Question

Which alcohol gives a precipitate immediately with Lucas reagent

Answer 1

In Lucas test, the alcohol is treated with a solution of anhydrous ZnCl₂ in concentrated HCl (Lucas reagent).

The principle of this test is based on the formation of alkyl chloride from alcohol by reaction with conc. HCl in the presence of ZnCl_2.

Reaction:

$\mathrm{ROH}+\mathrm{HCl}\stackrel{{\mathrm{ZnCl}}_2}{\to }\mathrm{R}-\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}$

Alkyl chloride are insoluble in water and their formation is indicated by the appearance of turbidity or cloudiness.

The rate of reaction of alcohol with HCl is in the order 3^o > 2^o > 1^o. Hence, tertiary alcohol gives turbidity or precipitate immediately.