Patent GB-808566-A (GB)
Title: A process for preparing aromatic carboxylic acids and the resulting acids
IPC: C07C51/265
Filed: 1955-09-19
Abstract: Aromatic carboxylic acids are made by oxidizing an alkyl-substituted aromatic compound having, as nuclear substituents, at least one alkyl group which is not subject to steric hindrance and has at least one hydrogen atom attached to its alpha carbon atom, and at least one further nuclear substituent comprising a carboxylic acid, carboxylic ester, ketonic, nitrile or nitro group or a halogen atom, in the liquid phase with molecular oxygen in the presence of an aliphatic carboxylic acid and an auxiliary agent, the molar ratio of the aromatic compound to the auxiliary agent being from 1:1 to 50:1 throughout the oxidation and the water content of the reaction mixture being maintained below 5 per cent by weight during the oxidation.  The correspondingly substituted aromatic carboxylic acids are obtained, the alkyl groups being oxidized to carboxylic acid groups.  The ketonic group may also be similarly oxidized. Thus p-nitrotoluene gives p-nitrobenzoic acid, p-acetyltoluene gives p-acetyl benzoic acid and/or p-toluic acid and finally terephthalic acid, and 3:4-dimethyl benzoic acid gives 4-methyl isophthalic acid, 3-methyl terephthalic acid, and benzene tri-carboxylic acid-(1:3:4). Other alkyl-substituted aromatic compounds mentioned are the o-, m- and p-isomers of toluic acid, ethyl benzoic acid, tolunitrile, chlorotoluene, and chloro ethyl benzene, m- and p-isopropyl benzoic acid, methyl naphthoic acid, 2:4- and 3:4-diethyl benzoic acids, 4-methyl-, 4-ethyl- and 6-ethyl-benzene dicarboxylic acids-(1:2), 4 - ethyl - benzene dicarboxylic acid-(1:3), m- and p-chlorocumenes, 1:3-dichloro-2-methyl benzene, 1- 2-dimethyl-3-chlorobenzene, and methyl-nitro-naphthalene.  The auxiliary agent is an alkyl-substituted benzene which oxidizes under the reaction conditions to yield oxidation products which do not interfere with the oxidation reaction contemplated, and which if oxidized separately under said reaction conditions in the absence of the alkylsubstituted aromatic compound, would have an oxidation rate of more than 1 gm. of auxiliary agent per litre of reaction mixture per hour.  The molar ratio of aromatic compound to auxiliary agent is maintained within the above-specified range during the reaction by adding fresh auxiliary agent to the reaction mixture when necessary.  Auxiliary agents mentioned are toluene, o-, m- or p-xylene, trimethylbenzenes, ethylbenzene, o-, m- or p-diethylbenzene, 1:3:5 - triethylbenzene, 1:2:6 - triethylbenzene, cumene, m- and p-diisopropylbenzene and 1:3:5-triisopropylbenzene.  The auxiliary agent may be such that the aromatic compound to be oxidized and the final product desired may be obtained from it successively, thus m- or p-diisopropyl benzene may be used as auxiliary agent in the oxidation of m- or p-isopropyl benzoic acid to isophthalic or terephthalic acid.  The aliphatic carboxylic acid may contain 1 to 4 carbon atoms in the molecule such as acetic acid or be a mixture of such acids.  Water may be removed from the reaction mixture by distillation to maintain the water content below 5 per cent by weight.  The molecular oxygen used may be pure or a mixture with inert gases such as air.  Catalysts such as manganese or cobalt salts or mixtures thereof which are soluble in the reaction mixture may be used.  An organic peroxide may be added to avoid or reduce the induction period at the beginning of the reaction.  In examples: (1) p-toluic acid is oxidized with oxygen in the presence of p-tert.-butyl toluene as auxiliary agent, cobalt acetate, manganese triacetate, di-tert.-butyl peroxide and butyric acid to give terephthalic acid and p-tert.-butyl benzoic acid; (2) p-chlorotoluene in the presence of p-tert.-butyl toluene; and (3) p-chlorocumene in the presence of p-tert. butyl cumene when oxidized under similar conditions both yield a mixture of p-chlorobenzoic acid and p-tert.-butyl benzoic acid; (4) p-isopropyl benzoic acid is similarly oxidized in the presence of cumene to give terephthalic acid and some benzoic acid.