Kinetics of bromination

Bromination is catalyzed by Lewis acids, and a study of the kinetics of bromination of benzene and toluene in the presence of aluminum chloride has been reported. Toluene is found to be about 35 times more reactive than benzene under these conditions. The catalyzed reaction thus shows a good deal less substrate selectivity than the uncatalyzed reaction, as would be expected on the basis of the greater reactivity of the aluminum chloride-bromine complex. 

The most extensive study of the effect of conditions upon the kinetics of bromination was made by Robertson et al.23l 279, who measured the rates of bromination of alkylbenzenes, acetanilide, aceto-p-toluidide, mesitylene, anisole and p-tolyl methyl ether in acetic acid at 24 °C. They found that at relatively high concentrations of bromine (A//40-M/100) the reaction is second-order in bromine, i.e. the rate equation is... 

Berliner et a/.284-8 have examined kinetics of bromination in aqueous acetic acid in an attempt to find the acid concentration at which the change in kinetic order principally occurs, though it follows from the earlier work that this will depend upon the aromatic reactivity. In 50 % acid the bromination of naphthalene was second-order overall284, and at constant ionic strength the rate coefficient showed a dependence on [Br-] according to equation (140)... 

More recently, the kinetics of bromination of benzene in water have been examined296. The reaction is second-order overall and the slope of the plot kobs... 

The kinetics of bromination with the complex formed between bromine and dioxan have been examined using benzene (which is unattacked) as solvent311, and it is probably appropriate to regard this as a catalysed bromination in view of the effect of dioxan upon the polarity of the bromine-bromine bond. With anisole, phenetole, and isopropoxybenzene, third-order kinetics are obtained, viz. 

The electrophilic bromination of ethylenic compounds, a reaction familiar to all chemists, is part of the basic knowledge of organic chemistry and is therefore included in every chemical textbook. It is still nowadays presented as a simple two-step, trans-addition involving the famous bromonium ion as the key intermediate. T]nis mechanism was postulated as early as the 1930s by Bartlett and Tarbell (1936) from the kinetics of bromination of trans-stilbene in methanol and by Roberts and Kimball (1937) from stereochemical results on cis- and trans-2-butene bromination. According to their scheme (Scheme 1), bromo-derivatives useful as intermediates in organic synthesis... 

As an analyst, if we were to perform a series of simple experiments to study the kinetics of bromine consumption, we would start the experiment (at a time we call t = 0) and then monitor the amount of bromine remaining as a function of time after t = 0. Probably the simplest way of monitoring this process would be to remove aliquots of reaction solution after various lengths of time, and titrate each of these, e.g. with thiosulfate, to determine the amount of bromine remaining in each sample. In effect, we say here that chemical kinetics is the study of the proportion of the matter that is initially present as a function of the reaction time-scale r. 

More recent studies of the kinetics of bromination of imidazoles coordinated to cobalt(III) have implied that, in aqueous solution, proton-abstraction from a Wheland intermediate is rate-determining at acidic pH values. In more nearly neutral conditions bromine addition becomes rate-... 

Gustafsson K, Bjork M. Burreau S, et al. 1999. Bioaccumulation kinetic of brominated flame retardants (polybrominated diphenyl ethers) in blue mussels Mytilus edulis). Environ Toxicol Chem 18(6) 1218-1224. 

The orientation of addition of iodine chloride to butadiyne, however, corresponds to an electrophilic attack. A study of the kinetics of bromination of various mono- and dialkyl-substituted butadiynes has been interpreted in terms of an electrophilic mechanism . The observed rates increased with increasing inductive electron-releasing power of the substituents. However, the effects are small, and under the conditions used (two-fold excess of hydrocarbon) it seems likely that significant polybromination occurred . Marked catalysis by bromide ion was observed and interpreted in terms of electrophilic attack by bromine on a complex of acetylene with Br ... 

The kinetics of bromination reactions are often complex, with at least three terms making contributions under given conditions. 

A nephelometric method was developed for the analysis of levallorphan tartrate in the presence of levorphanol tartrate. The method takes advantage of the kinetics of bromination of the allyl function on levallorphan which in acidic aqueous medium is faster than the bromination of the levorphanol. The bromination product is insoluble and hence lends itself to a turbidimetric determination (21). The procedure involves adding 1 mg of levallorphan tartrate (along with up to 10 mg of levorphanol tartrate) to 5 ml of 2.5N HCl and 15 ml of water in a glass stoppered 25-ml graduated cylinder. Bring to 25°C and add... 

The kinetics of bromine addition to alkenes can be more complex than the simple model in Figure 9.2 would suggest. Early studies revealed considerable kinetic variation with solvent, temperature, and the presence of additives such as water. The kinetic investigations can also be complicated because of the involvement of HBr3 (from Br2 and HBr) or of termolecular processes involving an alkene, Br2, and Br . The Br can be produced if the bromo-nium ion is intercepted by solvent or solvent anion (e.g., acetate ion). The equilibrium constant for dissociation of Bra" to Br2 and Br was determined to be 5 X 10 M in water and 2 x 10 M in methanol. Thus, a general kinetic expression for the addition of bromine to alkenes can be represented as " ... 

From a study of the kinetics of bromination of cycloalkenes, it was concluded that bromination of cyclohexene and cyclopentene proceeds via a cyclic bromonium ion whereas bromination of cyclo-octene proceeds via an a-bromocarbenium ion. Chlorination of conjugated dienes with iodobenzene dichloride gave mixtures of 1,2- and 1,4-dichlorides via radical reactions, e.g. a mixture of cis- and trans-3,4- and -3,8-dichlorocyclo-octenes was obtained from cyclo-octa-1,3-diene. ... 

Other reactions studied include reduction of the triple bond of a-acetylenic esters and nitriles by tributyltin hydride in methanol electroreductive cyclization of acetylenic halides at a mercury cathode the trimerization and tetramerization of cyclo-octyne in the presence of various transition metals the kinetics of bromination of alka-l,3-diynes, of permanganate oxidation of acetylenedicarboxylic acid, and of iodination of propiolic acid the participation of the triple bond in reactions of various acetylenes of the general formula (225) and the trimerization of but-2-yne with tolyl-palladium chloride to give a [Pg.48]

Aliphatic carboxylic acids may be similarly a-brominated in high yields (78-95%) by heating the acid with bromine and chlorosulfonic acid catalyst (0.05 equivalents) in 1,2-dichloroethane at 84 °C. No oxygen was required in the reaction. The kinetics of bromination were second order overall, first order in both bromine and the carboxylic acid, and the rate of reaction (v) is expressed by Equation 50. The rate constant (k) was proportional to the initial concentration of the chlorosulfonic acid catalyst at an early stage of the reaction. The rate of a-... 

Zhang, J., Unwin, P R. Kinetics of bromine transfer across Langmuir monolayers of phosphatidyletha-nolamines at the water/air interface. Phys. Chem. Chem. Phys. 2003, 5, 3979. 

The kinetics of bromination of substituted phenols and anisoles with pyri-dinium bromide perbromide are reported. The order in halogenating agent depends mainly on the position of a substituent apparent fractional orders for some orffio-substituted substrates arise from concentration effects. Cyclo-... 

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