Free radical bromination

BFRs work by releasing bromine free radicals when they are heated. These free radicals scavenge other free radicals that are part of the flame propagation process and thus reduce the rate of flame expansion and the extent of fire damage. Organobromine compounds are particularly well suited for this function because of the relatively low energy of the C - Br bond. When heated, these bonds break, and the relatively stable bromine free-radical is generated. The BFR s carbon skeleton is more or less irrelevant it is just a convenient way to carry the bromine atoms. Thus, most BFRs have simple carbon skeletons to which several bromine atoms are bonded. 

Know the meaning of substitution reaction, halogenation, chlorination, bromination, free-radical chain reaction, chain initiation, propagation, termination, combustion. 

Oxidation reactions wherein the bromine free radicals are used to oxidize leuco dyes to their colored forms. 

The absorption of light breaks the covalent bond holding together the bromine atoms together in the bromine molecule and produces free bromine atoms (i.e. bromine free radicals, Br ). 

The methyl radical then attacks a bromine molecule to produce a molecule of methyl bromide and to regenerate a bromine free radical, which proceeds to react as in step (2). 

There are two active centers in this sequence, the hydrogen and bromine free radicals, H and Br respectively. The overall, stoichiometrically-simple reaction is between H2 and Br2 to give two molecules of HBr. 

At elevated temperatures (250-400°C) bromine reacts with thiazole in the vapor phase on pumice to afford 2-bromothiazole when equimolecu-lar quantities of reactants are mixed, and a low yield of a dibromothiazole (the 2,5-isomer) when 2 moles of bromine are used (388-390). This preferential orientation to the 2-position has been interpreted as an indication of the free-radical nature of the reaction (343), a conclusion that is in agreement with the free-valence distribution calculated in the early application of the HMO method to thiazole (Scheme 67) (6,117). 

Bromine reacts with alkanes by a free radical chain mechanism analogous to that of chlorine There is an important difference between chlorination and brommation how ever Brommation is highly selective for substitution of tertiary hydrogens The spread m reactivity among pnmary secondary and tertiary hydrogens is greater than 10 ... 

Give the structure of the principal organic product formed by free radical bromination of each of the following... 

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

N Bromosuccimmide provides a low concentration of molecular bromine which reacts with alkenes by a mechanism analogous to that of other free radical halogenations... 

Halogenation (Section 11 12) Free radical halo genation of alkylbenzenes is highly selective for substitution at the benzylic position In the exam pie shown elemental bromine was used Alterna Lively N bromosuccinimide is a convenient re agent for benzylic bromination... 

Bromination. 1-Bromoalkanes are produced commercially by the anti-Markovnikow free-radical addition of HBr to a-olefins. These are further reacted with dimethyl amine to produce alkyldimethyl amines, which ultimately are converted to amine products for household cleaning and personal care. 

Technical-Grade Terephthalic Acid. All technical-grade terephthahc acid is produced by catalytic, hquid-phase air oxidation of xylene. Several processes have been developed, but they all use acetic acid as a solvent and a multivalent heavy metal or metals as catalysts. Cobalt is always used. In the most popular process, cobalt and manganese are the multivalent heavy-metal catalysts and bromine is the renewable source for free radicals (51,52). 

Flame Retardants. Flame retardants are added to nylon to eliminate burning drips and to obtain short self-extinguishing times. Halogenated organics, together with catalysts such as antimony trioxide, are commonly used to give free-radical suppression in the vapor phase, thus inhibiting the combustion process. Some common additives are decabromodiphenyl oxide, brominated polystyrene, and chlorinated... 

Pyrolysis. Pyrolysis of 1,2-dichloroethane in the temperature range of 340—515°C gives vinyl chloride, hydrogen chloride, and traces of acetylene (1,18) and 2-chlorobutadiene. Reaction rate is accelerated by chlorine (19), bromine, bromotrichloromethane, carbon tetrachloride (20), and other free-radical generators. Catalytic dehydrochlorination of 1,2-dichloroethane on activated alumina (3), metal carbonate, and sulfate salts (5) has been reported, and lasers have been used to initiate the cracking reaction, although not at a low enough temperature to show economic benefits. 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

In the pyrrole series, ester groups a to nitrogen are more readily hydrolyzed by alkali, but those in a /3 position more readily by acid. A methoxycarbonyl group in the 2-positlon is meta directing thus bromination yields mainly 4-bromo-2-methoxycarbonylpyrrole. Free radical chlorination with f-butylhypochlorite gives the 5-chloro derivative. 

Oxidation of azole anions can give neutral azole radicals which could, in principle, be tt (139) or a- (140) in nature. ESR spectra indicate structure (141 hyperfine splittings in G) for imidazolyl radicals, but both tt- and cr-character have been observed for pyrazolyl radicals. Tetrazolyl radicals (142 4 143) are also well known (79AHC(25)205). Oxidation of 2,4,5-triarylimidazole anions with bromine gives l,l -diimidazolyls (144) which are in equilibrium with the dissociated free radical (145) (70AHQ 12)103). 

UV spectra, 5, 517 bromination, 5, 540 dipole moment, 5, 523 free radical alkylation, 5, 543 free radical methylation, 5, 544 IR spectra, 5, 518, 519 irradiation, 5, 543 mass spectra, 5, 519 metabolism, 1, 234 monoprotonated tautomerism, 5, 509 oxidation, 5, 539 1-oxides... 

Free-radical reactions written in the simplest way imply no separation of charge. The case of toluene bromination can be used to illustrate this point ... 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Free-radical bromination is an important method of selective functionalization of hydrocarbons. The process is a chain reaction involving the following steps ... 

Besides hydrogen, other atoms and groups are susceptible to abstraction by free radicals. The most important from a synthetic point of view are bromine, iodine, sulfur. 

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