Catalytic Effects in the Bromination of Toluene

Abstract

Numerous previous investigations of the bromination of toluene have dealt with the effects of light, temperature, concentrations, catalysts, and solvents. Of various interpretative hypotheses hitherto advanced, none accounts adequately for all the phenomena reported.The observation in this laboratory that side-chain bromination of toluene is markedly facilitated by traces of organic peroxides and oxygen may be interpreted in a manner that permits the correlation of many hitherto unrelated, and some apparently discordant, data. In the light of the present and previous studies1 the authors believe that hydrogen bromide is a source of bromine atoms in the presence of oxygen and sufficiently active oxygen-carriers. The working hypothesis proposed is that sidechain bromination of toluene is a chain reaction initiated by bromine atoms, whereas nuclear bromination is a slow bimolecular reaction in which bromine atoms do not participate. It is the purpose of the present communication to show that this hypothesis is consistent with the earlier experimental data, and to report certain supplementary studies that also tend to support it.REVIEW OF THE LITERATURE It was early observed that nuclear and side-chain bromination of toluene are two separate and independent reactions and that their relative rates, and consequently the ratios of their respective products, vary widely with the experimental conditions.2 Ordinarily o-and p-bromotoluenes (hereafter referred to as "bromotoluenes") are the only products of nuclear bromination. m-Bromotoluene is formed only in very high bromine concentrations, and in the presence of a catalyst.3 Even then, the yield is only a fraction of one per cent. Likewise, polybromo compounds are not formed except in the presence of large amounts of catalyst, such as thirty mole per cent, of aluminum bromide,2 or beryllium bromide.4

Synthesis and thermomechanical behavior of (qua)ternary thiol-ene(/acrylate) copolymers

“…The thiol-ene polymerization reaction, first proposed by Kharasch and coworkers in 1938 (steps 1–4) [47], is shown in Figure 1, assuming the –ene cannot homopolymerize. Termination is generally thought to occur by radical recombination [48], as seen in steps 7–9.…”

Thiol−Ene Photopolymer Grafts on Functionalized Glass and Silicon Surfaces

“…Thiol-ene polymers form by a radical-based, step-growth mechanism in which multifunctional thiol and ene (usually vinyl or allyl ether) monomers are coupled. [16][17][18] In these studies, a stoichiometric mixture of a dithiol and a divinyl ether (ene) are polymerized using a photoinitiator to form linear polymer. The polymerization propagates by the addition of a thiyl radical to a vinyl functional group (step 1).…”

Kinetic Modeling of Thiol‐Ene Reactions with Both Step and Chain Growth Aspects

“…The step-growth nature of thiol-ene photopolymerizations was first suggested by Kharasch in 1938. [7] The polymerization reaction proceeds via propagation of a thiyl radical (-S) through the vinyl functional group.…”

“…[7] The polymerization reaction proceeds via propagation of a thiyl radical (-S) through the vinyl functional group. Rather Summary: A kinetic model is presented for thiol-ene crosslinking photopolymerizations including the allowance for chain growth reaction of the ene, i.e., homopolymerization.…”