Decolourization of bromine water. Image 2 of 2. Bromine water (Br2) reacted with cyclohexene (C6H10, centre) in a test tube. The ultraviolet lamp (left) provided the energy needed for the reaction and the stop clock (right) timed the reaction. This is an addition reaction (electrophilic) in which bromine adds across the double bond in the cyclohexene, forming clear 1,2- dibromocyclohexane (C6H10Br2). For an image at the start of the reaction, see image A500/265.