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