Advices For Studying Organic Chemistry

A Mechanism for the Elimination Reaction of Menthyl Chloride

E2 Elimination Where The Only Eligible Axial Cyclohexane β-Hydrogen is From a
Less Stable Conformer

1

4 3 2

Et−O −

H 3 C 1 CH(CH 3 ) 2

3 2

4

2-Menthene (100%)

Menthyl chloride

(more stable conformation)

H

H

Cl

H

Elimination is not possible for this

conformation because no hydrogen

is anti to the leaving group.

H

H 3 C CH(CH 3 ) 2

Cl

CH 3

H CH(CH^3 )^2

H H

H

Menthyl chloride

(less stable conformation)

Elimination is possible for this

conformation because the green

hydrogen is anti to the chlorine.

(^)
3) The more stable conformation of menthyl chloride:
i) The alkyl groups and the chlorine are equatorial.
ii) For the chlorine to become axial, menthyl chloride has to assume a
conformation in which the large isopropyl group and the methyl group are also
axial.
ii) This conformation is of much higher energy, and the free energy of activation
for the reaction is large because it includes the energy necessary for the
conformational change.
ii) Menthyl chloride undergoes an E2 reaction very slowly, and the product is
entirely 2-menthene (Hofmann product).