ROH + HX -> RX + H2O. Reactivity of HX: HI > HBr > HCl (halide nucleophilicity + bond strength). Reactivity of ROH: 3° > 2° > 1° > CH3OH. Mechanism: 3° = SN1 (carbocation); 1° = SN2. Lucas test $\frac{ZnCl2}{conc. HCl}$: Tests alcohol class. ZnCl2 acts as Lewis acid to protonate OH and assist leaving. 3° -> immediate cloudiness (alkyl chloride insoluble in reagent). 2° -> turbidity in 5-10 minutes. 1° -> no turbidity at RT (may need heating). Darzen's reaction: ROH + SOCl2 -> RCl + SO2 + HCl. Preferred method because by-products are gases (easy purification). With PCl5: ROH + PCl5 -> RCl + POCl3 + HCl. With PCl3: 3ROH + PCl3 -> 3RCl + H3PO3.