Question

What is Lucas reagent? Write down Lucas test for distinction between primary, secondary and tertiary alcohols.

Hint:

Lucas test is based on the stability of carbocations formed during substitution: \(3^\circ>2^\circ>1^\circ\) alcohols.

Answer 1

Step 1: Lucas reagent Lucas reagent consists of: \begin{itemize} \item Concentrated HCl \item Anhydrous ZnCl\(_2\) \end{itemize} ZnCl\(_2\) acts as a Lewis acid and facilitates the substitution of the \(-\text{OH}\) group by Cl\(^-\).
Step 2: Principle of Lucas test Alcohols react with Lucas reagent to form insoluble alkyl chlorides. Turbidity appears due to the formation of these alkyl chlorides.
Step 3: Distinction among alcohols \begin{itemize} \item Tertiary alcohols: Immediate turbidity (reaction is fastest due to stable carbocation). \item Secondary alcohols: Turbidity appears after a few minutes. \item Primary alcohols: No turbidity at room temperature (react only on heating). \end{itemize}