Methods of Preparation

Haloalkanes are prepared industrially and in the laboratory by five broad routes, each starting from a different, readily available class of organic material: from alcohols (the most versatile route, via three distinct reagent families), from alkenes (by simple addition of a halogen acid), from alkanes (by free-radical halogenation, needing light or heat to start), by halogen-exchange on an existing haloalkane (swapping one halogen for another without touching the carbon skeleton), and from silver salts of fatty acids (the Hunsdiecker reaction, which shortens the carbon chain by exactly one carbon). Recognising which of these f …

Alcohols convert to haloalkanes with three reagent classes. (a) Hydrogen halide: ethanol + HCl over anhydrous ZnCl2 gives chloroethane + water; the mixture of concentrated HCl and anhydrous ZnCl2 is called Lucas reagent. tert-Butyl alcohol + HI (generated in situ from KI/H3PO4) gives tert-butyl iodide + water. Reactivity of the halo acid follows HI > HBr > HCl, and reactivity of the alcohol follows tertiary > secondary > primary. (b) Phosphorus halides: alcohols react with PX5 or PX3 (PBr3 and PI3 are usually generated in situ from red phosphorus + Br2/I2); ethanol + PCl5 gives chloroethane + POCl3 + HCl, while 3 ethanol + PCl3 gives 3 chloroethane + H3PO3. (c) Thionyl chloride: ethanol + SOCl2 in the presence of pyridine gives chloroethane + SO2 (up) + HCl (up); this reaction i …

From alkenes: alkenes react with the halogen acids HCl, HBr or HI to give a haloalkane, the addition following Markovnikov's rule (the halogen ends up on the more substituted carbon). From alkanes: alkanes react with Cl2 or Br2 in the presence of UV light by a free-radical substitution mechanism, giving a mixture of mono-, di- and poly-substituted haloalkanes. Chlorination of methane, for instance, successively gives chloromethane, dichloromethane, trichloromethane and tetrachloromethane (each step losing HCl); because these four prod …

Two named reactions convert one haloalkane into another by swapping the halogen. (a) Finkelstein reaction: a chloro- or bromoalkane heated with a concentrated solution of sodium iodide in dry acetone gives the iodoalkane, e.g. bromoethane + NaI (acetone, heat) gives iodoethane + NaBr; this is an SN2 reaction, driven forward because NaBr/NaCl are insoluble in dry acetone and precipitate out, removing them from the equilibrium. (b) Swarts reaction: a chloro- or bromoalkane heated with a metallic fluoride such as …

Silver salts of fatty acids, refluxed with bromine in CCl4, give the bromoalkane with loss of one carbon as CO2: silver propionate + Br2 (CCl4, reflux) gives bromoethane + CO2 + AgBr. This route is valuable precisely because it shortens the carbon chain by exa …