Title: 8.13 Substitution and Elimination as Competing Reactions
18.13Substitution and Eliminationas Competing
Reactions
2Two 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
3Two Reaction Types
How can we tell which reaction pathway is
followedfor a particular alkyl halide?
b-elimination
H
H
X
Y
nucleophilic substitution
4Elimination 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.
5Example
NaOCH2CH3 ethanol, 55C
CH3CHCH2
(87)
(13)
6Figure 8.11
E2
Br
7Figure 8.11
SN2
Br
8When 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
9Uncrowded Alkyl Halides
Decreased crowding at carbon that bears the
leaving group increases substitution relative to
elimination.
CH3CH2CH2Br
NaOCH2CH3 ethanol, 55C
CH3CHCH2
CH3CH2CH2OCH2CH3
(9)
(91)
10But 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)
11When 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
12Weakly Basic Nucleophile
Weakly basic nucleophile increases substitution
relative to elimination
secondary alkyl halide weakly basic nucleophile
KCN
pKa (HCN) 9.1
DMSO
(70)
13Weakly Basic Nucleophile
Weakly basic nucleophile increases substitution
relative to elimination
secondary alkyl halide weakly basic nucleophile
I
NaN3
pKa (HN3) 4.6
(75)
14Tertiary 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.
15Example
2M sodium ethoxide in ethanol, 25C
99
1