Free radical substitution. Test tubes with a mixture of hexane and bromine before (left) and after (right) being exposed to sunlight. Free radical substitution reactions involve one atom in a molecule being replaced by another, in this case a hydrogen from the hexane with a bromine. The sunlight causes bromine (brown) molecules to split (homolytic fission), forming bromine free radicals. These react with the hexane forming bromohexane (colourless).