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Organic Chemistry 7
Reactions of Halogenoalkanes:
Nucleophilic Substitution and Elimination Reactions
IB Topics 10.5, 20.2
p. 371-374, 393-399
1
Overview of SN Reactions
Halogenoalkanes undergo nucleophilic substitution (SN) reactions
when they react with a nucleophile.
A substitution reaction is one where one atom on a reactant
is replaced by another atom from the second reactant.
The nucleophile replaces the halogen in the organic
reactant, and the halide ion is removed in the products
(and is often called the “leaving group”).
CH3Br +
–
OH
common nucleophiles:
→ CH3OH +
–
Br
OH–
NH3
CN–
H2O
2
Overview of SN Reactions
Predict the products and write the chemical equation for
these substitution reactions of halogenoalkanes:
1. 1-chloropropane reacts with hydroxide ions
2. 2-bromo-2-methylpropane reacts with hydroxide ions
3. 2-chlorobutane reacts with cyanide ions
3
Overview
Nucleophiles
of SN Reactions
Nucleophiles
The carbon-halogen bond is a polar covalent bond due to the presence of
the electronegative halogen atoms.
∂+
an electron deficient region is
created around the carbon atom
bonded to the halogen
C
X
electrons shift towards X and away from C
A nucleophile is an electron rich species which is attracted
to a region of electron deficiency.
Nucleophiles are small particles that have a lone electron
pair and may also have a negative charge.
examples: OH–
CN–
H 2O
NH3
4
Overview of SN Reactions
Breaking the Carbon-Halogen Bond
C X
When the C – X bond breaks during
the substitution reaction, both
electrons in the bond move towards
the halogen atom.
This type of bond breaking is called
heterolytic fission.
C
+
X
–
The halogen atom
becomes a halide ION.
It is also known as the
leaving group in an SN
reaction.
5
Overview of SN Reactions
Curly Arrows
Curly arrows are used to show the movement of electron
pairs when we describe mechanisms in organic reactions.
example 1:
heterolytic fission of the
C-X bond
example 2:
attraction of a nucleophile to
an electron deficient region
C
X
The tail of the curly arrow
shows where the electron
pair originates.
C
The head shows the place
where the electron pair
goes.
–
OH
6
Overview of SN Mechanisms
Mechanisms and Rate Determining Steps
A “mechanism” is a series of smaller steps that occurs during a reaction:
example:
Step 1:
CH3Br + OH– → [CH3BrOH]–
Step 2:
[CH3BrOH]– → CH3OH + Br–
overall:
CH3Br + OH– → CH3OH +Br–
Idea 1: One step in every mechanism is the SLOWEST STEP in the
reaction and is called the RATE DETERMINING STEP (RDS).
Idea 2: The rate of any chemical reaction depends on the rate of the slowest
step in the reaction mechanism. You can’t go any faster than the slowest step!
7
Overview of SN Mechanisms
Two Types of SN Mechanisms
SN1
SN2
one reactant in the
rate determining step
two reactants in the
rate determining step
(RDS is “unimolecular”)
(RDS is “bimolecular”)
tertiary halogenoalkanes
primary halogenoalkanes
RDS = step 1 in both mechanisms
8
SN2 Mechanisms
primary halogenoalkane + nucleophile
reaction of bromoethane in dilute aqueous sodium hydroxide
C2H5Br + OH– → C2H5OH + Br–
1. the reactants
nucleophile
primary halogenoalkane
9
SN2 Mechanisms
C2H5Br + OH– → C2H5OH + Br–
2. nucleophile approach
3. “transition state”
nucleophile approaches
from the side opposite
to the halogen
C–Br bond starts to break at
the same time as the C–O
bond starts to form
10
SN2 Mechanisms
C2H5Br + OH– → C2H5OH + Br–
4. products
halide ion leaves
(the leaving group)
new organic product
(alcohol)
(dative covalent bond forms
between the C and O atoms)
breaking the C–Br is a
heterolytic fission ...
the halogen takes
both the electrons
and forms a negative
ion
11
SN2 Mechanisms
CH3Cl + NaOH → CH3OH + NaCl
CH3Cl + OH– → CH3OH + Cl–
Using structural formulas and curly arrows, show the SN2 mechanism.
H
H
H C Br
H
reactants
–
OH
H C
H
–
Cl–
Cl
OH
H
H C OH
H
transition state
products
12
SN1 Mechanisms
tertiary halogenoalkane + nucleophile
reaction of 2-bromo-2methylpropane in dilute aqueous sodium hydroxide
(CH3)3CBr + OH– → (CH3)3COH + Br–
1. the reactants
hydroxide ion
(or other nucleophile)
(not used in step 1)
tertiary halogenoalkane
steric hindrance = bulky alkyl
groups block the direct attack of
the nucleophile
13
SN1 Mechanisms
(CH3)3CBr + OH– → (CH3)3COH + Br–
2. breaking the C-Br bond
C–Br breaks
via heterolytic fission
bromide ion
leaves
carbocation
forms
14
SN1 Mechanisms
(CH3)3CBr + OH– → (CH3)3COH + Br–
3. nucleophile attack
nucleophile
approaches
carbocation
dative bond
forms between
nucleophile and
carbon atom
final product
15
SN1 Mechanisms
(CH3)3CBr + OH– → (CH3)3COH + Br–
Using structural formulas and curly arrows, show the SN1 mechanism.
