8.13 Substitution and Elimination as Competing Reactions

1
8.13Substitution and Eliminationas Competing
Reactions
2
Two Reaction Types
Alkyl halides can react with Lewis bases in two
different ways nucleophilic substitution or
elimination.
b-elimination

H

H
X
Y
nucleophilic substitution
3
Two Reaction Types
How can we tell which reaction pathway is
followedfor a particular alkyl halide?
b-elimination

H

H
X
Y
nucleophilic substitution
4
Elimination versus Substitution
A systematic approach is to choose as a
referencepoint the reaction followed by a
typical alkyl halide(secondary) with a typical
Lewis base (an alkoxideion).

• The major reaction of a secondary alkyl
  halidewith an alkoxide ion is elimination by the
  E2mechanism.

5
Example
NaOCH2CH3 ethanol, 55C

CH3CHCH2
(87)
(13)
6
Figure 8.11
E2
Br
7
Figure 8.11
SN2
Br
8
When is substitution favored?
Given that the major reaction of a
secondaryalkyl halide with an alkoxide ion is
elimination by the E2 mechanism, we can expect
the proportion of substitution to increase with

• 1) decreased crowding at the carbon
  that bears the leaving group

9
Uncrowded Alkyl Halides
Decreased crowding at carbon that bears the
leaving group increases substitution relative to
elimination.

• primary alkyl halide

CH3CH2CH2Br
NaOCH2CH3 ethanol, 55C

CH3CHCH2
CH3CH2CH2OCH2CH3
(9)
(91)
10
But a crowded alkoxide base can favor elimination
even with a primary alkyl halide.

• primary alkyl halide bulky base

CH3(CH2)15CH2CH2Br
KOC(CH3)3
tert-butyl alcohol, 40C

CH3(CH2)15CH2CH2OC(CH3)3
CH3(CH2)15CHCH2
(87)
(13)
11
When is substitution favored?
Given that the major reaction of a
secondaryalkyl halide with an alkoxide ion is
elimination by the E2 mechanism, we can expect
the proportion of substitution to increase with

• 1) decreased crowding at the carbon
  that bears the leaving group
• 2) decreased basicity of the nucleophile

12
Weakly Basic Nucleophile
Weakly basic nucleophile increases substitution
relative to elimination
secondary alkyl halide weakly basic nucleophile
KCN
pKa (HCN) 9.1
DMSO
(70)
13
Weakly Basic Nucleophile
Weakly basic nucleophile increases substitution
relative to elimination
secondary alkyl halide weakly basic nucleophile
I
NaN3
pKa (HN3) 4.6
(75)
14
Tertiary Alkyl Halides
Tertiary alkyl halides are so sterically
hinderedthat elimination is the major reaction
with allanionic nucleophiles. Only in
solvolysis reactionsdoes substitution
predominate over eliminationwith tertiary alkyl
halides.
15
Example


2M sodium ethoxide in ethanol, 25C
99
1