An allylic or benzylic radical is a radical that is stabilised by resonance because its unpaired electron is delocalised onto an adjacent group. In an allylic radical the electron spreads onto a neighbouring carbon–carbon double bond (C=C); in a benzylic radical it spreads onto an adjacent aromatic ring. They matter because this extra delocalisation makes them especially stable and easy to form, more so than even a tertiary radical, which directs reactions to those positions.

烯丙基或苄基自由基是因共振而稳定的自由基,其未成对电子离域到相邻基团上。在烯丙基自由基中,电子扩展到相邻的碳碳双键(C=C)上;在苄基自由基中,电子扩展到相邻的芳环上。它们之所以重要,是因为这种额外的离域使其格外稳定且易于生成,甚至比叔自由基更甚,从而把反应导向这些位置。