Name: Acidic Nature of Phenol
Uploaded: 2019-07-19
Duration: 29 min 6 s
Description: Does the resonance play any role in the acidity of phenol? What is the importance of Pka in the comparison of Acidic strength? Let's watch this video.

Question

Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.

Accepted Answer

Acid catalysed dehydration of primary alcohols to ethers occurs by $S_{N} 2$ reaction involving nucleophilic attack by the alcohol molecule on the protonated alcohol molecule.

Under these conditions, $2 °$ and $3 °$ alcohols, however, give alkenes rather than ethers. The reason being that due to steric hindrance, nucleophilic attack by the alcohol molecule on the protonated alcohol molecule does not occur. Instead, protonated $2 °$ and $3 °$ alcohols lose a molecule of water to form stable $2 °$ and $3 °$ carbocation. These carbocations prefer to lose a proton to form alkenes rather than undergoing nucleophilic attack by alcohol molecules to form ethers.

$\underset{2^{o} Alcohol}{{({CH}_{3})}_{2} CH - OH} \overset{H^{+}}{\to} {({CH}_{3})}_{2} CH - \overset{+}{O} H_{2} \underset{- H_{2} O}{\to} {({CH}_{3})}_{2} \overset{+}{C} H \underset{- H^{+}}{\to} \underset{Propene}{{CH}_{3} - CH = {CH}_{2}}$

$\underset{3^{o} Alcohol}{{({CH}_{3})}_{3} C - OH} \overset{H^{+}}{\to} {({CH}_{3})}_{3} C - \overset{+}{O} H_{2} \underset{- H_{2} O}{\to} {({CH}_{3})}_{3} \overset{+}{C} \underset{- H^{+}}{\to} \underset{2 - methyl propene}{{({CH}_{3})}_{2} C = {CH}_{2}}$

Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.

Important Questions on Alcohols, Phenols and Ethers

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