Question

An unknown alcohol is treated with the "Lucas reagent" to determine whether the alcohol is primary, secondary or teritiary. Which alcohol reacts fastest and by what mechanism:

• A. tertiary alcohol by $S_{N}1$
• B. secondary alcohol by $S_{N}2$
• C. tertiary alcohol by $S_{N}2$
• D. secondary alcohol by $S_{N}1$

Answer 1

The correct option is A tertiary alcohol by $S_{N}1$

The reaction of alcohol with Lucas reagent $(HCl+ZnC{l}_2)$ is mostly $S_{N}1$ reaction and the rate of reaction is directly proportional to the stability of carbocation formed in the reaction.
Since, $3^o$ alcohol forms $3^o$ carbocation (most stable) hence, it will react fastest by $S_{N}1$ reaction.
The rates of reaction with Lucas reagent $(HCl+ZnC{l}_2)$ follows follows the order $3^{\circ }$ alcohol > $2^{\circ }$ alcohol > $1^{\circ }$ alcohol.

Mechanism of Lucas test reaction​
Acid-Base reaction with $ZnC{l}_2$
Attack of chloride ion