Question

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

• A. Secondary alcohol by SN₁
• B. Tertiary alcohol by SN₁
• C. Secondary alcohol by SN₂
• D. Tertiary alcohol by SN₂

Hint:

Lucas reagent test works on carbocation stability, making tertiary alcohols the fastest-reacting.

Answer 1

The Correct Option is B

Step 1: Understanding Lucas Test
- Lucas test differentiates alcohols based on their reaction with ZnCl₂/HCl.
Step 2: Order of Reactivity
- Tertiary alcohols react fastest via the S_N1 mechanism.
Final Answer: Tertiary alcohol via S_N1.

Answer 2

The alcohol that reacts fastest with Lucas reagent is a tertiary alcohol, and the reaction proceeds via the SN₁ mechanism.

Lucas reagent is a solution of zinc chloride (ZnCl₂) in concentrated hydrochloric acid (HCl). It is used to distinguish between primary, secondary, and tertiary alcohols based on their reactivity.

Tertiary alcohols react fastest because they form a stable tertiary carbocation intermediate upon protonation and loss of water. This carbocation formation is the rate-determining step of the SN₁ mechanism.

In the SN₁ mechanism, the alcohol is first protonated by the acidic medium, making the hydroxyl group a better leaving group. Then, the leaving group departs, forming a carbocation. The carbocation quickly reacts with the chloride ion (Cl⁻) from Lucas reagent to form the corresponding alkyl chloride.

Secondary alcohols react slower due to less stable carbocations, while primary alcohols hardly react under these conditions because primary carbocations are highly unstable and the reaction may proceed through a much slower SN₂ pathway.

Therefore, tertiary alcohols react fastest with Lucas reagent through an SN₁ mechanism.