20.19 : 自由基取代：烯丙基氯化

Generally, alkenes undergo an electrophilic addition reaction when treated with halogens at low temperatures.

However, allylic substitution reactions occur at low halogen concentrations, and under radical conditions such as high temperatures or in the presence of radical initiators.

For example, propene undergoes allylic chlorination when reacted with chlorine at 400 ^oC in the gas phase.

Allylic hydrogen gets substituted because the sp³ allylic C–H bond is weaker than the sp² vinylic C–H bond.

Additionally, the formed allylic radical intermediate is resonance stabilized.

Allylic chlorination follows a chain mechanism, including initiation, propagation, and termination steps.

In the initiation step, the chlorine molecule dissociates into two highly reactive chlorine atoms.

In the first propagation step, the chlorine atom abstracts allylic hydrogen, leading to the formation of an allylic radical intermediate.

In the second propagation step, the allylic radical reacts with a chlorine molecule to form an allyl chloride and a chlorine atom, which further propagates the reaction.

Finally, the reaction gets terminated by coupling any two radicals, thereby depleting the reactive intermediates.