THE INFLUENCE OF THE CONCENTRATION OF CHLORINE-CONTAINING CONDENSING AGENTS IN THE SYNTHESIS OF CARBOXYLIC ACID ANILIDESТ

Abstract

Chlorine-containing condensing agents (PCl3, TiCl4, SiCl4) are widely used in the synthesis of carboxylic acid arylamides by the reaction of arylamines with carboxylic acids in stoichiometric amounts, and often in excess (40–150 mol % relative to the latter). To find the optimal amount of the condensing agent used, the acylation of aniline with 3-hydroxy-2-naphthoic acid in boiling ortho-xylene was studied in the PCl3 concentration range of 0–10 mol % from 3-hydroxy-2-naphthoic acid. It has been established that phosphorus tri-chlorochloride plays the role of a condensing agent and a catalyst, while 3-hydroxy-2-naphthoic acid anilide is formed along two routes with different rates, which can be separated kinetically. The first route includes the rapid synthesis of acid chloride, its interaction with aniline to obtain the target product. The second route is implemented due to phosphorous acid, which is formed from phosphorus trichloride and is a true acylation catalyst. The maximum yield of anilide 3-hydroxy-2-naphthoic acid decreases with increasing catalyst concentration, approximately propor-tional to the amount of aniline phosphite formed from it in the mass, which, apparently, is not reactive when interacting with the starting acid. Similar patterns were also found in the acylation of aniline with benzoic and salicylic acids in the presence of phosphorus trichloride; benzoic acid in the presence of titanium tetrachloride; 3-hydroxy-2-naphthoic acid in the presence of silicon tetrachloride. They allow a new look at the role of these products (PCl3, TiCl4, SiCl4) as condensing agents and/or ca­talysts in the synthesis of amides of carboxylic acids. Taking into account the obtained results, in practice it is better to use PCl3 as a catalyst in an amount not exceeding 2–2.5%, or even to replace it with phosphorous acid. This allows to completely get rid of the release of hydrogen chloride, significantly reduce the consumption rates for raw materials, simplify the entire technological process, reduce the amount of waste, ensure the yield of 3-hydroxy-2-naphthoic acid anilide close to quantitative, create a direct catalytic amidation technology that fully meets the criteria «green» chemistry processes.