The photochlorination of hydrocarbon is unselective, although the reactivity of the C-H bonds is tertiary>secondary>primary. At 30 °C the relative reaction rates of primary, secondary and tertiary hydrogen atoms are in a relative ratio of approximately 1 to 3.25 to 4.43. The C-C bonds remain unaffected.
Upon radiation the reaction involves alkyl and chlorine radicals following a chain reaction according to the given scheme:
Chain termination occurs by recombination of chlorine atoms. Impurities such as oxygen (present in electrochemically obtained chlorine) also cause chain termination.
The selectivity of photochlorination (with regard to substitution of primary, secondary or tertiary hydrogens) can be controlled by the interaction of the chlorine radical with the solvent, such as benzene, tert-butylbenzene or carbon disulfide. Selectivity increases in aromatic solvents. By varying the solvent the ratio of primary to secondary hydrogens can be tailored to ratios between 1: 3 to 1: 31. At higher temperatures, the reaction rates of primary, secondary and tertiary hydrogen atoms equalize. Therefore, photochlorination is usually carried out at lower temperatures.