Q25E - Organic Chemistry (Mcmurry)

Question

Question: Evidence for the intermediate carbocations in the acid-catalyzed dehydration of alcohols comes from the observation that rearrangements sometimes occur. Propose a mechanism to account for the formation of 2,3-dimethyl-2-butene from 3,3-dimethyl-2-butanol.

Step-by-Step Solution

Answer

The mechanism followed as:

Step 1: Acid-catalyzed dehydrogenation

Acid-catalyzed hydrogenation of alcohol is done by E1 or E2 elimination, in which the hydroxyl group is protonated. This is followed by hydrolysis, and then the formation of alkene occurs.

Step 2: Formation of intermediate

In the reaction of alcohol with sulphuric acid, which is an acid catalyst, the formation of an alkyloxonium ion occurs, followed by hydrolysis. This is the removal of water which is a slow rate-determining process. The carbocation formation occurs, followed by the protonation of hydrogen by water. The formation of alkene occurs with the hydronium ion.

Step 3: Mechanism

1. Protonation

2. Loss of water

3. Alkyl shift

4. Formation of an alkene