HALOALKANES AND HALOARENES
HALOALKANES
AND
HALOARENES
Chapter Outline:
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Prerequisites
Learning Objectives
Introduction
Classification
Nomenclature
Nature of C-X Bond
Methods of preparations aryl halides
Uses of some polyhalogen compounds
Summary
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HALOALKANES AND HALOARENES
PREREQUISITES
w Organic compounds i.e., hydrocarbons are classified into different types based on the type of functional
group present in it.
w If the hydrogen in hydrocarbon is replaced by halogen then they are called halo compounds.
w Organic compound it named according to IUPAC system.
w Prefix + Root word + 10 suffix + 20 suffix.
w Depending on the no.of carbon to which the0 carbon is attached it is classified as 10, 200, 30 and
4 0.
Primary if carbon attached to 1 carbon, 2 – if carbon attached to 2 carbons and 3 – if carbon
attached to 3 carbons and 40 – if carbon attached to four carbons.
w Carbon adjacent to functional group called α- carbon, next called β next γ and so on.
w Carbon with +ve charge called carbocation and –ve charge called carbanion.
w Carbon attached to four different atoms or groups called chiral carbon or stereo center or asymmetric
carbon.
w The light which travels only in one direction called plane polarized light.
w The compound which rotate plane polarized light is called optically active compound, if its rotates to
right called dextro- rotatory or (+) isomer and to left called laevo rotatory or (-) isomers.
w A pair of non super imposable mirror images are called enantiomers.
w If the configuration of the compounds remains same even after the reaction then it is called retension
in configuration.
w If 50 : 50 mixture of two configurations obtained after the reaction then is called racemization.
LEARNING OBJECTIVES
In this chapter we are going to discuss about.
w Classification of halo alkanes and arenes
w Nomenclature of these compounds
w Methods of preparation of mono halo alkanes, tri halo- alkanes and halo arenes.
w Physical and chemical properties of these compounds.
w About the nature of carbon halogen bond.
w Uses of poly halogen compounds.
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HALOALKANES AND HALOARENES
INTRODUCTION
w Compounds formed by the replacement of one or more hydrogen atoms of the aliphatic and aromatic
hydrocarbon by same no.of halogen atoms are called aliphatic halogen derivatives, which are
commonly called alkyl halides and aryl halides respectively.
CLASSIFICATION
w These are classified in to three types based on the carbon to which the halogen is attached. Primary
alkyl halide if halogen is attached to 10 carbon. 20 alkyl halide if halogen is attached to 20 carbon. 30
alkyl halide if halogen is attached to 30 carbon.
Eg: Ethyl bromide where bromine is attached to10carbon.
H
CH3
C
Br
Ethyl bromide (10 Alkyl halide)
Br
2- Bromo propane (20 Alkyl halide)
Br
2- Bromo - 2- Methyl propane (30 Alkyl halide)
H
CH3
CH3
C
H
CH3
CH3
C
CH3
w Depending on the no.of halogens these may be classified as mono, di, tri, or poly halogen
compounds.
X
C 2H 5X
CH2X
CH2X
Monohalo Alkane
Dihalo Alkane
X
CH2 X
CH X
Monohalo arene
X
Dihalo arene
Trihalo Alkane
CH2X
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HALOALKANES AND HALOARENES
Mono halo compounds
w The general formula of mono halo alkanes is CnH2n+1 X (saturated)
w These can be classified as 10, 20 and 30 halo alkanes. Depending upon the hybridization of the
carbon atom to which halogen is attached these are of different types.
(a) Alkyl halides: In which the halogen attached carbon undergo sp3 hybridization
Eg: CH3 – Br,
C2 H5Br
H
H
C
Br
H
Methyl bromide
H
H
H
C
C
H
H
Br
Ethyl bromide
(b) Allylic halides: In which the halogen is attached to sp3 carbon which in turn bonded to a
double bonded carbon.
Eg: CH2 = CH – CH2 –Cl → 3 – Chloro-1- propene. (or) allylic chloride
H
CH2 = CH
C
Cl
H
3 – Chloro-1- propene
(c) Benzylic halide: In which the halogen is attached to sp3 carbon which in turn attached to aromatic ring.
CH2
Cl
Chloro phenyl methane
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HALOALKANES AND HALOARENES
(d) Vinylic halide: In which the halogen is attached to sp2 carbon i.e., a double bonded carbon.
H
CH2 = C
Br
Vicinal dihalide
(e) Aryl halide: In which the halogen is attached to sp2 carbon in an aromatic ring.
Br
Bromo benzene
Di halo compounds
w General formula of these compounds is Cn H2n X2 (Saturated).
w Vicinal dihalide: If the two halogens are attached to adjacent carbon.
