Question: The correct order of reactivity of alcohols with Lucas reagent is:
Step 1 (Governing Principle & Reaction Mechanism): [0.5 Mark]
The reaction of alcohols with Lucas reagent (conc. $HCl$ + anhydrous $ZnCl_2$) proceeds via an $S_N1$ mechanism. The rate-determining step involves the formation of a carbocation intermediate. The stability of the carbocation determines the reactivity of the alcohol:
$$R-OH + HCl \xrightarrow{ZnCl_2} R-Cl + H_2O$$The stability order of carbocations is: Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$).
Step 2 (Step-by-Step Logical Deduction): [0.25 Mark]
- $3^\circ$ Alcohols: Form stable $3^\circ$ carbocations rapidly; react immediately at room temperature (turbidity appears instantly).
- $2^\circ$ Alcohols: Form $2^\circ$ carbocations; react moderately (turbidity appears in 5–10 minutes).
- $1^\circ$ Alcohols: Form unstable $1^\circ$ carbocations; do not react at room temperature (no turbidity unless heated).
Step 3 (Final Concluded Answer): [0.25 Mark]
Based on the stability of the carbocation intermediate, the correct order of reactivity is:
$$3^\circ > 2^\circ > 1^\circ$$Correct Option: (D)
💡 EL10 Concept Insight: Think of the carbocation as a "hungry" intermediate; since $3^\circ$ carbocations are the most stable and easiest to form, they "grab" the chlorine atom the fastest, while $1^\circ$ carbocations are too unstable to form easily, making them the slowest to react.