Concept:The Lucas test (\(\text{HCl} + \text{ZnCl}_2\)) differentiates alcohols by the rate at which they form insoluble alkyl chlorides. This rate depends on the \(\text{S}_N1\) mechanism, which requires carbocation formation.
Formula:$$ \text{Reactivity order: } 3^{\circ} \text{ (instant)} > 2^{\circ} \text{ (5 mins)} > 1^{\circ} \text{ (requires heat)} $$
Solution:- We must identify the classification of each given option:
- A. 2-butanol = Secondary.
- C. 2-methyl-1-butanol = Primary.
- D. 2,2-dimethyl-1-butanol = Primary (neopentyl-type).
- B. 2-methyl-2-butanol has the -OH and a methyl group on the same carbon (C2). It is a Tertiary alcohol.
- Tertiary alcohols form the most stable carbocations and react almost instantaneously with Lucas reagent.
Why other options are incorrect:Primary and secondary alcohols react much slower because their respective carbocation intermediates are far less stable and take higher activation energy to form.
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