Question

Lucas reagent, used to distinguish between primary, secondary, and tertiary alcohols, is a mixture of:

• A. Conc. HCl + anhydrous ZnCl$_2$
• B. Conc. HNO$_3$ + AgNO$_3$
• C. Pd + BaSO$_4$
• D. NH$_4$OH + AgNO$_3$

Hint:

Lucas Test Summary: Tertiary alcohol → Immediate turbidity
Secondary alcohol → Turbidity after few minutes
Primary alcohol → No turbidity at room temperature

Answer 1

The Correct Option is A

Concept: Lucas reagent is commonly used in organic chemistry to distinguish between primary, secondary, and tertiary alcohols. The reagent is a mixture of: \[ \text{Concentrated HCl + Anhydrous ZnCl}_2 \] In this test, alcohols react with hydrogen chloride in the presence of zinc chloride to form alkyl chlorides. The rate of formation of turbidity indicates the class of alcohol.

Step 1: Role of ZnCl$_2$. Anhydrous ZnCl$_2$ acts as a Lewis acid catalyst. It activates the alcohol group and facilitates substitution by chloride ion.

Step 2: Observation for different alcohols.
• Tertiary alcohols react immediately, producing turbidity quickly.
• Secondary alcohols react slowly and produce turbidity after a few minutes.
• Primary alcohols react very slowly or show no turbidity at room temperature.

Step 3: Eliminate other options.
• Conc. HNO$_3$ + AgNO$_3$ → Not used in Lucas test.
• Pd + BaSO$_4$ → Used in Rosenmund reduction.
• NH$_4$OH + AgNO$_3$ → Tollen's reagent for aldehydes. Therefore, Lucas reagent is a mixture of concentrated HCl and anhydrous ZnCl$_2$.