<P> The most popular conversion is effected by reacting the alcohol with thionyl chloride (SOCl 2) in the "Darzens halogenation", which is one of the most convenient laboratory methods because the byproducts are gaseous . Both phosphorus pentachloride (PCl 5) and phosphorus trichloride (PCl 3) also convert the hydroxyl group to the chloride . </P> <P> Alcohols may likewise be converted to bromoalkanes using hydrobromic acid or phosphorus tribromide (PBr). A catalytic amount of PBr 3 may be used for the transformation using phosphorus and bromine; PBr 3 is formed in situ . </P> <P> Iodoalkanes may similarly be prepared using red phosphorus and iodine (equivalent to phosphorus triiodide). The Appel reaction is also useful for preparing alkyl halides . The reagent is tetrahalomethane and triphenylphosphine; the co-products are haloform and triphenylphosphine oxide . </P> <P> Two methods for the synthesis of haloalkanes from carboxylic acids are the Hunsdiecker reaction and the Kochi reaction . </P>

Due to presence of c – x polar bond in alkyl halide alkyl halides are