When 3 ethyl benzonitrile undergoes photochemical reaction (UV) with bromine (1 equivalent), we get a monobrominated product 1 bromo −1 ( 3 - cyanophenyl) ethane (A) as the major product.
The reaction follows benzylic free radical substitution mechanism which has a 2∘ benzylic free radical intermediate (stable due to resonance with the benzene ring). (i) Initiation step In this step radicals are generated via homolytic fission of covalent bond.
(ii) Propagation step 3-methyl benzonitrile reacts with bromine radical to give 1 -bromo-1-(3-cyanophenyl) ethane along with H*.
(iii) Termination stepH∙ and Br∙ reacts to form HBr. H∙+Br*⟶HBr