GB946400A - Process for the manufacture of sorbic acid - Google Patents

Abstract

Sorbic acid of a high degree of purity is obtained by reacting ketene with crotonaldehyde at 25 DEG to 80 DEG C. in the presence of a reaction solution comprising an inert diluent, a preformed polyester reaction product of ketene and crotonaldehyde and a C4-C18 fatty acid salt of a bivalent transition metal selected from Group II to VIII of the Periodic System, the reaction being carried out in the absence of any substantial amount of acetic anhydride, (preferably below 1% by weight of acetic anhydride based on the reaction solution), withdrawing at least a portion of the reaction product, distilling to recover the polyester formed, heating the polyester and then converting it in known manner into sorbic acid. An excess of crotonaldehyde may be used and preferably the reaction solution comprising the reaction product of crotonaldehyde and ketene, the inert diluent, crotonaldehyde and the bivalent transition metal salt is circulated, a portion of the reaction solution is withdrawn from the cycle and at the same time there is added to the circulating reaction solution the inert diluent and crotonaldehyde in a volume equal to that of the withdrawn solution, the polyester formed is separated from the withdrawn portion of the reaction solution by distillation and the distillate composed of the inert diluent and crotonaldehyde is recycled into the reaction solution. The crotonaldehyde used should be substantially anhydrous and the reaction is preferably effected at between 40 DEG and 60 DEG C. Specified inert diluents are benzene, toluene, xylene, methylene chloride and nitrobenzene and specified metal salt catalysts are salts of iron, nickel, mercury, cobalt, cadmium and zinc with butyric, isobutyric, valeric, isovaleric, a -methylbutyric, diethylacetic, caproic, caprylic, capric, lauric, 2-ethylcaproic, stearic, palmitic and oleic acid, e.g. zinc isobutyrate. The reaction solution is preferably conducted through a column filled with filling material, e.g. Raschig rings, through which the ketene is flowing in countercurrent, and is discharged at the lower end of the column and then conducted through a heat exchanger so as to control the exothermic reaction of the absorbed ketene with the crotonaldehyde. The reaction products may be isolated and the inert solvent and any excess crotonaldehyde recovered by passing part of the reaction solution preferably under reduced pressure through a continuously operating distillation column and after evaporation of the mixture of crotonaldehyde and solvent the polyester is passed continuously through a tube heated to 80-150 DEG C. for from 30 minutes to 4 hours and such heating may also be carried out before or during the distillation step, if desired under superatmospheric pressure. The thermally treated polyester is then after treated by thermal splitting or alkaline hydrolysis and subsequent splitting off of water in an acid medium to yield sorbic acid which may be further purified, e.g. by distillation and/or crystallisation. Specifications 788,098, 854,383 and 883,492 are referred to.