GB371946A - Improvements in or relating to the manufacture of aliphatic anhydrides - Google Patents

Abstract

Carbonyl chloride for use in the production of acid anhydrides from carboxylic acids is obtained by passing a mixture of carbon monoxide and chlorine over charcoal. The carbonyl chloride may be obtained and used in the nascent state by passing a mixture of the vapour of the carboxylic acid, carbon monoxide, and chlorine over charcoal or silica gel at 200-350 DEG C.ALSO:The anhydrides of aliphatic acids are obtained by causing the vapours of the acids to react with carbonyl chloride. The reaction is greatly accelerated by elevated temperatures, such as 150-350 DEG C. The reaction vessel may be filled with porous or absorbent bodies such as charcoal or silica gel. To avoid the formation of considerable quantities of the organic acid chloride, at least two molecules of acid should be used per molecule of phosgene. The vapours issuing from the reaction vessel may be condensed, and the crude anhydride then purified by fractional distillation, or the anhydride may first be treated with anhydrous sodium acetate or acetate to remove acetyl chloride and hydrogen chloride. The phosgene may be produced by passing a mixture of chlorine and carbon monoxide over a catalyst such as charcoal, in which case the product is then treated with the acid vapour. Or the phosgene may be employed in a nascent state, e.g. by passing a mixture of chlorine, carbon monoxide and acetic acid vapour over silica gel or active charcoal at a temperature between 200 DEG and 350 DEG C. The invention may be applied to the production of acetic anhydride or the anhydrides of higher aliphatic acids such as propionic or butyric acids. In the example, which illustrates the production of acetic anhydride, it is indicated that the crude vaporous product is cooled in a condenser, and the vapours escaping condensation are scrubbed successively with acetic acid and benzene to recover acetyl chloride, and thereafter with water to recover hydrogen chloride; the crude anhydride collected in the condenser may be subjected to any convenient treatment to remove the hydrogen and acetyl chlorides present therein, and to separate as far as possible the anhydride from the unchanged acetic acid contained therein, e.g. it is first heated under reflux and thereafter subjected to fractional distillation; the acetic acid used for scrubbing may be purified in a similar way.