Bromine atoms, from decomposition

The allylic bromination of an olefin with NBS proceeds by a free-radical chain mechanism. The chain reaction initiated by thermal decomposition of a free-radical initiator substance that is added to the reaction mixture in small amounts. The decomposing free-radical initiator generates reactive bromine radicals by reaction with the N-bromosuccinimide. A bromine radical abstracts an allylic hydrogen atom from the olefinic subsfrate to give hydrogen bromide and an allylic radical 3 ... 

Bromine-atom atomic resonance absorption spectrometry (ARAS) has been applied to measure the thermal decomposition rate constants of CF3Br in Kr over the temperature range 1222-1624 K. The results were found to be consistent with recently published theory. The formation of cyclopent[a]indene and acenaphthylene from alkyl esters of biphenyl-mono- and -di-carboxylic acids has been observed in flash vacuum pyrolyses at 1000-1100 °C. The kinetics and mechanisms of free-radical generation in the ternary system containing styrene epoxide, / -TsOH, and i-PrOH have been examined in both the presence and absence of O2. ... 

Rice, Fryling, and Weselowski (J. Amer. Chem. Soc., 1924, 46, 2405) make all reaction rates proportional to the concentration of what they call residual molecules, which have to be formed endothermically from one of the reactants. The proportion of these increases with temperature and accounts for the increase in reaction rate. Something of this kind may be true in special cases, for example, in the formation of HBr the residual molecule would be the bromine atom. But this resolution into atoms is only the limiting case of ordinary activation, and it is difficult indeed to see what the residual molecule could be, or what tautomeric change could occur in the simple decomposition of hydrogen iodide or nitrous oxide. 

Tetrachlorophthalic anhydride gave a relatively low yield of products derived from tetrachlorobenzyne. The pyrolysis tube was badly carbonized evidently extensive decomposition of the anhydride, the aryne, or the chlorinated products had occurred. Tetrabromophthalic anhydride gave a still lower yield of products. These are not listed in the table because none of them retained all four bromine atoms and could be definitely ascribed to reactions of tetrabromobenzyne, although tri-bromo- and dibromonaphthalene were present in appreciable amounts. Tetraphenylphthalic anhydride also gave low yields of products of the reaction of tetraphenylbenzyne with pyridine. This was not because of the stability of the anhydride and its reluctance to form the aryne, but rather because the aryne preferred to stabilize itself intramolecularly. The behavior of tetraphenylphthalic anhydride is discussed in another Section. 

Decomposition of the peroxide (step 1) to yield free radicals is a well-known reaction. The free radical thus formed abstracts hydrogen from hydrogen bromide (step 2) to form a bromine atom. The bromine atom adds to the double bond (step 3), and, in doing so, converts the alkene into a free radical. 

The roles of manganese in TPA manufacture are better understood than in the Witten process, and include decomposition of the CH2COOH radical (derived from the acetic acid solvent) and regeneration of the bromine atom promoter [13], In an effort to eliminate halogen compounds which are highly corrosive to oxidation equipment, use of acetaldehyde [14] and paraldehyde [15] has been developed. These aldehyde promoters are ultimately converted to acetic acid in high yield. For economic reasons, these aldehyde processes have been abandoned in favor of the bromine-promoted Amoco process. 

The reaction to form the adduct. In the case of the vinyl halides, Is exothermic by 35 kcal mole for all three halogen substituents (203, 205), [which can be compared to an exothermlclty of 32 kcal mole In the case of ethene (76, 205)]. Another decomposition pathway for these adducts Is the elimination of a chlorine or bromine atom (76), although the elimination of an F atom from the OH-vlnyl fluoride adduct Is endothermic by <19 kcal mole (203). 

Theoretical calculations of the stabilities of various hexahaloantimonates have shown that the rupture of an Sb-Cl bond requires more energy than that of an Sb-Br bond, and the relative positions of chlorine and bromine atoms within a mixed halogen counterion can affect the ease of bond breakage. Model compound studies of the reaction between p-methyl benzyl chloride, triethyl aluminium, and 2,4,4-trimethylpent-l-ene (dimeric isobutylene) indicated that the principal chain termination reactions involve hydridation and ethylation resulting from counterion decomposition. ... 

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