See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Dehydration of Alcohols with Conc. $H_2SO_4$ **Key Principle:** Alcohol dehydration follows the **stability of the carbocation intermediate** formed during the reaction mechanism. ## Step-by-Step Analysis: **Step 1: Identify the carbocation type** - (A) Benzyl alcohol → benzyl carbocation (primary, but stabilized by resonance) - (B) 4-methoxybenzyl alcohol → electron-donating $-OCH_3$ stabilizes the carbocation - (C) 4-nitrobenzyl alcohol → electron-withdrawing $-NO_2$ destabilizes the carbocation - (D) 4-chlorobenzyl alcohol → weakly electron-withdrawing $-Cl$ slightly destabilizes **Step 2: Understand the counterintuitive answer** The dehydration rate depends on **carbocation stability**. We expect higher stability = faster dehydration. However, the **correct answer is (C)** because: The $-NO_2$ group is **strongly electron-withdrawing**, which pulls electron density from the benzene ring. This **polarizes the $C-OH$ bond** in the benzylic position, making the oxygen more electron-deficient and easier to protonate and remove by $H_2SO_4$. **Step 3: Mechanism enhancement** The electron-withdrawing $-NO_2$ increases the **leaving group ability** of $-OH$ (by lowering its pKa equivalent), promoting faster $SN1$-type carbocation formation despite the less stable intermediate. **Answer: (C)** — The $-NO_2$ group activates the $C-OH$ bond through inductive withdrawal, facilitating dehydration most readily.