Advices For Studying Organic Chemistry

A Mechanism for the Reaction

Acid-Catalyz ed Dehydration of Secondary or Tertiary Alcohols: An E1 Reaction
Step 1

+

Protonated alcohol

fast

2 o or 3o Alcohol Strong acid Conjugate base

The alcohol accepts a proton from the acid in a fast step.

C O H

H

O H H A + A−

R

R'

C

H

C

R

R'

C

H

+

(R' may be H) (typically sulfuric or^

phosphoric acid)

Step 2

slow

The protonated alcohol loses a molecule of water to become a carbocation.

C O H

H

+ OH

R H

R'

C

H (rate determining)

C C +

H R'

R

+

This step is slow and rate determining

Step 3

The carbocation loses a proton to a base. In this step, the base may be another

molecule of the alcohol, water, or the conjugate base of the acid. The proton

transfer results in the formation of the alkene. Note that the overall role of the

acid is catalytic (it is used in the reaction and regenerated).

Alkene

C C

H R'

R

+ +

fast C CR'

R

A − + H A

(^)

2. The order of free energy of activation for dehydration of alcohols is 3° > 2° > 1° >
   methyl: