20.22: Radical Substitution: Allylic Chlorination
Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic chlorination, forming 3‐chloropropene.
The radical substitution reaction of allylic chlorination follows a chain mechanism similar to alkane halogenations and involves initiation, propagation, and termination steps. The initiation step involves the dissociation of the chlorine molecule into two chlorine atoms. The propagation step involves two chain-propagation steps. In the first chain-propagation step, the chlorine atom abstracts the allylic hydrogen atom, forming a resonance stabilized allylic‐radical intermediate. In the second chain-propagation step, the allyl radical intermediate reacts with a chlorine molecule, forming an allyl chloride and a chlorine atom. The chlorine atom formed in the second chain-propagation step further abstracts allylic hydrogen and propagates the reaction. In the termination step, the radicals react with each other to form non-radical products and stop the reaction.