Question

Why is the reactivity of all the three classes of alcohols with conc. \[\ce{HCl}\] and \[\ce{ZnCl2}\] (Lucas reagent) different?

Answer 1

The alcohol combine with \[\ce{HCl}\] to form protonated alcohol. The positive charge on oxygen weakens the C – O bond leading to its cleavage.

(i) 

\[\begin{array}{cc}
\phantom{...............}\ce{H}\\
\phantom{...............}|\\
\ce{R - \underset{\bullet\bullet}{\overset{\bullet\bullet}{O}} - H + HCl ⇌[fast] R - \underset{\bullet\bullet}{O}^+ - H + Cl-}
\end{array}\] 

(ii) 

(iii) 

\[\ce{R+ + Cl- ->[fast] RCl}\]

The rate-determining step in the above mechanism is (ii), which is a slow step reaction. The stability of carbocation will determine the reactivity of the reaction. Since the order of stability of carbocation is: Pri. < sec. < tert. Hence the order of formation of alkyl halide in the above reaction is pri. < sec. < tert.