chromic acid, aqueous K2Cr2O7, or
Cr2O3.
65
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
R
R
RCHOH
K2Cr2O7 or
R
CrO3
C
O
Cr+3
Acetone
2º
KMnO4 lead to C – C bond breakage.
Tertiary alcohols are not oxidized at all under condition, but in presence of acid.
They are rapidly dehydrated to alkene conditions but in presences of acid they
are rapidly dehydrated to alkene, which are then oxidized.
R
RCHOH
1)
HCrO4
H
R2CHOCrO3H
HOH
2º
R
2)
R
C
R
O -CrO3H
R
H
OH
C
HCrO3
H2O
Acetone
H2O
R
RCHOH
3)
Cr+3
R2C
O
Cr+4
Cr+4
R2C
O
Cr+3
2º
R
RCHOH
4)
2º
R
R
R
C
O
O
H
R
C
O
HCrO3
H2O
Cr
HO
O
Analysis of alcohol.
1- alcohol dissolve in H2 SO4.
2- Alcohol do not decolorize bromine in CCl4.
3- Alc. oxidized by CrO4 in H2SO4 within two minute to give blue- green or
opaque colour. And tertiary alcohol do not oxidized
4- Lucase test used to different 1º, form 2º, from 3º based on the reactivity of
alcohols Toward HX (Lacus regent ZnCL2 in HCL, 3º react immediately ,
2º react within 5min. 1º does not react at room temp.
66
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
5- Iodoform test is used to know the presence of by treatment of alcohol with
iodine and Sodium hydroxide NaOH.
Lucase test for distinguishbetween 1º, 2º, or 3ºtertiary .
OH
R
ZnCl2
H
No reaction
HCl
H
OH
R
Cl
ZnCl2
H
HCl (2-5)min.
R
R
H2O
R
Cl
OH
ZnCl2
R
H
R
R
HCl
67
R
R
H2O
R
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Ethers and epoxides:
Ethers:Either is of general formula R-O-R, R-O-Ar or Ar- O- Ar. To name ethers,
name each group attached to oxygen and follow then by ethers.
CH3
CH3CH2OCH2CH3
H3C
Ethyl ether
C
CH3
O
CH3
H3C
CH3
C
H
O
isopropyl Phenyl ether
methylterbutyl ether
O
Phenyl ether
If one group has on simple name, the compound may be named as an alkoxy
derivative:HO
CH3CH2CH2CHCH2CH3
CH2CH2
OC2H5
OCH3
2-Ethoxyethanol
3-methoxyhexane
COOH
C2H5O
4-ethoxybenzoic acid
O
CH3
anisole
Physical properties:
Ethers possess a small dipole moment. This dipole moment does not affect
the b.p of ethers, it have much lower b.p for ethers are very slightly soluble in
water diethyl ether is highly volatile and flammable.
Preparation of ethers:
1- Williamson synthesis: for preparation of symmetrical and asymmetrical
ethers, as aryl alkyl ethers as well as di alkyl ethers.
68
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
This reaction is substitution reaction of alkyl oxide or phenoxide ion for halide
ion.
Aryl halide cannot be used because of their reactivity toward nucleophile
substitutions: Sodium alkoxide prepare by direct action of sodium metal on dry
alcohol:
While sodium phenoxides are made by the action of aq. Sodium hydroxide on
phenol because the acidity of phenolic hydrogen.
69
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
For preparation of ethyl tret- butyl ethers.
For second reaction elimination reaction will be occur
Primary alkyl halide undergo subs. Reaction rather than elimination
dehydrogenation 3º 2º, 1º.
2- preparation of (Ether ) Alkoxy mercuration – demecuratoin :
By reaction of alkones with mercuric Trafford acetate in presence of alcohol to
give compounds which reduced to ether.
70
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
According to Markonickov’s rule without rearrangement.
3- preparation of ether . cleavage acid
Cleavage reaction occurs only under rigorous conditions concentrated acids
(usually HI on HBr) and high temp.
Cleavage involved nucleophile attack by halide ion on the protonated ether
71
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Alkyl group tends to undergo SN2 displacement whereas a tertiary Alkyl
group tends to undergo SN1 displacement.
2. Electrophilic aromatic subs in aromatic ethers.
Discussed is previously alkoxy - OR is other , Para directing and
moderately activating gr much stronger activator than –R but much weaker than
OH.
OR
OR
H
Y
H
Y
II
I
Cyclic ethers ;O
O
O
O
O
1,4-dioxane
1,4-dioxene
72
tetrahydrofurane
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Epoxides
Epoxides are compounds containing three membered ring
They are ethers with unusual properties.
Most important Epoxides is ethylene Epoxides which is prepared by
catalytic oxidation of ethylene by air.
