15.1.1—Preparation of halogenoalkanes

Halogenoalkanes can be made by radical substitution of alkanes with Cl₂/Br₂ under UV, electrophilic addition of X₂ or HX to an alkene, or substitution of an alcohol using a hydrogen halide or halogenating reagent.

Choose the route from the starting material: alkane needs UV radicals, alkene consumes π bonds, and alcohol replaces –OH. Conditions are part of the reaction, not optional decoration.

Ethene + HBr → bromoethane; ethanol + PCl₅ → chloroethane + POCl₃ + HCl; ethane + Br₂ requires UV light and gives bromoethane plus HBr.

Do not swap aqueous and ethanolic conditions or call every halogenoalkane preparation an addition reaction.