Allylic Substitution

$$\text{R--CH}_2\text{--CH=CH}_2 \xrightarrow{\text{NBS / } h\nu \text{ or } \Delta} \text{R--CH(Br)--CH=CH}_2 + \text{Succinimide}$$

$$\text{R--CH}_2\text{--CH=CH}_2 \xrightarrow{\text{Cl}_2 / 500^\circ\text{C}} \text{R--CH(Cl)--CH=CH}_2 + \text{HCl}$$

Concept: Substitution of a hydrogen atom at the allylic position (the $$sp^3$$ carbon directly attached to a double bond, $$\text{C--C=C}$$) by a halogen atom without undergoing addition across the double bond.

• Proceeds via a free radical mechanism involving a resonance-stabilized allylic free radical intermediate. High temperature or low concentration of halogen (maintained by NBS) favors free radical substitution at the allylic position over electrophilic addition across the double bond. Because allylic radicals exhibit resonance, allylic rearrangement can yield a mixture of isomeric products when unsymmetrical alkenes are used.