REACTIVITY OF BROMINE ATOM COMPLEXES WITH ORGANIC-COMPOUNDS

Complexes of bromine atoms with bromo compounds (C2H5Br·Br, CH2Br2·Br, CHBr3·Br, and CBr4·Br) were produced by pulse radiolysis of the corresponding bromoalkane dissolved in cyclohexane. With benzene as solvent, the complex C6H6·Br was produced. These complexes oxidize phenols to phenoxyl radicals and abstract reactive hydrogens from various compounds. Absolute rate constants for these reactions were determined by following the decay of the Br atom complexes (at 370, 390, 425, 480 and 540 nm, respectively) as a function of substrate concentration. The rate constants varied from 105 to 1010 M-1 s-1 and were strongly dependent on the nature of the Br atom complex and of the organic substrate. Hammett's plots of the rate constants (k) for substituted phenols vs the substituent constants (σp), according to the equation log k = ρσp, yielded ρ values of -4.2 for the reactions of C6H6·Br and -3.1, -1.9, and -1.3 for C2H5Br·Br, CH2Br2·Br, and CHBr3·Br, respectively. On the other hand, substituents on the benzene ring in XC6H5·Br exerted only minor effects on the reactivity of this complex with phenol and with cumene. The rate constants for reaction of C6H6·Cl, C6H6·Br, and C6H6·I with phenol were found to be 9 × 108, 3.5 × 107, and ∼105 M-1 s-1, respectively. © 1990 American Chemical Society.