Alkane decolourisation of bromine water. Image 1 of 3. Start of the reaction of bromine water (Br2, brown) with hexane (C6H14, an alkane) in a test tube (centre). The ultraviolet lamp (left) provides the energy needed for the reaction and the stopwatch (right) times the reaction. This is a substitution reaction, where one bromine atom replaces a hydrogen atom in the hexane, forming bromohexane (C6H13Br) and hydrogen bromide (HBr). The ultraviolet light powers this reaction by splitting the bromine molecules to form bromine free radicals that react with the hexane. For a sequence of the decolourisation, see images A500/722-724.