Question

Preparation of ethers by acid dehydration of alcohol a suitable method for:

• A. primary alcohol
• B. secondary alcohol
• C. tertiary alcohol
• D. all of these

Answer 1

Answer: (A)

primary alcohol

Acid-catalysed dehydration of $1^{\circ }$ alcohols to ethers occurs by $S{N}_2$ reaction involving the nucleophilic attack by the alcohol molecule on the protonated alcohol molecule.
Under these conditions, $2^{\circ }and{3}^{\circ }$ alcohols, however, give alkenes rather than ether. The reason being that due to steric hindrance, nucleophilic attack by the alcohol molecule on the protonated alcohol molecule does not occur. Instead protonated $2^{\circ }and{3}^{\circ }$ alcohols lose a molecule of water to form stable $2^{\circ }and{3}^{\circ }$ carbocations. These carbocations prefer to lose a proton to form alkenes rather than undergoing nucleophilic attack by alcohol molecules to form ethers.
Similarly, $3^{\circ }$ alcohols give alkenes rather than ethers.