Question

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 which mechanism?

• A. Tertiary alcohol by $S_{N}2$
• B. Secondary alcohol by $S_{N}1$
• C. Tertiary alcohol by $S_{N}1$
• D. Secondary alcohol by $S_{N}2$

Answer 1

Answer: (C)

Tertiary alcohol by $S_{N}1$

Lucas test is a test to recognize whether the given alcohol is primary, secondary or tertiary in nature. Lucas reagent is a mixture of conc. $HCl$ and dry anhydrous $ZnC{l}_2$. In this reaction, a carbocation is formed as an intermediate and thus the stability and ease of carbocation formation determine the rate of the reaction as that's the slow step of the mechanism. Hence the order of reactivity is tertiary alcohol > secondary alcohol by $S_{N}1$ mechanism.

Thus with Lucas reagent, tertiary alcohol reacts fastest and by $S_{N}1$ mechanism.