Question
An unknown alcohol is treated with the “Lucas reagent” to determine whether the alcohol is primary, secondary or tertiary. Which alcohol reacts fastest and by what mechanism :
A.
secondary alcohol by $${S_N}1$$
B.
tertiary alcohol by $${S_N}1$$
C.
secondary alcohol by $${S_N}2$$
D.
tertiary alcohol by $${S_N}2$$
Answer :
tertiary alcohol by $${S_N}1$$
Solution :
Tertiary alchols reacts fastest with lucas reagnet as the rate of reaction is directly proportional to the stability of carbocation formed in the reaction. Since most stable $${3^ \circ }$$ carbocation is formed in the reaction hence it will react fastest further tetriary alcohols appears to react by $${S_N}1$$ mechanism.
$$\eqalign{
& {\text{step}}\,{\text{1}}{\text{.}}\,\,\,{\left( {C{H_3}} \right)_3}C - OH + H - Cl \rightleftarrows {\left( {C{H_3}} \right)_3} - \mathop {\text{O}}\limits^{\text{ + }} {H_2} + C{l^ - } \cr
& {\text{step}}\,2.\,\,\,{\left( {C{H_3}} \right)_3}C - \mathop {\text{O}}\limits^{\text{ + }} {H_2} \rightleftarrows {\left( {C{H_3}} \right)_3}{C^ + } + {H_2}O \cr
& {\text{step}}\,3.\,\,\,{\left( {C{H_3}} \right)_3}{C^ + } + C{l^ - } \to \mathop {{{\left( {C{H_3}} \right)}_3}C - Cl}\limits_{{\text{tert - Butyl chloride}}} \cr} $$