A-Bromo-o-xylene

During the workup of the o-xylene oxidation run, a strong lachrymator made its presence felt. This was probably a-bromo-o-xylene, although it was not detected in the low voltage mass spectrum. We suspected that a strong peak at mass 104, undoubtedly caused chiefly by a fragment ion derived from o-methylbenzyl alcohol by loss of H20 (I), might also contain a contribution from benzocyclobutene from the interaction of a-bromo-o-xylene with the indium tube used to introduce samples into the spectrometer. To test this possibility, benzyl bromide and a-bromo-o-xylene were run separately under the same conditions. 

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,... 

Despite our failure to find any supporting spectral evidence, the suspected presence of a-bromo-o-xylene and the absence of o-methyl-benzyl acetate in the oxidation products from o-xylene suggest a solvolysis rate for this benzylic halide lower than for the isomeric methylbenzyl bromides. 

Xylene (150 g. or 1.41 moles) is heated to 130° in an oil bath while 252. g. (3.15 moles) of bromine is added slowly through a tube passing below the surface of the o-xylene. (Hood.) After the addition of the bromine, the reaction mixture is poured into an evaporating dish and placed in a vacuum desiccator over potassium hydroxide until the hydrogen bromide has been removed. The product is distilled, and the a-bromo-o-xylene is collected at 215-218° in 80% yield. 

An interesting, halogen-exchange reaction takes place when [Ni(Et2dtc)2] is refluxed in 1,2-dichloroethane with an excess of a,a -di-bromo-o-xylene and a-bromo-a -chloro-o-xylene. The products, a,a -... 

A mixture of 200 g. (1.08 moles) of 4-bromo-o-xylene [Org. Syntheses, 28, 22 (1948)], 14 g. of copper wire, and 600 ml. of concentrated ammonium hydroxide solution containing 12 g. of cuprous chloride is placed in a bomb and heated to 195° for 14 hours with agitation. The pressure is 900-1000 lb. The organic layer is separated from the resulting reaction mixture, and to the organic layer is added 40 ml. of 40% aqueous sodium hydroxide solution. The mixture is steam-distilled to remove the 3,4-dimethylaniline which is separated from the distillate and further purified by solution in 500 ml. of 8% aqueous hydrochloric acid, extraction with ether, and reprecipitation from the aqueous solution with 160 ml. of 40% alkali. The resulting alkaline solution is steam-distilled. The amine is separated from the distillate, dried, and distilled under vacuum. The product boils at 116-118°/22-25 mm., and the yield is 103 g. (79%). 

Near the end of the steam distillation a white waxy product begins to collect in the condenser. This is apparently a di-bromo-o-xylene. 

The crude product contains a small amount of unchanged 4-bromo-o-xylene which is removed by the ether extraction. 

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. 

The scheme shown below describes the synthesis of divinyltetramethyl-siloxane bisbenzocyclobutene (DVS-bisBCB) monomer. The BCB hydrocarbon (75) may be made from the pyrolysis of a-chloro-o-xylene (16). Treatment of the hydrocarbon with bromine provides 4-bromo-BCB with excellent yield. Palladium-catalyzed coiq>ling of 4-bromo-BCB with divinyltetramethylsiloxane produces the monomer DVS-bisBCB. Distillation of the monomw can lower ionic inq)urities to the ppb level if required (77). Die viscosity of DVS-bisBCB monomer is 100 cP at room tenqimture as measured with a Brookfield viscometer at 20 rpm. 

Tetramethyl-dipbenylamine, thick oil, bp — partially decomp 340—45° prepd from asym o-xylene, bromo-Zn-ammonia Amm bromide heated in a tube at 300-310° (Ref 2)... 

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