Ester manufacture

Abstract

Phenol sulphonate esters can be made in a known process by reacting an alkali metal phenol sulphonate salt with an acyl halide in a hydrocarbon solvent at elevated temperatures, but the use of anhydrous materials were strongly advocated because any residual water hydrolyses the acyl chloride in a competitive reaction. However, it becomes particularly difficult and expensive to dehydrate phenol sulphonate salts below about 2% w/w water on a commercial scale and the product obtained by reaction from such partly dehydrated salts can be comparatively impure or reduced in yield. Consequently, changes to the process such as omitting the solvent or using a different acylating agent have been proposed, but these alternatives introduce their own respective manufacturing problems such as entrainment of viscous acyl chloride and anhydride in the product or introduction of a cumbersome and hence expensive recovery process involving an extra distillation step amongst others. The present invention provides a surprisingly convenient modification to the known process in which the esterification and product separation steps are incorporated into a cycle together with an acyl halide preparation step in which residual carboxylic acid and anhydride in solution after product separation is preferably augmented with a replenishing amount of carboxylic acid and then reacted with thiony halide. The resultant ester is readily separable and is obtained in comparatively good yield.