Organic Chemistry

11.1 The E2 Reaction

Just as there are two important nucleophilic substitution reactions— and

are two important elimination reactions: E1 and E2. The reaction of tert-butyl

bromide with hydroxide ion is an example of an E2 reaction; “E” stands for elimination

and “2” stands for bimolecular. The product of an elimination reaction is an alkene.

The rate of an E2 reaction depends on the concentrations of both tert-butyl bromide

and hydroxide ion. It is, therefore, a second-order reaction (Section 10.2).

The rate law tells us that both tert-butyl bromide and hydroxide ion are involved in

the rate-determining step of the reaction. The following mechanism agrees with the

observed second-order kinetics:

We see that an E2 reaction is a concerted, one-step reaction: The proton and the

bromide ion are removed in the same step, so no intermediate is formed.

mechanism of the E2 reaction

CH 2 CCH 2 CH+ 2 O + Br−

HO−

H

Br Br–^ is eliminated

a proton is

removed

CH 3

CH 3

CH 3

CH 3

rate = k[alkyl halide][base]

2-methylpropene

CH 3 C CH 3 ++HO− CH 2 CH 2 O + Br−

Br

CH 3

CH 3

CH 3

tert-butyl bromide

SN 2 —there

SN 1

Section 11.1 The E2 Reaction 401

INVESTIGATING NATURALLY

OCCURRING ORGANOHALIDES

Organohalides isolated from marine organisms

have been found to have interesting and potent biological ac-

tivity. Compounds produced in nature are called natural

products. Bengamide Ais a natural product that comes from

an orange encrusting sponge. This compound, as well as a

host of analogs, has unique antitumor properties that are cur-

rently being exploited in the development of new anticancer

jasplankinolide

CH 3

O

N

H

O

N

HN

O

O

O

OH

Br

HN

cyclocinamide A

OO

N

H

N

NH H

NH 2

HN

HO

H

N

H

N

O

O

O

O

N

Cl

Br

HN

drugs. Jaspamide, also found in a sponge, modulates the for-

mation and depolymerization of actin microtubules. Micro-

tubules are found in all cells and are used for motile events,

such as transportation of vesicles, migration, and cell divi-

sion. Jaspamide is being used to further our understanding of

these processes. Notice that each of these naturally occurring

compounds has six asymmetric carbons.

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