CH2Cl 1,2- Dinchloro ethane
CH2Cl NOMENCLATURE
Isomerism
Alkyl halides exhibit
Chain isomerism
Positional isomerism
Chain isomerism
w Alkyl halide with minimum 4 carbon atom exhibit chain isomerism.
Eg: n-butyl chloride and isobutyl chloride are chain isomers.
CH3 CH2 CH2 CH2 n-butyl chloride
Cl
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HALOALKANES AND HALOARENES
Formula
Comman Name
CH3Cl
Methyl chloride
Chloro methane
n-propyl chloride
1-chloro propane
Iso propyl chloride
2-chloro propane
t-pentyl iodide
2-iodo-2-methyl butane
CH3 CH2 CH2Cl
CH3
CH
CH3
Cl
CH3
CH3
C
CH2
CH3
IUPAC Name
l
(CH3)3 C CH2 Br
neo-pentyl bromide
CH2 = CHCl
Vinyl chloride
Chloro ethene
CH2 = CH - CH2Cl
1-Bromo -2,2- dimethyl propane
Allyl chloride
3-Bromo propene
CH2Cl
Benzyl chloride
Chloro phenylmethane
Cl
O-Chloro toluene
1-chloro-2-methyl benzene
(or)
2-chloro toluene.
CH3 - CHCl2
ethylidene chloride
1, 1-dichloro ethane.
CH2Cl - CH2Cl
ethylene dichloride
1, 2-dichloro ethane
O-dichloro benzene
1, 2-dichloro benzene
m-dichloro benzene
1, 3-dichloro benzene
p-dichloro benzene
1, 4-dichloro benzene
CH3
Cl
Cl
Cl
Cl
Cl
Cl
Positional isomerism
Alkyl halides with minimum 3 carbon atoms (mono halides) and 2 carbon atoms (dihalides) exhibit
positional isomers.
Eg: a) 1-chloro propane and 2-chloro propane
CH3 CH2 CH3 CHCl CH2 CH3
Cl
1-chloro propane
2-chloro propane
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HALOALKANES AND HALOARENES
b) 1,1-dichloro ethane and 1, 2-dichloro ethane.
CH3 CH2Cl
CHCl2 1,1- dichloro ethane
CH2Cl
1,2- dichloro ethane
w If there is any double bond in the molecule then may exhibit geometrical isomerism, and if there is
any chiral carbon in the molecule they may also exhibit optical isomerism.
NATURE OF C-X BOND
w The bond between carbon and halogen in alkyl halide is polar in natural since halogens are more
electronegative than carbon. So carbon bears a partial positive charge and halogen partial negative
charge.
H
H
C
X = F, Cl, Br, I
X
H
w Carbon halogen bond length increases as we move from C - F to C - I
CH3
F < CH3
Cl < CH3
Br < CH3
I
Br > CH3
I
w Bond energy increases from C-F to C-I
CH3
F > CH3
Cl > CH3
Methods of preparation of mono halogen compounds
From alcohols
w Alkyl halides are best prepared from alcohols. The reaction follow either
SN1 or SN2 mechanism.
1) When alcohol is treated with dry HCl and anhydrous ZnCl2 corresponding alkyl halide is formed. Mixture of (1:1) dry HCl and anhydrous ZnCl2 is called Lucas reagent.
anhydrous
R OH + dry HCl Alcohol
ZnCl2
RCl2 + H2O
The reactivity of alcohols towards HX is allyl, benzyl > 30 > 20 >10 and the reactivity of halogen acids
is HI > HBr > HCl > HF.
R
OH + HX RX + H2O
w (Reaction of
10 and 20 alcohols with HCl requires a catalyst like ZnCl2 but 30 react with HCl at room
temperature).
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HALOALKANES AND HALOARENES
w Alcohols react with PX3 and PX5 to give corresponding alkyl halide. PBr5 and PI5 are highly unstable due to steric hinderence, so only chlorides are prepared by this method.
3 ROH + PX3 3RX + H3PO3
ROH + PX5 RX + POX3 + HX
w Alcohols react with thionyl chloride to give pure alkyl halide because the other two products
are escapable gases. This process is called Darzens procedure. Bromide and iodide are not prepared because SOBr2 and SOI2 does not exist.
RCl + HCl + SO2
ROH +SOCI2 w Alkyl bromides and iodines can be prepared by treating alcohol with red phosphorous with
Bromine or iodine.
