Mechanism of 3-halogenoaniline photolysis in methanol

The photolysis of 3-fluoro-, 3-chloro- and 3-bromoaniline (3-FA, 3-ClA, 3-B rA) in methanol leads to the formation of 3-anisidine and aniline with quan tum yields depending on the nature of the halogen. Photosubstitution is the major reaction in the case of 3-FA, but photoreduction also occurs. The la tter is more important with 3-BrA than with 3-FA and 3-ClA. Oxygen does not affect the formation of 3-anisidine but reduces that of aniline. Several t ransients are observed by laser flash photolysis. With each derivative, a b road absorption band with a maximum around 410 nm appears at the end of the pulse. Based on the quenching of oxygen, these bands are assigned to the t riplet-triplet absorptions. The anilino radical cation PhNH2.+ (lambda(max) = 410 and 430 nm) is produced by excitation of the three halogenoanilines. Its formation is not influenced by oxygen. The characteristic absorption o f Br-2(.-) is observed by irradiation of 3-BrA in the presence of Br- (5 x 10(-3) M). The solvated electron and the long-lived halogenoanilino radical cations (XPhNH2.+) are detected. From the quantum yields of aniline, 3-ani sidine, PhNH2.+ and Br-2(.-) formation in the absence and presence of oxyge n, the following mechanism can be proposed: 3-anisidine is produced after h eterolytic cleavage of the C-X bond from the excited singlet state and addi tion of MeOH. Aniline is also yielded from the excited singlet state, after heterolytic and/or homolytic cleavage of the C-X bond. In the former case PhNH2.+ is formed as an intermediate. With 3-BrA, the homolytic cleavage of the C-Br bond is proved by the formation of Br-2(.-).