Lucas reagent is a mixture of concentrated hydrochloric acid $(\mathrm{HCl})$ and zinc chloride $\left(\mathrm{ZnCl}_2\right)$. It is used for the differentiation of alcohols based on their reactivity. The test is particularly used to distinguish between primary, secondary, and tertiary alcohols.
- Tertiary alcohols react almost immediately with Lucas reagent to form alkyl chlorides (substitution of the hydroxyl group with a chlorine atom).
- Secondary alcohols react after some time, typically within 5-10 minutes.
- Primary alcohols react very slowly, if at all, and may require heating.