Organic Chemistry

6th Edition Chapter 9

Paula Yurkanis Bruice

Elimination Reactions
of Alkyl Halides
Competition Between
Substitution and
Elimination

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In addition to substitution, an alkyl halide can undergo an
elimination reaction

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 The E2 Reaction

Hydroxide cannot act as a nucleophile in this reaction

because of the bulky tertiary halide. Rather, hydroxide
acts as a base and abstracts a proton.

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The removal of a proton and a halide ion is called
dehydrohalogenation:

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 An E2 reaction is also called a b-elimination or a
1,2-elimination reaction:

The weaker the base, the better it is as a leaving group

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The Regioselectivity of the E2 Reaction

The major product of an E2 reaction is the most stable

alkene:

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Reaction coordinate diagram for the E2 reaction of
2-bromobutane and methoxide ion

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 The Zaitsev Rule

The more substituted alkene product is obtained when a

proton is removed from the b-carbon that is bonded
to the fewest hydrogens

The most stable alkene is generally (but not always) the

most substituted alkene

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Conjugated alkene products are preferred over the
more substituted alkene product:

Do not use Zaitsev’s rule to predict the major product

in these cases 11
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 Bulky bases affect the product distribution resulting in
the Hofmann product, the least substituted alkene:
Hofmann product

Zaitsev product

However, it takes a lot of steric hindrance for the less

stable product to be the major product:

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 Sect 9.9: bulky leaving groups -
- Hofmann Elimination
This give the anti-Zaitsev product (least
substituted product is formed)!

b  b _
CH3 CH2 CH2 CH CH3 OH CH3 CH2 CH CH CH3 6%
CH3 N+ CH3 heat

CH3 +

CH3 CH2 CH2 CH CH2 94%

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 Orientation of elimination:
regiochemistry/ Hofmann’s Rule
• In bimolecular elimination reactions in the
presence of either a bulky leaving group or a
bulky base, the hydrogen that is lost will come
from the LEAST highly-branched b-carbon.
More branched Less branched
H H H H
 b  b
H C C C C H
CH3
H X H

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Product from previous slide
H

H C H H
H C C
C H
CH3

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Another exception to Zaitsev’s rule

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Consider the elimination of 2-fluoropentane…

A carbanion-like transition state

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Therefore, the major product of an E2 elimination reaction
is the more stable alkene except if the reactants are
sterically hindered or the leaving group is poor

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The E1 Reaction

A unimolecular reaction:

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The E1 reaction, like the SN1 reaction, involves a
carbocation intermediate:

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How does a weak base like water remove a proton from
an sp3 carbon?

1) The presence of a positive charge greatly reduces the

pKa

2) Hyperconjugation weakens the C-H bond by electron

density

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The major product in an E1 reaction is generally the
more substituted alkene

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Reaction coordinate diagram for the E1 reaction of
2-chloro-2-methylbutane

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Because the first step is the rate-determining step, the rate of an
E1 reaction depends both on the ease with which the carbocation
is formed and how readily the leaving group leaves

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Because the E1 reaction forms a carbocation
intermediate, we need to consider carbocation
rearrangement

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 Competition Between E2
and E1 Reactions

An E2 is favored by a high concentration of strong base and an

aprotic polar solvent
An E1 is favored by a weak base and a protic polar solvent
Review Section 8.10 27
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 Stereochemistry of the E2 Reaction
The bonds to the eliminated groups (H and X) must be
in the same plane

The anti elimination is favored over the syn elimination

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Another view of the E2 reaction mechanism

The best overlap of the interacting orbitals is achieved

through back-side attack
Anti elimination avoids repulsion of the electron-rich
base 29
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Consider the stereoselectivity of the E2 reaction

The alkene with the bulkiest groups on opposite sides of the

double bond will be formed in greater yield, because it is the more
stable alkene 30
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Reaction coordinate diagram for the E2 reaction of
2-bromopentane and ethoxide ion

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When only one hydrogen is bonded to the b-carbon, the
major product of an E2 reaction depends on the structure
of the alkene

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 Elimination from Cyclic Compounds
In an E2 reaction, groups to be eliminated must be in
axial positions

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The hydrogen that is removed and the leaving group
both have to be in the axial position, therefore…

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E1 Elimination from Cyclic Compounds

No axial orientation required for the E1 reaction.

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 Competition Between
Substitution and Elimination
Alkyl halides can undergo SN2, SN1, E2, and E1

1) decide whether the reaction conditions favor SN2/E2 or SN1/E1

•SN2/E2 reactions are favored by a high concentration of a

good nucleophile/strong base

•SN1/E1 reactions are favored by a poor nucleophile/weak

base

2) decide how much of the product will be the substitution product

and how much of the product will be the elimination product
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Consider the SN2/E2 conditions

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A bulky alkyl halide or a sterically hindered nucleophile
encourages elimination over substitution

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High temperature favors elimination over substitution:

Why? Because elimination is entropically favorable.

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The E2 reaction should be used if you want to
synthesize an alkene:

Hydroxide

Water

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