ROH Red P/X2
RX
X2 = Br2 or I2
wAlkyl bromide can be obtained by the action of sodium bromide on alcohol in presence of H2SO4
ROH + NaBr + H2SO4 RBr + NaHSO4 + H2O
Alcohol
Alkyl bromide
w Alkyl iodide can be obtained by heating alcohol with sodium or potassium iodide in 95%
phosphoric acid
ROH Alcohol
95%H3PO4
NaI
RI
Alkyl iodide
From hydrocarbon
w This reaction takes place in presence of sunlight or heat and follows free radical mechanism.
Reaction of F2 with alkanes is explosive therefore they are prepared by halogen exchange methods. Iodination occurs only in presence of an oxidizing agent such as HgO, HIO3 HNO3 because direct reaction is a reversible reaction.
RH + X2 hn
Alkane
RX + HX
Alkyl halide
wAddition
of halogen acid to alkane follow Markownikoff rule and the mechanism is electrophilic
addition. At high temperature addition of halogen becomes reversible and hence does not occur.
R
CH = CH2 + HBr
R
Propane
CH
CH3
Br
2- Bromo Propane
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HALOALKANES AND HALOARENES
C = C + HX Ethylene
C
C
H
X
Ethyl halide
Halogen exchange method
w Alkyl chlorides and bromides are converted into their iodides or fluorides by treating them with NaI/
acetone or inorganic fluorides like Hg2F2 respectively. Conversion of alkyl halides to iodides or
fluorides is called Finkelstein reaction.
RCl
or
NaI/acetone
RI + NaCl
or Alkyl lodide
NaBr
RBr
RCl
or
RBr
NaI/acetone
RF + Hg2Cl2
or
Alkyl fluoride
Hg2Br2
w Alkyl
fluorides which cannot be prepared by Finkelstein reaction can be prepared by treating
chlorides or bromides with mercurous fluoride or antimony fluoride or AgF. This reaction is called
swarts reaction.
2CH3Cl + Hg2F2 2CH3F + Hg2Cl2
Methyl chloride
Methyl flouride
CH3Br + AgF CH3F + AgBr
Methyl flouride
Methyl Bromide
From silver salts of fatty acid
w When silver salts of fatty acids are treated with Br2 in CCl4 gives corresponding alkyl bromides. This
method is called Borodine Hunsdicker method. This reaction follow free radical mechanism. Yield of
alkyl chloride is less than alkyl bromide.
RCOOAg + Br2 CCl4
2RCOOAg + I2 RBr + AgBr + CO2
RCOOR + CO2 + 2AgI
Ester
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HALOALKANES AND HALOARENES
Physical properties
w Alkyl halides are colorless when pure. However bromides and iodides develop color when exposed
to light. Many volatile halogen compounds have sweet smell.
1. Methyl fluoride, chloride and bromide and ethyl chloride are gases at room temperature remaining are color less liquids up to C18 and beyond them are color less solids.
2. Alkyl halides are very slightly soluble in water. Even though these have polar nature they are insoluble in polar solvents as they have no ability of forming H – bonding with water. These are soluble in organic solvents like benzene, ether etc.
3. Alkyl halides have high boiling points than alkanes equal molecular weights the order is RI >RBr > RCl > RF. With the increase in size of alkyl group boiling points increases
and with the increase in branching boiling points decreases.
1 – Bromo butane → 375K ,
2 – Bromo butane → 364K,
2 – bromo – 2 – methyl propane → 346K.
4. Fluoro and chloro compounds are lighter than water where as bromo and iodo compounds are heavier than water. Density decreases with the increase in the size of alkyl group.
5. Dipole moment decreases as electronegativity of halogen decreases. But fluorides have lower value because its small atomic size.
RCl > RF > RBr > RI
Chemical properties
Haloalkanes
Haloalkanes
Nucleophilic substitution reactions
Elimination reaction
Reaction with metals
Reduction reaction
Friedel - craft alkylation
Nucleophilic substitution reactions:
w Alkyl halide on treatment with aqueous NaOH or KOH corresponding alcohol is formed.
RX + KOH (eq)
ROH + KX
Alkyl halide
Alcohol
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HALOALKANES AND HALOARENES
w Alkyl halide on treatment with moist silver oxide alcohol is formed.
RX + AgOH ROH + AgX
Alcohol
Alkyl halide
w Alkyl halide on treatment with dry silver oxide gives ether.
2RX + Ag2O ROR + 2AgX
dry silver oxide
Alkyl halide
Ether
w Alkyl
halides react with sodium alkoxides to give ethers. This reaction is called William son’s
synthesis.
|
RX + R ONa Alkyl halide
ROR + NaX
Ether
Sodium
alkoxides
w Alkyl halide reacts with KCN and AgCN to give cyanides and isocyanides as the major products
respectively. AgCN is covalent in nature and nitrogen free to donate electron pair forming isocyanide.
