Lucas' reagent is a solution of zinc chlorie in concentrated hydrochloric acid, used to classify alcohols of low molecular weight. The reaction is a substitution in which the chlorine replaces the hydroxyl group. Even though this reaction is normally very unfavorable, the zinc ion complexes with the hydroxyl group (by accepting a lone electron pair from O of -OH), making it a better leaving group. The remaining carbocation then combines with the chloride ion to form a chloroalkane.