step 1
CH3
CH3
H3C C+
H3C C Br
+
Br–
CH3
CH3
carbocation
step 2
CH3
CH3
H3C C+
CH3
H3C C OH
OH–
CH3
16
SN1 and SN2 Mechanisms
Key Features
SN1
SN2
type of halogenoalkane
tertiary
primary
number of reactants in the
first step (the RDS)
1
2
type of intermediate that
forms
carbocation
tranistion state
17
SN1 and SN2 Mechanisms
Practice 1: 1-chloropropane reacts with dilute aqueous sodium hydroxide.
Using structural formulas and curly arrows, show the mechanism for this reaction.
18
SN1 and SN2 Mechanisms
Practice 2: 2-chloro-2-methylbutane reacts with dilute aqueous sodium hydroxide.
Using structural formulas and curly arrows, show the mechanism for this reaction.
19
Secondary Halogenoalkanes
Secondary halogenoalkanes can undergo either SN1 or SN2 mechanisms.
Draw both mechanisms for the reaction of
2-chloropropane in dilute aqueous sodium hydroxide.
20
Factors Affecting Rates of SN Reactions
1. Type of Halogenoalkane
tertiary
(SN1 mechanism)
formation of carbocation
intermediate requires
less energy so the rate is
faster
>
secondary
(SN1 or SN2
mechanism)
>
primary
(SN2 mechanism)
formation of the
transition state
intermediate requires
more energy so the rate
is slower
21
Factors Affecting Rates of SN Reactions
2. Type of Halogen
I > Br > Cl > F
A carbon-halogen bond breaks during SN reactions of
halogenoalkanes. The strength of this bond has an affect on the rate.
Bond Enthalpy Values (kJ mol-1)
C-I 228
C-Br 290
C-Cl 346
C-F 467
The C-I bond is the weakest, so less energy is needed to break.
Iodoalkanes will therefore react the faster.
The C-F bond is the strongest, requiring more energy to break.
Fluoroalkanes therefore react more slowly.
22
Factors Affecting Rates of SN Reactions
3. Type of Nucleophile
–
CN
>
–
OH
> NH3 > H2O
In general, anions tend to be better nucleophiles than their
corresponding neutral molecules.
They have a higher electron density and are more readily attracted to
a region of electron deficiency.
Nitrogen containing nucleophiles are better nucleophiles than those
with oxygen. Their covalent bonds are less polar, so they will donate
their lone electron pairs more readily.
23
Factors Affecting Rates of SN Reactions
Measuring the Rate of SN Reactions
If silver nitrate is added to the reaction
mixture, the Ag+ ions will form a precipitate
with the halide ions that are released.
Ag+(aq) + Cl–(aq) → AgCl(s)
Ag+(aq) + Br–(aq) → AgBr(s)
Ag+(aq) + I–(aq) → AgI(s)
By measuring the amount of precipitate
formed over time, the rate of the
reaction may be compared.
http://www.sciencephoto.com
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SN Mechanisms Using Different Nucleophiles
nucleophile = OH–
general equation:
halogenoalkane + hydroxide* → alcohol + halide ion
CH3CH2Br +
OH–
→
CH3CH2OH +
–
Br
* The hydroxide ion is present in dilute aqueous solutions
of NaOH, KOH, etc.
(The metal ion (Na+ or K+) is often omitted from the equations
because it is a spectator ion.)
This reaction occurs at ROOM TEMPERATURE.
25
SN Mechanisms Using Different Nucleophiles
nucleophile = H2O
general equation:
halogenoalkane + water → alcohol + aqueous hydrogen halide*
CH3CH2Br + H2O → CH3CH2OH + HBr*
* aqueous hydrogen halides = acids (e.g. hydrobromic acid)
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SNSNMechanisms
MechanismsUsing
UsingDifferent
Other Nucleophiles
Nucleophiles
nucleophile = H2O
Good news!
You do not need to know the mechanism for the SN
reactions of halogenoalkanes with water.
But you DO need to explain why the
OH– ion is a better nucleophile than H2O.
attracted to an electron-deficient
region (i.e. with a (partial) + charge)
negatively charged ion
∴ stronger attraction to a +
region than a neutral
molecule
neutral molecule
∴ weaker attraction to a +
region than an ion with a –
charge
∴ rate of reaction is faster
∴ rate of reaction is slower
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SN Mechanisms Using Different Nucleophiles
nucleophile = H2O
Try this:
Deduce the equation for the reaction of 2-bromo-2-methylbutane with water.
Name the organic product.
