Conversion to Halogenoalkanes

The -OH group of an alcohol can be replaced by a halogen atom to make a halogenoalkane. Different reagents are used for each halogen.Chloroalkanes: phosphorus(V) chloride
PCl₅ reacts with alcohols at room temperature:C₂H₅OH + PCl₅ → C₂H₅Cl + POCl₃ + HClEthanol gives chloroethane. The misty (steamy) fumes of hydrogen chloride given off are also used as a test for an -OH group. Tertiary alcohols can also be converted into chloroalkanes simply by shaking with concentrated hydrochloric acid at room temperature.Bromoalkanes: potassium bromide and 50% concentrated sulfuric acid
The acid reacts with KBr to make hydrogen bromide in situ (in the reaction mixture), which then reacts with the alcohol:KBr + H₂SO₄ → KHSO₄ + HBr
C₂H₅OH + HBr → C₂H₅Br + H₂OThe acid is diluted to about 50% because concentrated sulfuric acid would oxidise some HBr to bromine.Iodoalkanes: red phosphorus and iodine
The alcohol is heated under reflux with red phosphorus and iodine, which react to make phosphorus(III) iodide, PI₃, in situ:3C₂H₅OH + PI₃ → 3C₂H₅I + H₃PO₃In each case the halogenoalkane is separated and purified afterwards, usually by distillation.