Competitive bromination

Fig. 1 Concentration dependence of rate-constant ratios measured by competitive brominations (data from Grosjean et al., 1973).

In the first example, we studied the competitive bromination of two disubsti-tuted trisacetylacetonates of chromium. The dichloro compound (X = C1) reacts with iV-bromosuccinimide much more rapidly than the dinitro compound (X =N02). The reaction is thought to involve electrophilic substitution. This substantial influence of substituents on the reaction rate is undoubtedly an electronic rather than a steric effect. 

Recent redeterminations of free radical heats of formation by other methods (competitive bromination and iodination kinetic studies) have shown that this technique has, in the past, yielded erroneously low Arrhenius parameters. Several reasons for this have been suggested chain propagation by benzyl radicals. 

Competitive brominated polystyrene Source Speciality Chemicals Magazine, reproduced with permission. ... 

Figure 13-13 Chemistry of a classical consecutive-competitive bromination reaction subject to mixing effects.

Although alkenes typically react with chlorine and bromine by addition at room tern perature and below (Section 6 14) substitution becomes competitive at higher tempera tures especially when the concentration of the halogen is low When substitution does occur It IS highly selective for the allylic position This forms the basis of an industrial preparation of allyl chloride... 

These reactions occur on the benzylic hydrogens because these hydrogens are much more reactive. Competition experiments show, for example, that at 40°C a benzylic hydrogen of toluene is 3.3 times as reactive toward bromine atoms as the tertiary hydrogen of an alkane and nearly 100 million times as reactive as a hydrogen of methane. 

Other mechanisms must also operate, however, to account tor the fact that 5-10% of the product is formed with retained configuration at the chiral center. Isotopic labeling studies have also demonstrated that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with that of the bromination reaction ... 

No product derived from the transannular hydrogen abstraction is observed in the addition of bromotrichloromethane because bromine-atom abstraction is sufficiently rapid to prevent effective competition by the intramolecular hydrogen abstraction. 

This activation of the ortho position is most strikingly illustrated in the reactivity of 2,5-dimethylthiophene, which competitive experiments have shown to undergo the SnCb-catalyzed Friedel-Crafts reaction more rapidly than thiophene and even 2-methylthiophene. The influence of the reagent on the isomer distribution is evident from the fact that 2-methoxythiophene is formylated and bromi-nated (with A -bromosuccinimide) only in the 5-position. Similarly, although 3-bromo-2-methylthiophene has been detected in the bromi-nation of 2-methylthiophene with bromine, only the 5-isomer (besides some side-chain bromination) is obtained in the bromination of alkylthiophenes with A -bromosuccinimide. ° However, the mechanism of the latter type of bromination is not established. No lines attributable to 2-methyl-3-thiocyanothiophene or 2-methyl-3-chIoro-thiophene could be detected in the NMR spectra of the substitution products (5-isomers) obtained upon thiocyanation with thiocyanogen or chlorination with sulfuryl chloride. 2-Methyl- and 2-ethyl-thiophene give, somewhat unexpectedly, upon alkylation with t-butyl chloride in the presence of Feds, only 5-t-butyl monosubstituted and... 

The other bromine atom comes from another bromine-containing molecule or ion. This is clearly not a problem in reactions with benzylic species since the benzene ring is not prone to such addition reactions. If the concentration is sufficiently low, there is a low probability that the proper species will be in the vicinity once the intermediate forms. The intermediate in either case reverts to the initial species and the allylic substitution competes successfully. If this is true, it should be possible to brominate an alkene in the allylic position without competition from addition, even in the absence of NBS or a similar compound, if a very low concentration of bromine is used and if the HBr is removed as it is formed so that it is not available to complete the addition step. This has indeed been demonstrated. ... 

Iodine can, however, be displaced cleanly with bromine<150) and with chlorine if iodine monochloride or sulfuryl chloride and benzoyl peroxide are used as chlorinating agents.reactivity ratios for the displacement of bromine by chlorine from substituted bromo-benzenes have been found to be in the order p-phenyl > o-methoxy > p-chloro > un-... 

Hypohalous acids, e.g. HO8-—Br8+ (bromine water), were thought to add on in very much the same way, but there is some evidence that the actual electrophile may well be the halogen itself, e.g. Br2, and that both 1,2-dibromide (35a) and 1,2-bromhydrin (35 b) are then obtained by competition of Br and H20 for the... 

Fig. 5 Reactivity-structure relationship for the bromination of monosubstituted stilbenes (data from Ruasse and Dubois, 1972). The curvature shows the X-dependence of the competition between carbocation and bromonium ion pathways.

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