But KCN is ionic in nature and provide cyanide ion in solution because C – C bond is more stable
than C – N bond.
R–X + KCN Alkyl halide
R–CN + KX
(alc)
Alkyl cyanide
R–X + AgCN R–NC + AgX
Alkyl halide
Alkyl isocyanide
w Alkyl halides react with KNO2 and AgNO2 to give alkyl nitrites and nitro alkanes respectively as the
major products.
R–X + KNO2 R–ONO + KX
Alkyl halide
Alkyl nitrites
R–X + AgNO2
R–NO2 + AgX
Nitro alkanes
Alkyl halide
w Alkyl halide reacts with ammonia to give a mixture of 1o, 2o, 3o and quaternary ammonium salts.
R–X + NH3
RX + R-NH2
RX + R2NH
R3N + RX
R-NH2 + HX
1o Amine
R2NH + HX 2o Amine
R3N + HX
R4N+ X-
3o Amine
Quaternary ammonium salts
w This reaction is called Hormann ammonolysis of alkyl halides.
w Alkyl halides react with silver salt of fatty acids to give esters.
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HALOALKANES AND HALOARENES
R|X+ RCOOAg RCOOR| + AgX
Ester
silver acetate
Alkyl halide
w Alkyl halides react with KSH and K2S to give thioalcohol or mercaptan and thioether respectively.
RX + KSH RSH + KX
Alkyl halide
Thioalcohol
2RX + K2S RSR + 2KX
Alkyl halide
Thioether
w Alkyl halide reacts with sodium salt of hydrazoic acid to give alkylazide.
R - X + NaN3 RN3 + NaX
Alkyl halide
Alkyl azide
w Alkyl halide reacts with (C6H5)3 P to give phosphonium salt.
[ R(C6H5)3P ]+X- RX + (C6H5)3P Alkyl halide
Phosphonium salt
Elimination reaction:
w Alkyl halides are converted to alkenes by treating than with alcohol KOH or NaNH2 or KNH2. The
reaction proceeds through E1 or E2 mechanism.
Alc.KOH
CH3 - CH2 - X CH2 = CH2
Ethylene
Alkyl halide
Reaction wirh metals:
w Alkyl halides react with sodium in presence of dry ether to give higher alkanes.
2RX + 2Na Dry ether
R - R + 2NaX
w Alkyl halides react with magnesium in presence of dry ether to give Grignard’s reagent. [Reactivity
order is RI > RBr > RCl]
RX + Mg Dry ether
RMgX
w Alkyl halides react with lithium in presence of dry ether to give alkyl lithium.
Dry ether
RX + 2Li Alkyl halides
RLi + LiX
Alkyl lithium
w Alkyl halide reacts with zinc in presence of dry ether to give higher alkanes. This reaction is called
Frankland reaction.
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HALOALKANES AND HALOARENES
2RX + Zn Dry ether
R - R + ZnX2
w Ethyl chloride reacts with an alloy of sodium and lead to form TEL (Tetra ethyl lead) which is used
as antiknocking agent in petroleum industry.
4C2H5Cl + 4Na/Pb (C2H5)4Pb + NaCl + 3Pb
Reaction wirh metals:
w Alkyl halide on reduction with reducing agents like Zn/acid or LiAlH4 or H2/Ni or Pd gives alkanes.
RX Alkyl halide
Zn/HCl
LiAlH4
R - H
Alkane
Friedel-Craft alkylation:
w Alkyl halides undergo Friedel – crafts alkylation in presence of anhydrous AlCl3 with benzene to
give alkyl benzene .
R
+R-X
AlCl3
+ HX
Alkyl halides
Benzene
Methods of preparation of tri halogen compound (Chloroform)
Haloform reaction
w Compounds containing CH3CO – group or any compound which on oxidation gives such group
reacts with alkali and halogen to give halo form.
CH3 - C - R + 4NaOH + 3X2
∆
O
=
=
O
O
R - C - O + CHX3 + 3NaX + 3H2O
Reaction of bleaching powder with ethyl alcohol and acetone
w Bleaching powder undergoes hydrolysis to give halogen and weak base. Halogen oxidizes ethyl
alcohol to trichloro acetaldehyde and acetone to tri chloro acetone which on reaction with base to
give chloroform.
