Organic Chemistry

d.

c.

Stamps issued in honor of English

Nobel laureates: (a) Sir Derek

Barton for conformational analysis,

1969; (b) Sir Walter Haworth for the

synthesis of vitamin C, 1937; (c) A. J.

P. Martin and Richard L. M. Synge

for chromatography, 1952;

(d) William H. Bragg and William L.

Bragg for crystallography, 1915 (the

only father and son to receive a

Nobel Prize).

Section 11.6 Elimination from Cyclic Compounds 419

In contrast, the conformer of menthyl chloride that undergoes elimination is the less

stable conformer because when the Cl and H are in the required axial positions, the

methyl and isopropyl groups are also in axial positions.

Notice that when menthyl chloride or trans-1-chloro-2-methylcyclohexane under-

goes an E2 reaction, the hydrogen that is eliminated is not removed from the -carbon

bonded to the fewest hydrogens. This may seem like a violation of Zaitsev’s rule, but

the rule states that when there is more than one -carbon from which a hydrogen can

be removed, the hydrogen is removed from the -carbon bonded to the fewest hydro-

gens. In the preceding two reactions, the hydrogen that is removed has to be in an axial

position and only one -carbon has a hydrogen in an axial position. Therefore, that

hydrogen is the one that is removed even though it is not bonded to the -carbon with

the fewest hydrogens.

PROBLEM 12

Why do cis-1-bromo-2-ethylcyclohexane and trans-1-bromo-2-ethylcyclohexane form

different major products when they undergo an E2 reaction?

PROBLEM 13

Which isomer reacts more rapidly in an E2 reaction,cis-1-bromo-4-tert-butylcyclohexane

or trans-1-bromo-4-tert-butylcyclohexane? Explain your choice.

E1 Elimination from Cyclic Compounds

When a substituted cyclohexane undergoes an E1 reaction, the two groups that are

eliminated do not have to both be in axial positions, because the elimination reaction is

not concerted. In the following reaction, a carbocation is formed in the first step. It

then loses a proton from the adjacent carbon that is bonded to the fewest hydrogens—

in other words, Zaitsev’s rule is followed.

H

Cl

CH 3

CH 3 + Cl−

+ H+

+

H

CH 3 CH 3 OH

E1 conditions

CH 3

CH 3

CH 3

not in an

equatorial

position

equatorial

H

Cl

H

H

H

Cl

H

+ H 2 O

CH 3

H CH 3

trans-1-chloro-2-methylcyclohexane

H

CH 3

axial

axial

HO− E2 conditions

more stable less stable

+ Cl−

b

b

b

b

b

a.

An E1 reaction involves both syn and

anti elimination.

b.

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