Preparing haloalkanes

From alcohols: the –OH group is replaced by halogen using concentrated halogen acids, phosphorus halides (PCl₃, PCl₅, PBr₃, PI₃) or thionyl chloride (SOCl₂). Alcohols are easy to obtain, so this is the preferred route.

SOCl₂ is the reagent of choice for chlorides: the by-products SO₂ and HCl are gases that leave the mixture, so the alkyl chloride is obtained pure.

With HCl, primary and secondary alcohols need ZnCl₂ as a catalyst, while a tertiary alcohol reacts on simple shaking with concentrated HCl at room temperature. With a given halogen acid the order of reactivity is 3° > 2° > 1°.

Bromides are made with constant boiling HBr (48%). Iodides form in good yield when the alcohol is heated with NaI or KI in 95% orthophosphoric acid; H₂SO₄ is avoided because it turns KI into HI and then oxidises HI to I₂. PBr₃ and PI₃ are made in situ from red phosphorus with Br₂ or I₂.

None of these methods works for aryl halides: the C–O bond of a phenol has partial double bond character and is too strong to break.

Free radical chlorination or bromination of an alkane gives a hard-to-separate mixture of isomeric mono- and polyhalo products, so the yield of any single one is low. (CH₃)₂CHCH₂CH₃ has four kinds of hydrogen and gives four monochloro isomers.

From alkenes: HCl, HBr or HI add to give an alkyl halide; propene can give two products, and Markovnikov's rule picks the major one. Br₂ in CCl₄ adds to give a colourless vic-dibromide, and the loss of the reddish brown bromine colour is a test for a C=C bond.

Finkelstein reaction: an alkyl chloride or bromide with NaI in dry acetone gives the alkyl iodide. NaCl or NaBr is insoluble in dry acetone and precipitates, which pulls the equilibrium forward (Le Chatelier's principle).

Swarts reaction: heating an alkyl chloride or bromide with a metallic fluoride (AgF, Hg₂F₂, CoF₂ or SbF₃) gives the alkyl fluoride.