Why is the reactivity of all the three classes of alcohols with conc. HCl and `ZnCl_(2)` (Lucas reagent) different ?

Explanation

The alcohols combine with HCl to form protonated alcohol. The positive charge on oxygen weakens the C-O bond leading to its cleavage.

(iii) `R^(+)+Cl^(-) overset("fast) to 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. `lt` sec. `lt` tert. Hence the order of formation of alkyl halide in the above reaction is pri.`lt` sec. `lt` tert.