Bromination of biphenyl

De la Mare and Maxwell199 measured the rate of bromination of biphenyl by hypobromous acid in 75 % aqueous acetic acid, in some cases catalysed by perchloric acid, at temperatures between —3.78 and +20.1 °C. They showed that whereas when mineral acid is present the brominating species is Br+ (or a solvate), in the absence of mineral acid it is BrOAc which is a highly reactive brominating species giving Ea = 7.9 (this value is only approximate since it also includes a contribution from bromination by HOBr), and the appropriate kinetic equation is then... 

At 0.9 °C the rate of bromination of biphenyl relative to benzene was approximately 1,270, compared to 26.9 in the presence of mineral acid, and this latter value is fairly close to that obtained with 50 % aqueous dioxan. The possibility that the positive brominating species might be protonated bromine acetate, AcOHBr+, was considered a likely one since the reaction rate is faster in aqueous acetic acid than in water, but this latter effect might be an environmental one since bromination by acidified hypobromous acid is slower in 50 % aqueous dioxan than in... 

An investigation of the relative rates of bromination of benzene, toluene, m-and p-xylene by bromine in acetic acid, catalysed by mercuric acetate, revealed relative rates almost identical with those obtained with molecular bromine322, though as in the bromination of biphenyl by bromine acetate (p. 129) it is quite inconsistent for a much more reactive electrophile to have the same selectivity. Relative rates were (molecular bromination values in parenthesis) benzene 1.0 toluene, 480 (610) p-xylene, 2.1 x 103 (2.2 x 103) m-xylene 2.0 x 10s (2.1 x 10s). 

PBBs have also been marketed as mixtures of congeners, produced in this case by the bromination of biphenyl. Their main commercial use has been as lire retardants, for which purpose they were introduced in the early 1970s. The most widely known commercial PBB mixture was Firemaster, first produced in 1970 in the United States, with production discontinued in 1974 following the recognition of pollution problems. 

Direct Substitution in Biphenyl. Direct nitration, chlorination, and bromination of biphenyl can. be used to prepare certain derivatives.87 Direct nitration is probably the most satisfactory method of preparing large quantities of 2- and 4-nitrobiphenyl. In certain substitution reactions it is sometimes difficult to separate the mixture of... 

This method is a modification of that of Buckles, Hausman, and Wheeler.2 4,4 -Dibromobiphenyl has also been prepared by the bromination of biphenyl in water,3 carbon disulfide,4 and glacial acetic acid 6 by the bromination of a mixture of biphenyl-sulfonic acids in dilute sulfuric acid 6 by the action of sodium carbonate on the perbromide obtained from the reaction of di-azotized benzidine with bromine water 7 and by passing />-di-bromobenzene vapor through a red-hot tube.8... 

Using resonance structures of the intermediates, explain why bromination of biphenyl occurs at ortho and para positions rather than at meta. 

The same type of zeolite was applied in selective bromination of biphenyl. 

PBBs are normally prepared by direct bromination of biphenyl, using an aluminium chloride catalyst other manufacturing routes, using Friedal Craft processes, are possible [97]. The principal grade produced was basically a hexabromo- (Fire-... 

In these latter reactions. Rule 1 does not allow us to speak of secondary isotope effects due to isotopic substitution at the reaction site unless the possibility of a primary isotope effect on has been conclusively eliminated. This was done by Berliner and Schueller (171) in their study of the bromination of biphenyl with Br2 in 50% acetic acid. They observe that deuteration in the two para positions reduces the rate of bromination at these positions by a not very temperature-de-pendent 15%, and show it to be a genuinely secondary isotope effect on by means of a careful kinetic analysis. 

Obtain the energy of each cation that might be generated by electrophilic addition of Br to biphenyl (biphenyl+Br+). Which one is most stable Are there others of comparable stability Examine the structure of the most stable cation(s), and draw all of the resonance contributors needed to describe this ion(s). Predict the product(s) of biphenyl bromination. Will the reaction be highly selective, moderately selective or unselective ... 

Bromination of substituted biphenyls is more complicated since bromination of each ring may lead to different products. Repeat the analysis described above for each of the following reactions. 

H. Thoma, G. Hauschulz, E. Knorr and O. Hutzinger, "Polybrominated Dibenzofurans (PBDF) and Dibenzodioxins (PBDD) from the Pyrolysis of Neat Brominated Diphenylethers, Biphenyls and Plastic Mixtures of these Compounds,"Chemosphere, JJi, 277(1987). 

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. ... 

There are principally two different approaches of correlating experimental rate data of electrophilic substitution with reactivity indices (1) Correlating the index with the rate data of a given reaction, e.g. bromination. For example, a satisfying correlation of Dewar reactivity numbers with the log of rate constants of the bromination of benzene, naphthalene (1- and 2-position), biphenyl (4-position), phenanthrene (9-position), and anthracene (9-position) has been observed [55]. In correlations of this type the reactivity index corresponds to the reactivity constant in the Hammett equation while the slope of the linear correlation corresponds to the reaction constant (see also Sect. 3) (2) correlating the index with experimental a values. 

Resonance structures explain that bromination occurs in the ortho and para positions of the rings. The positively charged intermediate formed from ortho or para attack can be stabilized by resonance contributions from the second ring of biphenyl, but this stabilization is not possible for meta attack. 

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