Which of the following radicals are stabilised by resonance (delocalisation of the unpaired electron over more than one atom)?

A $CH X_{2} = CH - CH X_{2} X^{∙}$
B $CH X_{2} = CH X^{∙} (unpaired electron in the sp X_{2} orbital of C X_{1})$
C $C X_{6} H X_{5} - CH X_{2} X^{∙}$
D $C X_{6} H X_{5} X^{∙} (unpaired electron in the sp X_{2} orbital of the ring carbon)$

Answer: A, C

The allyl and benzyl radicals have the unpaired electron in a

p

orbital that overlaps with an adjacent

π

system, so it is delocalised. In the vinyl and phenyl radicals the unpaired electron is in an

s p^{2}

orbital lying in the plane of the molecule, orthogonal to the

π

system, so there is no delocalisation. Answer: 1 and 3.