Organic Chemistry

secondary alkyl halides, which, in turn, are more reactive than primary alkyl halides in

both E1 and E2 reactions (Section 11.2).

We have seen that the weakest bases are the best leaving groups (Section 10.3).

Therefore, for series of alkyl halides with the same alkyl group, alkyl iodides are the

most reactive and alkyl fluorides the least reactive in E1 reactions.

Because the E1 reaction forms a carbocation intermediate, the carbon skeleton

can rearrange before the proton is lost, if rearrangement leads to a more stable car-

bocation. For example, the secondary carbocation that is formed when a chloride ion

dissociates from 3-chloro-2-methyl-2-phenylbutane undergoes a 1,2-methyl shift to

form a more stable tertiary benzylic cation, which then undergoes deprotonation to

form the alkene.

In the following reaction, the initially formed secondary carbocation undergoes a

1,2-hydride shift to form a more stable secondary allylic cation:

PROBLEM 5

Three alkenes are formed from the E1 reaction of 3-bromo-2,3-dimethylpentane. Give the

structures of the alkenes, and rank them according to the amount that would be formed.

(Ignore stereoisomers.)

PROBLEM 6

If 2-fluoropentane were to undergo an E1 reaction, would you expect the major product to

be the one predicted by Zaitsev’s rule? Explain.

relative reactivities of alkyl halides in an E1 reaction

increasing reactivity

most reactive RI > RBr > RCl > RF least reactive

Section 11.3 The E1 Reaction 411

C CHCH 3

CH 3 OH

+

CH 3 Cl

CH 3

C CHCH 3

CH 3 CH 3

CH 3

+

C CHCH 3

CH 3

CH 3

C CCH 3

CH 3

secondary carbocation tertiary benzylic cation

1,2-methyl

shift

2-methyl-3-phenyl-

2-butene

3-chloro-2-methyl-

2-phenylbutane

+ H+

CH 3 CH CHCH 2 CHCH 2 CH 3

+

Br

5-bromo-2-heptene

CH 3 CH CHCH 2 CHCH 2 CH 3

secondary carbocation

CH 3 CH CHCHCH 2 CH 2 CH 3

CH 3 CH CHCH CHCH 2 CH 3

secondary allylic cation

2,4-heptadiene

CH 3 OH

1,2-hydride

shift +

H++

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