Bromination of o-xylene

Subsequently, rate coefficients were determined for the zinc chloride-catalysed bromination of benzene, toluene, i-propyl-benzene, r-butylbenzene, xylenes, p-di-f-butylbenzene, mesitylene, 1,2,4-trimethyl-, sym-triethyl-, sym-tri-f-butyl-, 1,2,3,5-and 1,2,4,5-tetramethyl- and pentamethylbenzenes, all at 25.4 °C and in acetic acid, and it was shown that the reaction was inhibited by HBr.ZnCl2 which accumulates during the bromination and was considered to cause the first step of the reaction (formation of ArHBr2) to reverse320. The second-order coefficients for bromination of o-xylene at 25.0 °C were shown to be inversely dependent upon the hydrogen bromide concentration and the reversal of equilibrium (155)... 

The syntheses of 74 and 75 is outlined in Scheme 20. Bromination of o-xylene, using a catalytic amount of iodine, afforded 4,5-dibromo-o-xylene 71 as a crystalline solid after recrystallization from methanol [141]. The appearance of only one singlet for the aromatic hydrogen atoms in the H NMR spectrum of 71 confirms the regiochemical outcome of the bromination. Treatment of 71 with... 

Ellis K. Fields We have found o-methylbenzyl bromide among the oxidation products of o-xylene. Even though this bromide persists during the oxidation period and appears to solvolyze somewhat more slowly than other benzyl bromides, it releases its bromine eventually to initiate and propagate free radical chains. 

A mixture of 106 g. (1.0 mole) of o-xylene and 107 ml. of concentrated sulfuric acid is stirred and heated on a steam bath for 1.5 hours, and the resulting solution is dissolved in 1.5 1. of water. A solution of 473 g. of hydrated barium hydroxide in 750 ml. of water is added, and the resulting neutral solution is filtered hot and cooled to 50-60°. To this is added a solution of 192 g. (1.20 moles) of bromine and 200 g. of barium bromide in 700 ml. of water. (Hood.) The mixture is held at 50-60° for 15 hours, after which it is cooled and the precipitated barimn bromo-o-xylenesulfonate removed by filtration, washed with water, and dried. The yield of the salt (somewhat impure) is 63%. 

Catalysis. A procedure for the preparation of 4-bromo-o-xylene calls for addition of bromine over 3 hrs. to a stirred mixture of 500 g. of o-xylene with 12 g. of iron filings and a crystal of iodine. A note states that neither catalyst suffices by itself. 

Octaalkyl-substituted phthalocyanine [152] can be synthesized by bromin-ation of dialkylbenzene accessible via a Grignard reaction. The synthesis of the alkoxy-substituted phthalocyanines [153] starts from catechol. The reaction with alkylhalide is followed by a bromination in positions 4 and 5. Alkoxy-methyl-substituted phthalocyanines [154] are obtained by substitution of o-xylene with bromine and subsequent radical bromination of the two methyl groups of l,2-dibromo-4,5-dimethylbenzene. The alkoxy groups are introduced by reacting the tetrabromo compound with the corresponding alcoholate. 

Arrange the following five compounds in order of decreasing rate of bromination benzene toluene o xylene m xylene 13 5 tnmethylbenzene (the relative rates are 2 X 10 5 X 10" 5 X 10 60 and 1)... 

Cumming, Hopper, and Wheeler, Systematic Organic Chemistry, p. 351, Constable and Company, London, 1937 (description of a procedure for light-catalyzed bromination which is applicable to o-xylene). 

A. a,a,ot, a -Tetrabromo-o-xylene. In a 2-1. three-necked flask equipped with an oil-lubricated Trubore stirrer, a dropping funnel, a thermometer extending nearly to the bottom of the flask, and a reflux condenser (Note 1) attached to a gas absorption trap 2 is placed 117 g. (1.1 moles) of dry o-xylene (Note 2). An ultraviolet lamp such as a General Electric R.S. Reflector Type 275-watt sun lamp is placed about 1 cm. from the flask so as to admit the maximum amount of light. The stirrer is started, and the o-xylene is heated to 120° with an electric heating mantle. A total of 700 g. (4.4 moles) of bromine (n.f. grade) is added in portions from the dropping funnel to the reaction flask at such a rate that the bromine color is removed as fast as it is added. 

When the benzyl and xylyl bromides were brought in contact with indium tubes for sampling, the indium was quickly discolored and pitted. The spectra of the material so taken up were recorded despite the clear evidence of reaction between the indium and the bromide. Relative intensities in the low voltage spectra and suggested identities of the compounds responsible for the peaks are shown in Table II. The chief result of indium attack on a-bromo-o-xylene was expected to be removal of a bromine atom to produce a xylyl radical. If this were the case, the major stable products should be xylene polymers of molecular weight 210, 314,... 

It is convenient to seal a short inner tube inside the stem of the dropping funnel so that the rate of addition can be observed readily. The introduction of the bromine below the surface of the o-xylene through an extended stem, about 4-mm. inside diameter, results in better mixing of reactants and less loss of bromine vapors. 

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