Process for the preparation of acyl cyanides

#The present invention relates to a process for the preparation of certain acyl cyanides. Such compounds are useful as starting materials for the synthesis of herbicides.

#It is known that acyl cyanides can be prepared by reacting acyl halides with metal cyanides (compare Angew. Chem. 68, 425-448 (1956)). However, this process has a number of disadvantages. Thus, for example, it is expensive and can be carried out technically only with difficulty since it is a two-phase reaction in which a solid is reacted with a liquid or with a substance present in solution. Moreover, the reaction does not give a single reaction product but a mixture of substances which is difficult to separate and which also contains, in addition to the particular acyl cyanide, a relatively large amount of a corresponding dimer. Accordingly, the yields of the acyl cyanide are relatively low. A further disadvantage of this process is that the washing water obtained during working up has to be subjected to thorough purification before it is run off since it still contains considerable amounts of highly toxic metal cyanides which are used in excess during the reaction.

#Furthermore, it is known that aroyl cyanides can be synthesized by reacting arylcarboxylic acid chlorides with hydrocyanic acid, in the presence of pyridine as an acid-binding agent, in absolute ether (see Angew. Chem. 68, 425-448 (1956)). However, this process also is associated with several disadvantages. Thus, firstly, it is not generally applicable. Moreover, it is technically very involved because the operations with pyridine, which is highly toxic, and with ether, which is readily inflammable, demand particularly stringent safety precautions. Moreover, in this case also through purification of the washing water obtained during working up is unavoidable because of the pyridine dissolved therein. The fact that a considerable amount of dimeric aroyl cyanide is formed during the reaction is also a disadvantage since, as a result of this, both the yield of aroyl cyanide is greatly reduced and the isolation thereof is made more difficult.

#226.2 (1 mol) of benzoic acid anhydride, 140.6 g of benzoyl chloride and 50 g (1 mol) of 98% pure sodium cyanide were mixed in a 1 liter flask, whilst stirring. The mixture was warmed to 155.degree. C. in the course of 30 minutes. A slightly exothermic reaction took place. After 10 minutes, the reaction was stirred for a further 30 minutes at 155.degree.-160.degree. C., by external heating, and the reaction was brought to completion. The reaction mixture was cooled to 90.degree. C., a distillation apparatus was installed and the benzoyl cyanide formed was distilled off in vacuo. Yield: 130 g (99% of theory) of benzoyl cyanide; melting point 32.degree. C.

#After purification, the distillation residue gave 225 g (99% of theory) of benzoic acid anhydride and 59 g of sodium chloride. The benzoic acid anhydride can be used for a further batch without purification.

#295 g (1 mol) of 3-chlorobenzoic acid anhydride were mixed with 179 g (1 mol) of 3-chlorobenzoic acid chloride and 65 g (1 mol) of potassium cyanide and the mixture was slowly warmed to 190.degree.-210.degree. C. After the heat of reaction had subsided, the mixture was left at this temperature for a further 30 minutes and the potassium chloride was then removed by filtration. The filtrate was distilled. Yield: 162 g (98% of theory) of 3-chlorobenzoyl cyanide; boiling point 118.degree.-120.degree. C. at 14 mm Hg.