Interox Chemicals Limited

Philip J Turner

Martin Jeff

Kenneth M Dear

Kevan M Reeve

Vincent I Routledge

Process for the preparation of 4-sulfo-2-chlorobenzoic acid

Selective bromination of alkylarenes to the alpha monobrominated derivative is desirable as a precursor for the selective production of, for example, the corresponding alcohols. The product can be obtained by a photolytic reaction between the substrate, hydrogen peroxide and hydrogen bromide in approximately equimolar ratios in an organic solvent, but improvement in the yield of the product coupled with similar or improved selectivity in its production was either not achieved or was actually impaired by a range of variations in the ratios and by omitting the solvent. The invention employs mole ratios of H.sub.2 O.sub.2 :substrate of 0.1:1 to 0.4:1, preferably 0.2:1 to 0.33:1 and of H.sub.2 O.sub.2 :bromide of 1:1.2 to 1:2, preferably about 1:1.3 to 1:1.8 and progressive introduction of the H.sub.2 O.sub.2, either in the presence or absence of the solvent, while irradiating with light preferably having principal emissions in the range of 250 to 600 nm, and the process thereby achieves either or both of improved yield and improved selectivity of the alpha monobrominated product. The reaction mixture is preferably maintained at 50.degree. to 70.degree. C. The process is especially suitable for deactivated alkylarenes, such as o-nitrotoluene.

Bromination

It is desired to find an alternative way to produce benzophenones or substituted benzophenones which does so selectively and avoids the problems of previous methods that use, for example, large amounts of an aluminium chloride catalyst or nitric acid. In the invention process, a diphenyl methane starting material (DPM) in a hydrophobic phase is oxidized in a photolytic process by contact with an aqueous phase containing greater than 1 mole of HBr and at least 1.5 moles of H.sub.2 O.sub.2, both per mole of DPM, the radiation being capable of dissociating bromine to its radicals, and especially using light of wavelength 600 to 250 nm. Preferably, a reaction temperature of around 50.degree. to 65.degree. C., is used, the organic solvent comprising a suitably boiling chlorinated carbon or hydrocarbon. The H.sub.2 O.sub.2 is introduced progressively into the reaction mixture. The benzophenone product is recovered substantially in the organic phase, whereas most of the bromine/bromide is retained in the aqueous phase which can be re-employed in a subsequent oxidation of DPM, possibly after restoration of the bromide to its initial concentration. In a modification to the process, by suitably controlling the mole ratios of bromide and H.sub.2 O.sub.2 to DPM at about 1:1:1, and preferably using a reaction temperature of about ambient to 35.degree. C., the DPM can be substituted by a single bromine atom on the methane carbon also with good selectively, instead of being oxidized to the benzophenone.

Oxidation process

Selective bromination of alkylarenes to the alpha dibrominated derivative is desirable as a precursor for the selective production of, for example, the corresponding aromatic aldehyde. Improvements in the extent of selectivity of the product and/or its rate of production can be obtained by employing a photolytic reaction between the substrate, hydrogen peroxide and hydrogen bromide using mole ratios of H.sub.2 O.sub.2 : substrate of about 2.8:1 or higher, preferably 3,2: to 3.5:1 and of bromide:substrate of over 2.5:1, preferably about 2.8:1 to 3.2:1 and progressive controlled introduction of the H.sub.2 O.sub.2, the reaction being carried out in the presence of a substantial amount of an organic solvent, and preferably more than 5.5 volumes per volume of substrate, whilst irradiating with light preferably having principal emissions in the range of 250 to 600 nm. The reaction mixture is preferably maintained at 50.degree. to 60.degree. C. The reaction often takes from 5 to 12 hours, of which period hydrogen peroxide is introduced progressively into the reaction mixture during preferably the first 3 to 5 hours. The process is especially suitable for deactivated alkylarenes, such as o-nitrotoluene.

Oxidation of alcohols to ketones using bromine suffers from a side reaction of comparable speed in which a bromide substituted product is formed. Additionally, the use of bromine as reagent is relatively unpleasant and it creates a major waste disposal problem. The latter problem can be solved by generating bromine in situ by reaction between hydrogen peroxide and bromide ions or hydrogen bromide, but this inevitably increases the exposure of the alcohol to HBr/Br.sup.-. The selectivity of the process towards non-substituted oxidation depending upon the inherent deactivation or reactivity of the alcohol can be improved by irradiating the reactants with light of suitable frequency to generate bromine radicals in the mixture, and/or by controlling the rate of introduction of the hydrogen peroxide and controlling the extent to which mole ratio of HBr:alcohol is substoichiometric. Selectivity can also be improved by employing a two-phase reaction mixture, the alcohol being present to a considerable extent in the organic phase with HBr/Bromide being retained to a great extent in the aqueous phase. It is particularly beneficial for the selective oxidation of alcohols that are substituted by adjacent electronegative groups, for example chloropropanols.

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Widnes, United Kingdom

Philip J Turner