Preparation From Alcohols

Thionyl chloride, SOCl2, reacts with an alcohol to replace the hydroxyl group with chlorine, forming the alkyl chloride together with sulphur dioxide and hydrogen chloride. The special advantage is that both by-products, SO2 and HCl, are gases that leave the reaction mixture, so the alkyl chloride is obtained in a pure form without tedious separation. This makes thionyl chloride a cleaner reagent than reacting the alcohol with hydrochloric acid or phosphorus halides, which leave dissolved or solid residues. The no-water-removal option is wrong because the genuine benefit is the gaseous escape of by-products, not the absence of water handling. The high-temperature option is incorrect since the reaction is mild and does not demand extreme heating. The oxidation option is wrong because thionyl chloride performs substitution of OH by Cl, not oxidation of the alcohol to an aldehyde. Mechanistically the alcohol first attacks sulphur to form a chlorosulphite ester, which then loses sulphur dioxide as chloride displaces it from carbon, explaining why the by-products are gaseous. This pyridine-free version is often run with a base to mop up hydrogen chloride, and it is favoured over phosphorus trichloride or red phosphorus with halogens precisely because no liquid or solid residues contaminate the product. This matches the NCERT preparation of haloalkanes from alcohols. Sanity check: gaseous SO2 and HCl simply bubble away, leaving the pure chloride.