What is Lucas reagent?

Lucas reagent is a mixture of conc. HCl and anhydrous ZnCl$_2$. It is used to distinguish between primary, secondary, and tertiary alcohols.

What happens in the Lucas test?

The alcohol is converted into an alkyl chloride: $R-OH \xrightarrow[\text{ZnCl}_2]{\text{conc. HCl}} R-Cl$

The rate of this reaction depends on how easily the alcohol forms a carbocation.

Role of mechanism

In acidic medium, the $-OH$ group is protonated / activated and leaves as water. If a stable carbocation can form, the reaction proceeds by $S_N1$.

Carbocation stability order is: $3^\circ > 2^\circ > 1^\circ$

Therefore, the reactivity order in Lucas test is: $3^\circ \text{ alcohol} > 2^\circ \text{ alcohol} > 1^\circ \text{ alcohol}$

Which alcohol reacts fastest?

Since tertiary alcohol forms the most stable carbocation, it reacts the fastest.

Which mechanism is followed?

Tertiary alcohols react via $S_N1$ mechanism because tertiary carbocations are sufficiently stable.

Check options

• A: tertiary alcohol by $S_N1$ — Correct
• B: secondary alcohol by $S_N2$ — Incorrect
• C: tertiary alcohol by $S_N2$ — Incorrect; steric hindrance prevents $S_N2$
• D: secondary alcohol by $S_N1$ — Secondary may react by $S_N1$, but not fastest

Final answer

The alcohol that reacts fastest with Lucas reagent is tertiary alcohol, and it reacts by $S_N1$ mechanism.