Q33E - Organic Chemistry (Mcmurry)

Question

When 2-methyl-2,5-pentanediol is treated with sulfuric acid, dehydration occurs and 2,2 dimethyltetrahydrofuran is formed. Suggest a mechanism for this reaction. Which of the two oxygen atoms is most likely to be eliminated, and why?

Step-by-Step Solution

Answer

The oxygen atom is most likely to be eliminated at tertiary carbon because the resulting carbocation is more stable.

Step 1: Protonation of the tertiary hydroxyl group

The first step of the reaction is protonation, where the alcohol group gets protonated with sulphuric acid to form a tertiary hydroxyl group, as shown below:

Protonation

Step 2: Dehydration

The second step of the reaction is the dehydration reaction, where water leaves to form a more stable tertiary carbocation. A tertiary carbocation is more likely to be eliminated in the first step of the reaction because the resulting carbocation is more stable, as shown in the figure given below:

Dehydration

Step 3: Nucleophilic attack

In the third step of the reaction, there is a nucleophilic attack on the carbocation by the second hydroxyl group, as shown below:

                                                                                2,2-dimethyltetrahydrofuran     

                                     Mechanism of the given reaction