Question

Choose the incorrect statement.

• A. Relative ease of dehydration of alcohols on heating with a protic acid is Primary $>$ Secondary $>$ Tertiary
• B. Reaction of alcohols with anhydrides is carried out in the presence of small amounts of Conc. $\text{H}_2\text{SO}_4$ to remove the water formed
• C. p-Nitrophenol is more acidic than p-Cresol
• D. Cyclic $\text{C}_4\text{H}_7\text{OH}$ exists as four structural isomers

Hint:

Dehydration rates of alcohols always mirror carbocation stability profiles. Since a $3^\circ$ carbocation is far more stable than a $1^\circ$ intermediate, the ease of dehydration must be Tertiary $>$ Secondary $>$ Primary. Statement (A) states the exact reverse, revealing itself as the incorrect option.

Answer 1

The Correct Option is A

Concept: The dehydration of alcohols in the presence of an acid catalyst typically proceeds via an $\text{E}1$ mechanism involving the formation of a carbocation intermediate. The rate-determining step is the generation of this carbocation, meaning the relative ease of dehydration depends directly on carbocation stability: \[ \text{Stability order: Tertiary } (3^\circ) > \text{Secondary } (2^\circ) > \text{Primary } (1^\circ) \]

Step 1: Evaluate Statement (A).
Because tertiary carbocations are highly stabilized by hyperconjugation and inductive effect inputs, tertiary alcohols dehydrate under extremely mild acidic conditions. Primary alcohols require much harsher conditions. Therefore, the actual ease of dehydration follows the order: $\text{Tertiary} > \text{Secondary} > \text{Primary}$. This makes statement (A) incorrect, and thus the correct answer to this question.

Step 2: Verify why the other options are correct statements.

• Statement (C) is a valid chemical fact: The electron-withdrawing nitro group ($-\text{NO}_2$, $-I/-M$) stabilizes the phenoxide conjugate base, increasing acidity. The electron-donating methyl group ($-\text{CH}_3$, $+I$) destabilizes it, decreasing acidity.
• Statement (D) is a valid structural fact: The cyclic configurations for $\text{C}_4\text{H}_7\text{OH}$ include cyclobutanol, 1-methylcyclopropanol, 2-methylcyclopropanol, and cyclopropylmethanol.