Organic Chemistry I

β
-elimination
, or
1,2-elimination
. (The number 1,2- indicate that the atoms being removed
are on two adjacent carbons, not necessarily means C1 and C2 atoms.)
Chapter 8 Elimination Reactions | 277

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Figure 8.0c Alpha and Beta carbon
Beside the substrate, a base is required for elimination reactions, other than the nucleophile for substitution. Similar to
substitution reaction, elimination reactions also have different rate laws, and therefore involve different mechanisms.
The elimination mechanisms are E1 and E2. You may expect that E1 is the unimolecular reaction with first order rate
law, and E2 is the second order bimolecular reaction. That is correct. We will go through the mechanism in details first,
then compare between elimination and substitution.
278 | Chapter 8 Elimination Reactions

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8.1 E2 Reaction
8.1.1 E2 Mechanism
E2 mechanism is the
bimolecular elimination
mechanism, that the reaction rate depends on the concentration
of
both
substrate and base. We will take the elimination reaction of 2-bromo-2-methylpropane as an example for
discussion.
Figure 8.1a Bimolecular Elimination Reaction
It was mentioned earlier that HX is the side product of dehydrohalogenation, why there is no HX (HBr for
this reaction) in the reaction equation? It could be understood as that with the presence of excess base
(OH
–
) in the reaction mixture, HBr reacts with OH
–
to give H
2
O and Br
–
. The following discussion of the
mechanism will help you to understand this better.
E2 mechanism is also a single-step concerted reaction, similar to S
N
2, with multiple electron pair transfers happen at
the same time.
Figure 8.1b E2 Reaction Mechanism
8.1 E2 Reaction | 279

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Base, OH
–
, uses its electron pair to attack a
β
-hydrogen on
β
-carbon, and starts to form a bond; at the same time, the
β
C-H sigma bond begins to move in to become the π bond of a double bond, and meanwhile Br begins to depart by taking
the bonding electrons with it. A transition state is formed in the reaction process with partially breaking and partially
forming bonds. At the completion of the reaction, the C=C double bond and H
2
O molecule are fully formed, with Br
–
leaves completely.
Since both the substrate (halide) and the base are involved in the single-step mechanism, E2 is the second order
reaction.