GB824144A - An improved process for the carboxylation of phenols - Google Patents

Abstract

The invention comprises a process for the production of an hydroxy aromatic carboxylic acid by the reaction between an alkali metal salt of a phenol and carbon dioxide, under anhydrous conditions, in which a mixture of a phenol, an alkali metal salt of a phenol and an aliphatic, substantially water-insoluble, alcohol having at least 5 carbon atoms in the molecule is heated at, at least 120 DEG C. in the presence of carbon dioxide. The amount of free phenol in the solution should exceed 0.01 mol. per mol. of alkali metal salt and may be 0.1 to 5 mols. per mol. of p alkali metal salt. Suitable alcohols are alkanols containing 5-20 carbon atoms in the molecule in a straight or branched chain, e.g. isomeric amyl alcohols and mixtures thereof, iso-octanol, nonanol-2, isodecanol, lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, and eicosyl alcohol. The free phenol may be recovered in the aliphatic alcohol at the end of the reaction by adjusting the pH to about 5 and separating the organic layer. The alkali metal phenate may be prepared by any suitable process, e.g. reaction of an alkali metal hydroxide with a phenol in solution in the alcohol to be used. The phenol may be used in excess of the amount required to react with the alkali metal hydroxide or the excess phenol may be added after the metal phenate has been formed. Water formed during the process may be removed before or after the addition of excess phenol. A suitable solvent medium for use in the process of the invention is one containing 10-90% by volume of phenol and from 90-10% of the aliphatic alcohol both determined by the volume of that substance present in the liquid phase. The preferred solution is one containing substantially one mol. of alkali metal phenate for each two mols of a phenol and such quantity of aliphatic alcohol as is substantially equal in volume to the volume of phenol. The carboxylation may be carried out at temperatures between 120-200 DEG C. or higher, preferably at pressures greater than one atmosphere. Suitable monoor polyhydric phenols for use as starting materials are phenol, alkyl phenols, halogen-substituted phenols, aryl phenols, aralkyl substituted phenols, amino phenols, naphthols, long chain substituted phenols and nitrophenols. Examples are given of the preparation of salicyclic acid, 6-methyl salicyclic acid, 2-methyl - 5 - isopropyl salicylic acid, 3 - methyl-6-isopropyl salicylic acid, and 3,5-dichlorosalicylic acid.