Radical Substitution: Allylic Chlorination

Allylic chlorination occurs under radical conditions (high temperature or radical initiators) at low halogen concentrations, favoring substitution over addition. The allylic hydrogen is more reactive 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, making substitution thermodynamically favorable.

In the initiation step, the chlorine molecule dissociates into two highly reactive chlorine atoms. This step generates the radical species needed to begin the chain reaction. High temperatures or radical initiators provide the energy required for this homolytic cleavage of the Cl–Cl bond.

The first propagation step occurs when a chlorine atom abstracts an allylic hydrogen, forming a resonance-stabilized allylic radical intermediate. In the second propagation step, this allylic radical reacts with a chlorine molecule to form allyl chloride and regenerate a chlorine atom, which continues the chain reaction.

Allylic substitution is favored at high temperatures, in the presence of radical initiators, and at low halogen concentrations. These conditions promote radical formation and propagation. In contrast, electrophilic addition occurs at low temperatures with high halogen concentrations, following a different reaction pathway.

The allylic radical intermediate is resonance stabilized because the unpaired electron can delocalize across the allylic carbon and the adjacent pi system. This resonance stabilization lowers the energy of the intermediate, making its formation favorable and contributing to the overall reactivity of allylic hydrogens in radical substitution reactions.

The termination step occurs when any two radicals couple together to form non-radical products, depleting the reactive intermediates and stopping the chain reaction. Common termination reactions include chlorine atom coupling or recombination of allylic radicals with chlorine atoms, effectively ending the propagation cycle.

When propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic chlorination to form 3-chloropropene, also called allyl chloride. The chlorine atom substitutes the allylic hydrogen on propene, producing this characteristic product of radical substitution reactions under high temperature conditions.