Lucas' reagent

"Lucas' reagent" is a solution of anhydrous zinc chloride in concentrated hydrochloric acid.

A positive test is indicated by a change from clear and colourless to turbid, signalling formation of a chloroalkane.

The test was reported in 1930 and became a standard method in qualitative organic chemistry.

It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via an SN1 reaction:[3]

Tertiary alcohols react immediately with Lucas reagent as evidenced by turbidity owing to the low solubility of the organic chloride in the aqueous mixture.

Primary alcohols do not react appreciably with Lucas reagent at room temperature.

Lucas test: negative (left) with ethanol and positive with t -butanol