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HALOALKANES AND HALOARENES
(i)
CaOCl2 + H2O
Ca(OH)2 + Cl2
C2H5OH + Cl2
CH3CHO + 2HCl
CH3 CHO + 3Cl2
CCl3CHO + 3HCl
2CCl3CHO + Ca(OH)2
(ii)
2CHCl3 + (HCOO)2Ca
CaOCl2 + H2O
Ca(OH)2 + Cl2
CH3COCH3 + 3Cl2
CCl3COCH3 + 3HCl
2CCl3COCH3 +Ca(OH)2
2CHCl3 + (CH3COO)2Ca
Halogenations of alkanes
w Alkanes react with halogens in presence of sunlight to give different alkyl halides. This reaction is a
chain reaction since all the hydrogens are substituted by halogens. This reaction follows free radical
substitution
CH4 + Cl2
CH3Cl + Cl2
CH2Cl2 + Cl2
hn
hn
hn
CH3Cl + HCl
CH2Cl2 + HCl
CHCl3+ HCl
Chloroform
CHCl3 + Cl2
CCl4 + HCl
Reduction of carbon tetra chloride
w Chloroform is prepared by the reduction of carbon tetra chloride by iron fillings and water.
CCl4 + 2[H] CHCl3 + HCl
Carbon tetra chloride
Chloroform
From chloral hydrate
w Pure chloroform is obtained by the distillation of chloral hydrate with NaOH(aq).
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HALOALKANES AND HALOARENES
CCl3CH(OH)2 + NaOH(aq) CHCl3 + HCOONa + H2O
Physical properties
w CHCl3 and CHBr3 are sickly smelling liquid, while CHI3 is in the form of yellow hexagonal plates.
w Boiling points increases with increase in molecular weight.
[CHCl3 → 334K, CHBr3 → 422.5K, CHI3 → 392K]
w CHCl3, CHBr3 are sparingly soluble in water, but CHI3 is in soluble in waters.
Chemical properties of Haloform
w Dihalocarbene is a good electrophile and most of the reactions are attribute to it.
Reactivity order is CHI3 > CHBr3 >CHCl3 >>CHF3
O
CHCl3 + OH
O
:CCl3
O
:CCl3 + H2O
O
:CCl2 + Cl
Oxidation
w Chloroform
auto oxidises in presence of sunlight with air only to form phosgene gas (carbonyl
chloride). Phosgene gas is poisonous in nature so now a day chloroform is not used as an anesthetic.
To prevent this reaction 1% C2H5OH is used. The purity of chloroform can be tested by the addition
of AgNO3. If precipitate is seen then chloroform is impure.
CHCl3+ 1 O2
2
COCl2 + 2HCl
Carbonyl chloride
Chloroform
COCl2 + 2C2H5OH
(C2H5O)2CO + 2HCl
Ethyl carbonate
Reduction
w Chloroform on reduction with Zn/HCl in ethyl alcohol gives dichloro methane.
w Chloroform on reduction with Zn/H2O gives methane
Zn/HCl-C2H5OH
CHCl3 Chloroform
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CH2Cl2
Dichloro methane
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HALOALKANES AND HALOARENES
CHCl3 Zn/H2O
CH4
Methane
Chloroform
Hydrolysis
w Chloroform undergoes hydrolysis in presence of a base like NaOH or KOH to give sodium formate
or potassium formate.
CHCl3 + 4NaOH
Chloroform
HCOONa + 3NaCl + 2H2O
CHCl3 + 3NaOH
Chloroform
CH(OH)3
-H2O
CH(OH)3+ 3NaCl
HCOOH
NaOH
HCOONa
Nitration
w Chloroform reacts with nitric acid to form chloropicrin which is used as insecticide and a war gas.
Chloroform is used as a tear gas.
CHCl3+ HO - NO2 Chloroform
CCl3NO2 + H2O
Nitric acid
Chloropicrin
With acetone
w Chloroform
reacts with acetone in alkaline medium to form chloretone. Which is used as a
hypnotic.
CH3
O
CHCl3 + CH3 - C - CH3
Chloroform
OH
C
Acetone
H 3C
CCl3
Chloretone
Halogenation
w Chloroform reacts with chlorine in presence of sunlight to give carbon tetrachloride
w Chloroform reacts with silver to give acetylene.
CHCl3 Cl2/hu
(Pyrene).
CCl4
Carbon
tetrachloride
Chloroform
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HALOALKANES AND HALOARENES
2CHCl3+ 6Ag
CH CH + 6AgCl Acetylene
Reimer - Tiemann reaction
w Chloroform
reacts with phenol in presence of a base to give salicylaldehyde or ortho-hydroxy
benzaldehyde. The electrophile in this reaction is dichloro carbene.
OH
OH
CHO
+ CHCl3+ 3NaOH
+ 3NaCl + 2H2O
Chloroform
Salicylaldehyde
Phenol
Mechanism
Step -1: Formation of dichloro carbene.
O
CHCl3 + OH- :CCl3 + H2O
O
:CCl3
:CCl2 + Cl
Step-2: Attack of the electrophile on to benzene ring followed by hydrolysis with alkali.
