<P> Lucas' reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid . This solution is used to classify alcohols of low molecular weight . The reaction is a substitution in which the chloride replaces a hydroxyl group . A positive test is indicated by a change from clear and colourless to turbid, signalling formation of a chloroalkane . Also, the best results for this test are observed in tertiary alcohols, as they form the respective alkyl halides fastest due to higher stability of the intermediate tertiary carbocation . The test was reported in 1930 and became a standard method in qualitative organic chemistry . The test has since become somewhat obsolete with the availability of various spectroscopic and chromatographic methods of analysis . It was named after Howard Lucas (1885--1963), an American chemist . </P> <P> Lucas test in alcohols is a test to differentiate between primary, secondary, and tertiary alcohols . It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via an S 1 reaction: </P> <Dl> <Dd> ROH + HCl → RCl + H O </Dd> </Dl>

Why does the lucas test fail when the alcohol is not soluble in the reagent