Action of thionyl chloride on carboxylic acids in presence of pyridine.

Abstract

IN order to use thionyl chloride most effectively for the preparation of acyl chloride, RCO.Cl, a knowledge of the sequence of reactions occurring is desirable. Trichloroacetic acid reacts extremely slowly with thionyl chloride even at higher temperatures; but Carre and Libermann1 have shown that pyridine (1 mol.) helps the reaction. Mills and Human2 adopted the use of the base in the preparation of anilides and esters of the half-esters of phthalic acid. We have now shown that during the addition of the first half of the thionyl chloride (1 mol.) to an ethereal solution of cyclohexyl hydrogen phthalate (1 mol.) and pyridine (1 mol.), the following reactions occur: The second half of the thionyl chloride then converts the anhydride into the acyl chloride, and it was from the latter that Mills and Human obtained the anilide by adding the amine together with pyridine. Now the significant point is that although the formation of anhydride is very rapid3, the formation of acyl chloride from the anhydride is comparatively slow and requires a time which depends on the nature of R in the acyl group. It just happens that the anhydride of cyclohexyl hydrogen phthalate (the example quoted by Mills and Human) will give acyl chloride in yields approaching 80 per cent in the one hour of reaction time allowed by these workers; but the rate of formation of acyl chloride from the anhydride of 2-octyl hydrogen phthalate is definitely slower, and we were at a loss to account for the high yield of anilide reported by Mills and Human.