O
OH
O
+
+H
O
O
O
O
O
CCl2
+ :CCl2
OH
H
O
CHO
O
H
O
CHO
H+
CHCl2
H 2O
O
CHCl2
Hydrolysis
with alkali
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HALOALKANES AND HALOARENES
Carbylamine reaction
1) Chloroform react with primary amine (both aliphatic and aromatic ) to give isocyanides
NH2
NC
CHCl3 + 3KOH + +3KCl + 3H2O
Chloroform
Phenyl isocyanide
Aniline
w This is an identification test for primary amines, secondary amines and tertiary amines do not give
this test.
w Isocyanides have an offensive smell. The electrophile in this reaction is dichloro carbene
Chloroform reacts with aniline in presence of a base to give phenyl isocyanide
Nucleophilic substitutions reaction
w This reaction has been found to proceed by two different mechanisms.
w They are SN1 and SN2
Substitution nucleophilic bi-molecular (SN2)
w In this type rate depends on the concentration of both the reactants
w The attacking nucleophile interacts with alkyl halide leading to the breakage of carbon halogen bond
and form a new carbon oxygen (C-OH) bond.
w These two process takes place simultaneously without formation of an intermediate.
w The attacking nucleophile will come from one end and the leaving nucleophile leave from the other
end.
w This process is called inversion of configuration.
w In transition state the structure cannot be isolated because it is unstable.
w If these are bulky groups on carbon atom then they inhibit this type of mechanism.
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HALOALKANES AND HALOARENES
w The order of reactivity of alkyl halides towards SN2 is
methyl > primary > secondary > tertiary.
Substitution nucleophilic uni-molecular (SN1)
w In this type rate depend only on the concentration of alkyl halide.
w These reactions are generally carried out in polar protic solvent (water, alcohol, acetic acid etc).
CH3
(CH3)3CBr
Step - 1
CH3
CH3
+
CH3
+
+ OH-
Step - 2
+ BrCH3
(CH3)3COH
CH3
w This mechanism occurs in two steps. In step-1: The bond between carbon and halogen cleaves
which results in the formation of a carbocation.
w In step-2: The carbocation formed is attached by the nucleophile.
w Since step-1 is slow and reversible rate of reaction depends only on the concentration of
alkyl halide.
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HALOALKANES AND HALOARENES
w More the stability of carbocation greater will be the ease of formation of carbocation and and faster
will be the rate of reaction.
w The order of reactivity of alkyl halide towards SN1 is tertiary > secondary > primary >methyl
w Allylic and benzylic halides show high reactivity towards the SN1 reaction since carbocation formed
gets resonance stabilized.
H2C = CH
+
CH2
H2C+
CH2
+
CH = CH2
CH2
CH2
CH2
+
+
w For a given alkyl group the reactivity of halide follows the order same in both the mechanisms they
are R-I > R-Br >R-Cl > R-F
Stereo chemical aspects of nucleophilic substitution reaction
w A SN2 reaction proceeds with complete stereo chemical inversion while a SN1 reaction proceeds with
racemisation.
w In case of optically active alkyl halide the product formed by SN
2
mechanism has inverted configuration
when compared to the reactant. This is because the attacking nucleophile attacks from one end and
the outgoing nucleophile will leave from the other end.
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HALOALKANES AND HALOARENES
w Eg: When 2-bromo octane is made to react with sodium hydroxide (+) - octan-2-ol is formed.
w SN2 reactions are accompanied by inversion in configuration.
w In case of optically active alkyl halide the product formed by SN1 mechanism is accompanied by
racemisation.This is because the attacking nucleophile can attack carbocation from either side
resulting in a mixture of products one having the same configuration and other inversion.
Example:
w When 2-bromo butane is treated with sodium hydroxide butan-2-ol formation takes place.
CH3
CH3
OH-
HO
C 2H 5
CH3
OH-
H
HO
H
C 2H 5
C 2H 5
Elimination reaction
w When haloalkanes with β-hydrogen is treated with alcoholic KOH there is elimination of β-hydrogen
and halogen from α-carbon resulting in the formation of an alkene.
Br
OHCH3 CH2 CH2 CH CH3
CH3
CH2 CH CH
Pent 2 ene
(81%)
CH3
CH2
CH2
CH2
CH
CH2
Pent 1 ene
(19%)
w Since β-hydrogen is eliminated it is often called β-elimination.
w If there is availability of more than one β-hydrogen (i.e., β-carbon) then the major product will be
according to Zaitsev rule(Alexander Zaitsev a Russian chemist formulated this rule, also pronounced
as saytzeff)
w According to this rule in dehydrohalogenation reactions, the preferred product is that alkenes which
has the greater number of alkyl groups attached to the doubly bonded carbon atoms.
