Substitution and Elimination Competing Reactions SN1

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Substitution and EliminationCompeting
ReactionsSN1 SN2 vs. E1 and E2Formation of
Alkenes
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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
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Two Reaction Types
How can we tell which reaction pathway is
followedfor a particular alkyl halide?
b-elimination

H

H
X
Y
nucleophilic substitution
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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.

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Example
NaOCH2CH3 ethanol, 55C

CH3CHCH2
(87)
(13)
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Figure 1
E2
Br
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The weaker the base, the better it is as a
leaving group
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Regioselectivity of the E2 Reaction
The major product of an E2 reaction is the most
stable alkene
The greater the number of substituents, the more
stable is the alkene
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Reaction coordinate diagram for the E2 reaction
of 2-bromobutane and methoxide ion
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Figure 2
SN2
Br
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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

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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)
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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)
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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

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Weakly Basic Nucleophile
Weakly basic nucleophile increases substitution
relative to elimination
secondary alkyl halide weakly basic nucleophile
KCN
pKa (HCN) 9.1
DMSO
(70)
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Weakly Basic Nucleophile
Weakly basic nucleophile increases substitution
relative to elimination
secondary alkyl halide weakly basic nucleophile
I
NaN3
pKa (HN3) 4.6
(75)
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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.
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Example


2M sodium ethoxide in ethanol, 25C
99
1
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Putting things together Elimination vs.
Substitution