Advices For Studying Organic Chemistry

C

O

R

R

R δ+ H C

H

δ+ O

R

H

R δ+ C

H

H

R δ+

δ+ δ+

δ+

δ+ δ+

Transition state leading

(most stable)

to 3o carbocation

Transition state leading

to 2o carbocation

Transition state leading

(least stable)

to 1o carbocation

H

H

δ+ O H

H

δ+

3) Because this developing positive charge is least effectively delocalized in the

transition state leading to a 1° carbocation, the dehydration of a 1° alcohol

proceeds through a different mechanism ––– an E2 mechanism.

7.7C A MECHANISM FOR DEHYDRATION OF PRIMARY ALCOHOLS:

AN E2 REACTION

A Mechanism for the Reaction

Dehydration of a Primary Alcohol: An E2 Reaction

+

Protonated alcohol

fast

Primary

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−

H

H

C

H

C

H

H

C

H

+

(typically sulfuric or

phosphoric acid)

slow

rate determining

A base removes a hydrogen from the β carbon as the double bond

forms and the protonated hydroxyl group departs. (The base may be

another molecule of the alcohol or the conjugate base of the acid)

C O H

H

A − + A OH

H H

H

C

H

+

Alkene

C C

R'

R

+ H +

(^)

7.8 CARBOCATION STABILITY AND THE OCCURRENCE OF