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HALOALKANES AND HALOARENES
METHODS OF PREPARATIONS OF ARYL HALIDES
From benzene
w Benzene undergo electrophilic substitution of halogen in presence of lewis acids like FeCl3 or AlCl3
etc to form halobenzene.
w Iodobenzene can’t prepared by this method since the reaction is reversible, the formed HI must be
oxidized by using oxidising agents like HNO3, HIO4 etc...
w Fluorobenzene can be prepared by this method because Fluorine being more reactive.
From diazonium salt
w When a primary aromatic amine dissolved in cold aqueous mineral acid is treated with sodium nitrite,
a diozonium salt is formed.
Sandmeyer reaction
w When diozonium salt is treated with cuprous chloride or cuprous bromide gives chloro-benzene or
bromobenzene.
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HALOALKANES AND HALOARENES
Gattermann reaction
w When diozonium salt reacts with copper powder in presence of HCl or HBr form chrolobenzene or
bromobenzene.
Preparation of iodobenzene
w When diozonium salt is treated with potassium iodide iodobenzene is formed.
Preparation of fluoro benzene
w When diozonium salt is treated with fluoroboric acid fluorobenzene is formed.
Raschig process
w The commercial method of preparation chlorobenzene is by this process.
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HALOALKANES AND HALOARENES
Borodine - Hunsdicker reaction
w In this process silver salt of benzoic acid is distilled with Br2 in presence of CCl4 at 350K to give
bromobenzene.
Physical properties and Chemical properties
w Aryl halides are heavier than water, through polar but immiscible with water. In Aryl halides the
halogen is attached to sp2 hybridized carbon, the C-X in aryl halides have double bond character due
to conjugation resulting in 3 resonating structures.
w Due to this nature the reactions are not normal i.e., nucleophilic substitution reactions are difficult.
Even the length of carbon-halogen bond is less (169pm) when compared to haloalkanes (177pm).
Nucleophilic substitution reaction
w These reactions do not take under ordinary conditions. Possibility of SN1 mechanism is ruled out
because the phenyl cation formed will not be resonance stabilized.
Re-placement by hydroxyl group
w Chloro
benzene can be converted into phenol by heating in aqueous sodium hydroxide at a
temperature of 623K and a pressure of 300 atmospheres
Cl
OH
NaOH, 623K
300atm
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HALOALKANES AND HALOARENES
w If there is any electron withdrawing group on benzene ring at ortho and para position the reactivity
increases. But such type of effect is not observed when electron withdrawing group is present at
meta position.
Cl
OH
NaOH, 443K
H+
NO2
NO2
Cl
OH
NO2
NO2
NaOH, 386K
H+
NO2
NO2
w Presence of – NO2 group at ortho and para positions withdraws electron density from benzene ring
facilitating the attack of nucleophile, the carbanion formed is stabilized by resonance.
O2N
Cl
O2N
NO2
OH
NO2
NaOH
H 2O
NO2
NO2
w The negative change at ortho and para position with respect to halogen is stabilized by - NO2 group.
But this type of stabilization is not there if - NO2 group is present at meta position.
Re-placement by cyano group
w When
brome benzene is treated with copper cyanide in presence of dimethyl form amide at a
temperature of 470K gives cyano benzene.
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HALOALKANES AND HALOARENES
Br
+ CuCN
CN
DMF
+ CuBr
470K
Replacement by amino group
w When chloro benzene is treated with ammonia in cuprous oxide at a temperature of 475k and at a
pressure of 60 atom give aniline.
Cl
2
+ 2NH3 + Cu2O
475K
60atm
NH2
2
+ 2CuCl + H2O
Electrophilic substitution on benzene ring
w Halogen atom on benzene ring is deactivating but ortho para directing. Halogen atom increases
electron density at ortho and para positions due to +R(resonance effect) and +M(mesomeric effect)
effect. But these have –I effect so the electrophilic substitution reactions are slow in chloro benzene
when compared to benzene.
Halogenation
w Chloro benzene reacts with chlorine in presence of anhydrous Ferric chloride to give 1,4 – Di chloro
benzene (major) and 1,2 –di chloro benzene (minor)
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HALOALKANES AND HALOARENES
Nitration
w Chloro benzene reacts with nitric acid (Nitration mixture -1:1 HNO3 and H2SO4) to give
1-chloro- 4-nitro benzene (major) and 1-chloro -2- nitro benzene (minor)
Sulphonation
w Chloro benzene when treated with concentrated H2SO4 gives 4-chloro benzene sulphonic acid
(major) and 2-chloro benzene sulphonic acid (minor).
