Step 1 (Governing Principle & Reaction Mechanism): [0.5 Mark]
The reaction of alcohols with Lucas reagent (a mixture of anhydrous $\text{ZnCl}_2$ and concentrated $\text{HCl}$) follows an $S_N1$ mechanism. The rate of this reaction depends on the stability of the carbocation intermediate formed. The order of reactivity of alcohols towards Lucas reagent is:
$$ \text{Tertiary (3}^\circ\text{)} > \text{Secondary (2}^\circ\text{)} > \text{Primary (1}^\circ\text{)} $$Tertiary alcohols form stable carbocations readily and react with Lucas reagent at room temperature to produce immediate turbidity due to the formation of insoluble alkyl chlorides.
Step 2 (Analysis of Given Options): [0.25 Mark]
- (A) $\text{CH}_2 = \text{CH} - \text{CH}_2\text{OH}$: Primary ($1^\circ$) allylic alcohol. Requires heating.
- (B) $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$: Primary ($1^\circ$) alcohol. Does not react at room temperature.
- (C) $\text{C}_6\text{H}_5\text{CH}_2\text{OH}$: Primary ($1^\circ$) benzylic alcohol. Does not react at room temperature.
- (D) $(\text{CH}_3)_3\text{COH}$: Tertiary ($3^\circ$) alcohol. Forms a stable tertiary carbocation $(\text{CH}_3)_3\text{C}^+$.
Step 3 (Final Concluded Answer): [0.25 Mark]
Since $(\text{CH}_3)_3\text{COH}$ is a tertiary alcohol, it undergoes the substitution reaction instantaneously at room temperature.
Correct Option: (D) $(\text{CH}_3)_3\text{COH}$
💡 EL10 Concept Insight: Think of Lucas reagent as a "test for speed." Tertiary alcohols are like athletes who are already at the finish line (very stable), so they react instantly, while primary alcohols are like people who haven't even started running yet, so they need a lot of extra energy (heat) to get moving.