GB836525A - A process for the preparation of carboxylic acids - Google Patents

Abstract

Carboxylic acids are prepared by reacting mercaptans having at least three carbon atoms and in which a mercapto group is attached to a hydrocarbyl group through an aliphatic carbon atom with carbon monoxide in the presence of concentrated sulphuric acid and in the absence of added water, diluting the resulting reaction mixture with water in the absence of any substantial addition of free carbon monoxide, and separating the carboxylic acid from the resulting diluted reaction mixture. The reactants may be pretreated to remove water. The mercaptan may contain more than one mercapto group and may be substituted, e.g. by halogen, nitro or carbonyl groups; secondary and tertiary mercaptans are preferred. The sulphuric acid is above 90 and preferably above 95% concentration and is preferably in molecular excess, e.g. a molar ratio of H2SO4 to mercaptan of 2:1 to 10:1. Temperatures of - 10 DEG to 100 DEG C. and pressures up to 105 atmospheres and higher are mentioned. Preferably at least a substantial portion of the reaction mixture is maintained in the liquid phase. A solvent may be present during the reaction. Carbon monoxide may first be absorbed in the acid before the addition of the mercaptan. The reaction conditions of temperature and pressure are preferably maintained for some time after carbon monoxide absorption has ceased. If the mixture is appreciably above room temperature it is preferably cooled to a temperature not substantially above about 20 DEG C., and the pressure is preferably reduced to about atmospheric before diluting with water. The amount of water may be from 2 to 2,500, preferably 75 to 500% by volume of the acid originally charged.