Friedel - craft reactions
w Chloro benzene reacts with methyl chloride in presence of anhydrous AlCl3 to give
1-chloro – 4- methyl bengene (major )and 1-chloro -2-methyl benzene (minor).
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HALOALKANES AND HALOARENES
w Chloro benzene reacts with acetyl chloride to give 4 chloro acetophenone (major) and
2-chloro acetophenone(minor).
Reaction with metals
w Halo arenes react with metals like magnesium and lithium to form organo metallic compounds.
Br
+ Mg
Cl
+ 2 Li
Ether
MgBr
∆
Ether
Li
∆
Wurtz - fitting reaction
w Aryl halides react with alkyl halides in presence of sodium in dry ether to give alkyl benzene. This
reaction is called wurtz - fittig reaction.
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HALOALKANES AND HALOARENES
Fitting reaction
w Aryl halides react among themselves in presence of sodium in dry ether to give biphenyl. This reaction
is called Fitting reaction.
Ullmann reaction
w Two moles of iodo benzene reacts with copper at a temperature of 100°C to 350°C in presence of
nitro benzene to give biphenyl.
Reaction with chloral
w Chloro benzene reacts with chloral to give DDT ( dichloro diphenyl trichloro ethane).
USES OF SOME POLYHALOZEN COMPOUND
Dichloro methane (Methylene chloride)
w Widely used as a solvent, as a paint remover, as a solvent in the manufacture of drugs, as a propellent
in aerosols.
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HALOALKANES AND HALOARENES
Drugs
w Used as a
Aerosols
solvent in metal furnishing and cleaning.
w This effects the human central nervous system and when exposed it causes slight impaired hearing
and vision.
w Higher level of methylene chloride causes dizziness, nausea, tingling and numbness in fingers and
toes. Direct contact with eyes can burn the cornea.
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HALOALKANES AND HALOARENES
w Direct contact with skin causes intense burning and mild redness of the skin.
Trichloromethane (Chloroform)
w Used as a solvent for fats, alkaloids, iodine etc...
w Used In the production of freon refrigerant R-22.
w Inhaling the vapours depress central nervous system. Breathing 900 parts of chloroform per million
parts of air for a short time causes dizziness, fatigue and headache.
w Previously this is used as an anesthetic but not now because of the formation of a poisonous gas
phosgene (carbonyl chloride).
Tri iodo methane (Iodoform)
w Previously this is used as an antiseptic. But this nature is not due to iodoform but due to liberation of
iodine. Due to its smell this has been replaced by other compounds containing iodine.
Tetra chloro methane (Carbon tetrachloride)
w Used as a solvent.
w Used in the manufacture of refrigerants and propellents for aerosol cans.
w Used as a feed stock in the preparation of chlorofluoro carbons.
w The effect of CCI4 are dizziness, light headache, nausea, vomiting, permanent damage to
nerve cells and if it is severe it causes stupor, coma, unconsciousness or death.
w Exposure may irritate eyes, heart beat becomes irregular or may even stop.
w If released into air depletes ozone layers.
Freons
(Chloro floro compounds of mathane and ethane are called freous)
w Thesse are unstable, unreactive, non-toxic, non-corrosive and easily liquefiable gases.
freon 12 ( CCl2F2) is most common.
w When realised into stratosphere may deplete ozone layer.
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HALOALKANES AND HALOARENES
FDDT (Dichloro diphenyl trichloro ethane)
w Used as an insecticide.
w It is toxic to fish when mixed with water.
w DDT is not metabolised very rapidly by animals so it is deposited and stored in fatty tissues.
SUMMARY
w Alkyl and aryl halides may be classified as mono, di tri poly halo compounds depending upon the
no.of halogen atoms.
w The carbon halogen bond is polarized because halogen is more electronegative than carbon.
w Alkyl halides are prepared by the replacement of –OH group of alcohols using PCl3 or PCl5 or H or
SOCl2, addition of halogen acids to alkanes, halogenation of alkanes.
w Alkyl halides are prepared by electrophilic substitution or from benzene diazonium salts using proper
reagents.
w The
boiling points of aryl and alkyl halogen compounds are higher than the corresponding
hydrocarbons.
w These are slightly soluble in water but more soluble in organic, solvents.
w Alkyl and aryl halides undergo nucleophilic substitution, elimation reactions, reaction with metals,
and some miscellaneous reactions.
w Nucleophilic substitution is of two types SN1 and SN2.
w In SN1 racemisation is seen and in SN2 inversion in configuration is seen if the alkyl halides contain
chiral carbon.
w Different types of polyhalogen compounds are discussed with their uses and their environmental
hazards.
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