H2C
O2 , Ag , 250o
CH2
CH2
H2C
O
Ethylene epoxide
Preparation of epoxides
1- form halohydrins
X2 ,H2O
C
C
C
C
X
OH
+
H2O
C
C
OH
O
CH2
CH3
H
C
C
Cl2 ,H2O
CH3
H
H
H
C
C
OH
Cl
H
+
C
C
OH
Cl
HCl
+
H2O +
C
C
Conc. aq. OH
X
Ethylene epoxide
Example:H
+
O
X
Ethylene epoxide
Mechansim
Br
1)
H2C
Br
CH2
+ OH
+
H2O
OH
H2C
CH2
O
Br
2)
H2C
CH2
O
H
H
Br
H
C
C H
O
CH2
H2C
+
Br
O
Ethylene epoxide
73
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
2 -preparation of carbon-carbon double bonds.
The carbon-carbon bond can be oxidized directly to the epoxides by per
benzoic acid.
C
+
C
C6H5CO2OH
+ C6H5COOH
C
C
Peroxy benzoic acid
O
Example:H
C
CH2
+
H
C
C6H5CO2OH
CH2 + C6H5COOH
O
styrene
styrene oxide
Peroxy benzoic acid
O
cyclohexene
cyclohexene epoxide
Reaction of Epoxides
Epoxides are highly reactive due to the case of opining of highly strained
three membered ring (i. e. the molecule to highly unstable).
1. Acid catalyzed cleavage
Epoxides converted into protonated epoxides which can undergo attack by
any nucleophile
O
C
C
O
H
C
Z:
H
C
OH
C
C
Z
Example:-
74
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
+
H2O
+
H
CH2
H2C
H2C
OH OH
O
+
H
CH2
H2C
+
C2H5OH
+
CH2
H2C
O
OH
CH2
OC2H5 OH
+
H
CH2
H2C
O CH2 CH2OH
O
Phenol
O
C
C
H
C
H
OH
C
C
Z:
O
C
Z
+
H
H2C
CH2
H2C
O
H
H
O
CH2
O
H
+
H2C
+
CH2
H
H2C
O
H
OH2
CH2
OH
H2C
CH2
+
H
OH OH
Glygol
R
H
O
+
H2C
+
CH2
H
O
H
H2C
H
OR
CH2
OH
H2C
OR
CH2
+
H
OH
• Base catalysed cleavage:
Unlike ethers, epoxides can be cleavage undergo alkaline conditions
epoxides itself undergoes nucleophilic attack.
75
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
O
H
C
Z
C
C
OH
HZ
C
C
O
Z:
C
+
Z
Example
C2H5O Na
+
H2C
O
Sodium ethoxide
+
ONa
H2C
O CH2CH2OH
CH2
O
sodium phenoxide
NH3
2- Ethoxy ethane
C2H5O CH2CH2OH
CH2
H2C
+
2-phenoxyethanol
CH2
H2NCH2CH2OH
2-aminoethanol
O
Ethanol amine
3. Reaction with Grignard reagent
Discussed previously
To prepare primary alcohols With two carbons more than the alkyl or aryl
group of Grignard . Reagent (nucleophilic attack)
R
Mgx +
H2C
CH2
RCH2CH2OMg
H
RCH2CH2OH
O
example
CH3CH2CH2CH2Mg
+
H2C
CH3CH2CH2CH2CH2CH2OH
CH2
hexanol
O
MgBr
H2C
CH2CH2OH
CH2
2-phenylethanol
O
R
MgX +
H2C
CH2
RCH2CH2OMgX
H
RCH2CH2OH
O
76
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
:Z
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Orientation of Cleavage of Epoxides
For unsymmetrical epoxides, carbons are not equivalent. The attack is
depending chiefly on the catalyst (acid or base). In general, the nucleophile
attacks the more substituted carbon in acidic catalysed cleavage and the less
substituted carbon in base catalysed cleavage.
CH3
H3C
C
CH2
H218O
+
CH3
CH3ONa
H3C
O
C
CH2OH
18
OH
CH3
H3C
C
CH2
+
CH3OH
CH3
CH3ONa
H3C
O
C
CH2OH
OH
Both reaction follow SN2 mechanism. Cleavage of C-O bond and attack by the
nucleophilic occur in a single step.
Attack occurs not at the less hindered carbon , but at the carbon that can
best accommodate the positive charge.
Acid - catalyzed SN2 Cleavage
Z
O
C
Z:
H
C
Z
C
C
O
C
C
HO
H
Bond - breaking exceeds
bond- making :
positive charge on carbon
77
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬
Science of college, Dep. of Chem., Lecturers in Organic chemistry by Dr.Ihmood .kh .jebur 2017
Attack occurs at the less hindered carbon.
Base - catalyzed SN2 Cleavage
Z
Z:
+
H2C
Z
CH2
O
C
C
C
C
O
O
Bond- making on partcular
charge on carbon
Analysis of ethers
Ether dissolved in conc. H2SO4 (cold).
By Zeisel test involve cleavage by hydroxide and identification of the products
formed.
78
7102 -‫ هحاضرات الدكتور احوود خلف جبر الجبوري‬/‫كلية العلوم –قسن الكيوياء‬