Preparation and physical properties

The essentials

Haloalkanes are usually made from alcohols or from hydrocarbons. Haloarenes are made by electrophilic substitution of arenes or from diazonium salts.

Key points

• From alcohols: R–OH + HX (with ZnCl₂ for HCl); R–OH + PCl₃ or PCl₅; R–OH + SOCl₂ → R–Cl + SO₂ + HCl. Thionyl chloride is preferred because the by-products are gases and escape, leaving a pure product.
• From alkenes: addition of HX follows Markovnikov's rule; with HBr in the presence of peroxide, anti-Markovnikov addition occurs.
• Free-radical halogenation of alkanes gives mixtures and is not a good laboratory method.
• Haloarenes: benzene + Cl₂ with FeCl₃ (Lewis acid) gives chlorobenzene; diazonium salt + CuCl/HCl or CuBr/HBr gives the aryl halide (Sandmeyer reaction).
• Finkelstein reaction: R–Cl or R–Br + NaI in dry acetone → R–I; NaCl/NaBr precipitate and drive the reaction forward.
• Swarts reaction: R–Br or R–Cl heated with AgF, Hg₂F₂, CoF₂ or SbF₃ gives R–F.
• Boiling point: higher than parent alkanes; RI > RBr > RCl > RF. For isomers, more branching gives a lower boiling point.
• Density: increases with the number and atomic mass of halogen atoms; bromo and iodo compounds are denser than water.
• Solubility: only slightly soluble in water, because energy released by new attractions is less than that needed to break hydrogen bonds in water; soluble in organic solvents.
• p-Dichlorobenzene has a higher melting point than its o- and m-isomers because its symmetry allows better packing.

Common mistakes

• Assuming the peroxide effect works with HCl or HI; it applies only to HBr.