Q26E - Organic Chemistry (Mcmurry)

Question

Question:Acid-catalyzed dehydration of 2,2-dimethylcyclohexanol yields a mixture of 1,2-methylcyclohexane and isopropylidenecyclopentane. Propose a mechanism to account for the formation of both products.

Step-by-Step Solution

Answer

The mechanism followed for the formation of propan-2ylidene cyclopentane and 1,2-dimethylhexene is shown below:

Step 1: Acid-catalyzed dehydrogenation

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

Step 2: Formation of 1,2-dimethyl cyclohexene

2,2-dimethyl cyclohexene on reacting with acid results in catalyst protonation. This is followed by the loss of water and then by an alkyl shift. One of the methyl groups is shifted to the next carbon, followed by loss of the hydrogen ion and formation of 1,2 dimethyl cyclohexene.

Step 3: Formation of isopropylidene cyclopentane

Isopropylidene cyclopentanone reacting with an acid catalyst to form protonation. This followed by the loss of water and then followed by an alkyl shift. Both methyls are shifted to the next carbon. followed by the loss of the hydrogen ion, and the formation of isopropylidene occurs. Whereas for the formation of 1,2-dimethyl cyclohexane, only 1 methyl is shifted. The only difference is in the alkyl shift, as shown below: