Question

From amongst the following alcohols the one that would react fastest with conc HCl and anhydrous $ZnC{l}_2$?

• A. 2-butanol
• B. 2-methylpropan-2-ol
• C. 2-methylpropanol
• D. 1-butanol

Answer 1

The correct option is B 2-methylpropan-2-ol

The reaction of alcohol with Lucas reagent $(\text{conc.}HCl+\text{anhyd.}ZnC{l}_2)$ is mostly $S_{N}1$ reaction and the rate of reaction is directly proportional to the stability fo 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 $(\text{conc.}HCl+\text{anhyd.}ZnC{l}_2)$ follows follows the order $3^{\circ }$ alcohol > $2^{\circ }$ alcohol > $1^{\circ }$ alcohol.
2-methylpropan-2-ol is $3^o$ alcohol thus it will react fastest as compared to others.