Organic Chemistry

418 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and Elimination

3-D Molecules:

Axial chlorocyclohexane;

Equatorial chlorocyclohexane

Tutorial:

E2 Eliminations from cyclic

compounds

for a substituent in that position.) The less stable conformer, with the chloro sub-

stituent in the axial position, readily undergoes an E2 reaction.

Because one of the two conformers does not undergo an E2 reaction, the rate of an

elimination reaction is affected by the stability of the conformer that does undergo the

reaction. The rate constant of the reaction is given by Therefore, the reaction is

faster if is large (i.e., if elimination takes place by way of the more stable con-

former). If elimination has to take place by way of the less stable conformer, will

be small. For example, neomenthyl chloride undergoes an E2 reaction with ethoxide

ion about 200 times faster than menthyl chloride does. The conformer of neomenthyl

chloride that undergoes elimination is the morestable conformer because when the Cl

and H are in the required axial positions, the methyl and isopropyl groups are in the

equatorial positions.

Keq

Keq

k¿Keq.

H H

H

+ Cl−

Cl

Cl

H

HO−

E2 conditions

HO−

E2 conditions

no reaction

Keq

k′

equatorial

more stable

axial

axial

less stable

Sir Derek H. R. Barton (1918–1998)

was the first to point out that the

chemical reactivity of substituted

cyclohexanes was controlled by their

conformation. Barton was born in

Gravesend, Kent, England. He

received a Ph.D. and a D.Sc. from

Imperial College, London, in 1942

and became a faculty member there

three years later. Subsequently, he

was a professor at the University of

London, the University of Glasgow,

the Institut de Chimie des Substances

Naturelles, and Texas A & M

University. He received the 1969

Nobel Prize in chemistry for his work

on the relationship of the three-

dimensional structures of organic

compounds to their chemical

reactivity. Barton was knighted by

Queen Elizabeth II in 1972.

menthyl chloride

CH(CH 3 ) 2

CH 3 CH 2 OH

Cl

Cl−

Cl

H

CH 3

more stable CH(CH 3 ) 2

CH 3

CH(CH 3 ) 2

CH 3

less stable

CH 3 CH 2 O− E2 conditions

+ +

neomenthyl chloride

E2 conditions

H

CH(CH 3 ) 2

CH(CH 3 ) 2

Cl

CH 3

CH(CH 3 ) 2

Cl

more stable

CH 3

CH 3

H

less stable

CH 3 CH 2 O−

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