(CH3)2CBrCH2CH3 + H2O → (CH3)2C(OH)CH2CH3 + HBr
2-methylbutan-2-ol
28
SNSNMechanisms
MechanismsUsing
UsingDifferent
Other Nucleophiles
Nucleophiles
nucleophile = NH3
general equation:
halogenoalkane + NH3 → amine + hydrogen halide*
CH3CH2Br + NH3 → CH3CH2NH2 + HBr*
* aqueous hydrogen halides = acids (e.g. hydrobromic acid)
29
SNSNMechanisms
MechanismsUsing
UsingDifferent
Other Nucleophiles
Nucleophiles
nucleophile = NH3
Try these:
Deduce the equation for the reaction of 1-bromobutane with ammonia.
Name the organic product.
CH3CH2CH2CH2Br + NH3 → CH3CH2CH2CH2NH2 + HBr
1-bromobutane
butanamine
Deduce the equation for the reaction of 2-chloro-2-methylpropane with ammonia.
Name the organic product.
(CH3)3CCl + NH3 → (CH3)3CNH2 + HCl
2-chloro-2-methylpropane
1,1-dimethylethanamine
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SNSNMechanisms
MechanismsUsing
UsingDifferent
Other Nucleophiles
Nucleophiles
nucleophile = NH3
You are responsible only for the SN2 mechanism of a primary haloalkane with NH3.
CH3CH2Br + NH3 → CH3CH2NH2 + HBr
Step 1:
H H
H C C Br
H H
NH3
H H
H C C
Br
NH3
transition
state
H C C NH2
+ HBr
H H
Step 2:
H H
H H
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SNSNMechanisms
MechanismsUsing
UsingDifferent
Other Nucleophiles
Nucleophiles
nucleophile = CN–
general equation:
halogenoalkane + CN– → nitrile + halide ion
CH3CH2Br +
–
CN
→ CH3CH2CN +
bromoethane
propanenitrile
2 carbon atoms
3 carbon atoms
–
Br
NOTE: Nucleophilic substitution with cyanide ions as the nucleophile
will INCREASE the carbon chain length!
32
SNSNMechanisms
MechanismsUsing
UsingDifferent
Other Nucleophiles
Nucleophiles
nucleophile = CN–
You are responsible only for the SN2 mechanism of a primary haloalkane with CN–.
CH3CH2Br + CN– →
Step 1:
H H
H H
H C C Br
H H
CH3CH2CN + Br–
CN–
H C C
–
Br
CN
transition
state
H C C CN
–
Br
H H
Step 2:
H H
propanenitrile
+
H H
33
Elimination Reactions
No - not another nucleophilic substitution reaction ...
It’s time to look at ELIMINATION reactions!
34
Elimination Reactions
Elimination reactions involve the removal of a small molecule
from a larger molecule:
H H
H C C H
H H
H C C H + X Y
X Y
a double C=C bond forms
(alkene produced)
35
Elimination Reactions
Consider the effect of reaction conditions on the reaction between a
halogenoalkane and sodium hydroxide:
Condition 1: dilute aqueous NaOH, warm temperature:
warm
CH2CH3Br(aq) + NaOH(aq) →
CH3CH2OH(aq) + NaBr(aq)
nucleophilic substitution reaction
Condition 2: NaOH dissolved in ethanol, hot temperature:
hot
CH2CH3Br(aq) + NaOH(ethanol) → CH2CH2(aq) + NaBr(aq) + H2O
elimination reaction
36
Elimination Reactions - E2 Mechanism
CH3CH2Br(aq) + OH–(ethanol) → CH2CH2(aq) + Br–(aq) + H2O
E2: “concerted process” with 2 reactants in step 1 (bimolecular):
H H
H H
H C C H
H C C H
2
1
H Br
3
–
Br
+
+ H2O
–
OH
1 OH– accepts an H+
2 C-H e pair moves to form the
second bond between C-C
3 heterolytic fission of C-Br bond
37
Elimination Reactions - E1 Mechanism
CH3CH2Br(aq) + OH–(ethanol) → CH2CH2(aq) + Br–(aq) + H2O
E1: 2 step process with 1 reactant in step 1 (unimolecular):
Step 1:
H H
H C C H
H Br
Step 2:
H H
H C C+ H
H
H H
H C C+ H
+
–
Br
H
H H
H C C H
+ H2O
OH–
38
Summary - Reactions of Halogenoalkanes
reaction type
nucleophile
conditions
product
nucleophilic
substitution
OH–
aqueous OH–
warm
alcohol + X–
nucleophilic
substitution
H2O
warm
alcohol + HX
nucleophilic
substitution
NH3
aqueous,
concentrated NH3
high pressure
amine + HX
nucleophilic
substitution
CN–
ethanol solvent
heat under reflux
nitrile + X–
elimination
OH–
(acts as a base not a
nucleophile)
ethanolic
hot
OH–
alkene + X–
+ H2O
39
SN1 and SN2 Mechanism Images Credit
University of Surrey. “Sn1 Reactions.” Online video clip. YouTube.You Tube, 9 Sept. 2011.
Web. 15 May 2013.
<http://www.youtube.com/watch?v=JmcVgE2WKBE&list=PLC2B5E61AA6E401A4>
University of Surrey. “Sn2 Reactions.” Online video clip. YouTube.You Tube, 9 Sept. 2011.
Web. 15 May 2013.
<http://www.youtube.com/watch?v=h5xvaP6bIZI&list=PLC2B5E61AA6E401A4>
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