Bromine Atom Complexes with Bromoalkanes. Their Formation in the Pulse Radiolysis of Di-, Tri-, and Tetrabromomethane and Their Reactivity with Organic Reductants

Article abstract of DOI:10.1021/j100369a046

Bromine atoms were produced in the pulse radiolysis of neat dibromomethane (DBM) and bromoform and of cyclohexane solutions containing DBM, bromoform, carbon tetrabromide, or ethyl bromide.The Br atoms form complexes with dimethyl sulfoxide (λ_max 425 nm), with aromatic compounds, and with oxygen-containing compounds.In the absence of other complexing agents, since Br atoms do not abstract H from the solvents rapidly, they form complexes with their parent bromo compounds.The absorption maxima of these complexes are at 365 nm for C2H5Br*Br, 390 nm for CH2Br2*Br, 425 nm for CHBr3*Br, and 480 nm for CBr4*Br.The stability of RBr*Br appears to increase with the number of Br atoms in the molecule.These complexes act as oxidants towards p-methoxyphenol, 1,3,5-trimethoxybenzene, triphenylamine, and N,N,N',N'-tetramethyl-p-phenylenediamine.The cate constants for the oxidations were about 1E10 M^-1 s^-1 with CH2Br2*Br but only of the order of 1E8 - 1E9 M^-1 s^-1 with CBr4*Br.The initial products of the oxidation are the ion pairs between the radical cation of the organic substrate and the Br- ion.In the case of p-methoxyphenol the initial ion pair releases HBr under neutral or basic conditions to form the neutral p-methoxyphenoxyl radical.The Br atom complexes are also capable of abstracting H from weak C-H bonds.The benzylic and allylic hydrogens in hexamethylbenzene and cyclohexene are abstracted with rate constants near 1E9 by CH2Br2*Br and near 1E7 M^-1 s^-1 by CBr4*Br.The behavior of Br atoms is compared with that of Cl and I atoms.