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An unknown alcohol is treated with the “Lucas reagent” to determine whether the alcohol is primary, secondary or tertiary. Which alcohol reacts fastest and by what mechanism ?

1. secondary alcohol by S_N1
2. tertiary alcohol by S_N1
3. secondary alcohol by S_N2
4. tertiary alcohol by S_N2

Correct answer: tertiary alcohol by S_N1

Solution

Lucas reagent (conc. HCl + ZnCl2) reacts fastest with tertiary alcohols because they form the most stable carbocation, proceeding by an SN1 mechanism (immediate turbidity).

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