Question

Identify the reagent used in the Lucas test to distinguish between primary, secondary, and tertiary alcohols.

• A. Dilute \(HCl\) and \(ZnCl_2\)
• B. Conc. \(HCl\) and anhydrous \(ZnCl_2\)
• C. \(H_2SO_4\) and \(ZnCl_2\)
• D. Conc. \(HNO_3\) and \(ZnCl_2\)

Hint:

Lucas reagent = Conc. \(HCl\) + anhydrous \(ZnCl_2\). Tertiary alcohols react fastest due to stable carbocation formation.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The Lucas test is a qualitative chemical test used to classify alcohols as primary, secondary, or tertiary based on the rate of formation of alkyl chlorides.
Step 2: Detailed Explanation:
The Lucas reagent consists of a mixture of concentrated Hydrochloric acid (\(HCl\)) and anhydrous Zinc chloride (\(ZnCl_2\)).
The anhydrous \(ZnCl_2\) acts as a catalyst (Lewis acid) that helps in the cleavage of the C-OH bond.
Reactivity order: Tertiary > Secondary > Primary.
- Tertiary alcohols: Immediate turbidity.
- Secondary alcohols: Turbidity in about 5 minutes.
- Primary alcohols: No turbidity at room temperature.
Step 3: Final Answer:
The correct reagent is Conc. \(HCl\) and anhydrous \(ZnCl_2\).