Question

Lucas reagent is

• A. Con. HNO\(_3\) + ZnCl\(_2\)
• B. Con. HCl + ZnCl\(_2\)
• C. Con. H\(_2\)SO\(_4\) + ZnCl\(_2\)
• D. Acetic acid + ZnCl\(_2\)
• E. Oleium + ZnCl\(_2\)

Hint:

The Lucas test is based on the formation of an insoluble alkyl chloride layer.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
This is a direct question about the composition of a specific named reagent used in organic chemistry. The Lucas reagent is used to distinguish between primary, secondary, and tertiary alcohols.
Step 2: Detailed Explanation:
The Lucas test involves the reaction of an alcohol with the Lucas reagent. The reaction is an S\(_N\)1 reaction where the hydroxyl group of the alcohol is replaced by a chloride ion. The anhydrous ZnCl\(_2\) acts as a Lewis acid, which coordinates with the oxygen atom of the hydroxyl group, making it a better leaving group (H\(_2\)O). This facilitates the formation of a carbocation intermediate.
The composition of the Lucas reagent is a solution of anhydrous zinc chloride (ZnCl\(_2\)) in concentrated hydrochloric acid (HCl).
The rate of reaction depends on the stability of the carbocation formed:
Tertiary alcohols react almost instantaneously because they form a stable tertiary carbocation, resulting in immediate cloudiness or turbidity due to the formation of the insoluble alkyl chloride.
Secondary alcohols react more slowly, typically within 5-10 minutes, as they form a less stable secondary carbocation.
Primary alcohols do not react at room temperature because the primary carbocation is highly unstable.
Step 3: Final Answer:
The Lucas reagent is a mixture of concentrated HCl and anhydrous ZnCl\(_2\). This corresponds to option (B).