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

Concept: The Lucas test is used to distinguish between primary, secondary, and tertiary alcohols based on their reactivity with Lucas reagent. Lucas reagent is a mixture of: \[ \text{Conc. } HCl + \text{Anhydrous } ZnCl_2 \] \(ZnCl_2\) acts as a Lewis acid catalyst and facilitates the substitution reaction.

Step 1: Understand the reaction. Alcohols react with Lucas reagent to form alkyl chlorides. \[ ROH + HCl \xrightarrow{ZnCl_2} RCl + H_2O \]

Step 2: Reaction rate differences.
• Tertiary alcohols react immediately producing turbidity.
• Secondary alcohols react slowly (few minutes).
• Primary alcohols show no turbidity at room temperature. Thus, the reagent used in the Lucas test is: \[ \text{Conc. } HCl + \text{Anhydrous } ZnCl_2 \]