Patent GB-854383-A (GB)
Title: Process for the manufacture of sorbic acid
IPC: C07C51/09
Filed: 1957-06-07
Abstract: A condensation product is obtained by reacting ketene with crotonaldehyde at a temperature of at least 20 DEG C. in the presence of at least one cadmium, zinc, iron, nickel, mercury or cobalt salt of a fatty acid containing at least 4 and preferably not more than 18 carbon atoms.  The product has a molecular weight of about 1,000 to 3,000 and is probably a polyester.  The preferred temperature range for the reaction is 25-30 DEG C., although raised temperatures may be used.  The product is an intermediate in the preparation of sorbic acid (see Group IV(b)).  In examples, ketene is introduced into a stirred or agitated solution of crotonaldehyde in toluene containing zinc isovalerate, and the toluene and excess crotonaldehyde removed in vacuo.ALSO:Sorbic acid is prepared by a process wherein ketene is reacted with crotonaldehyde at a temperature of at least 20 DEG  C. in the presence of at least one cadmium, iron, nickel, mercury, cobalt or zinc salt of a fatty acid containing at least 4 and preferably not more than 18 carbon atoms, the vaporisable compounds are distilled off, and the residue, still containing the fatty acid salt is decomposed to sorbic acid either by alkaline hydrolysis followed by heating with a strong aqueous acid, or by thermal decomposition.  The catalysts are generally used in a proportion of 0,1-5% by weight, preferably 0,5-2% based on the crotonaldehyde.  The condensation reaction is preferably carried out in an inert solvent or an excess of crotonaldehyde.  It is often preferred to use an equimolecular quantity of ketene and crotonaldehyde in which the catalyst is dissolved, a small proportion of the crotonaldehyde acting as solvent and remaining unreacted.  Preferred temperatures are 25-30 DEG  C.  The alkaline hydrolysis is preferably carried out at 60-100 DEG  C., with a strong base.  The resulting solution contains salts of unsaturated hydroxycarboxylic acids, the free acids being largely precipitated on the addition of acid.  These free acids are dehydrated to sorbic acid by treating them with a strong aqueous acid preferably at 80-150 DEG  C.  Alternatively the ketene/crotonaldehyde condensation product, still containing the catalyst may be converted to sorbic acid by thermal decomposition, suitably at 180-220 DEG  C.  A catalytic amount, preferably 1-5%, of a substance having an alkaline reaction may be present.  It is preferred to carry out the thermal decomposition in the presence of an inert substance boiling above 150 DEG  C. and preferably above 180 DEG  C. at atmospheric pressure, and by heating at 150-300 DEG  C.  A base compound is preferably present, generally 1-15% calculated as metal hydroxide, based upon the quantity of the ketene-crotonaldehyde reaction product.  The inert diluent is suitably added in a quantity of 1-15 parts by weight per part of ketene crotonaldehyde reaction product.  It is preferred to use diluents or solvents which are liquid at normal temperature and from which pure sorbic acid precipitates on cooling, and it is especially advantageous to use substances which are capable of forming azeotropic mixtures with sorbic acid.  The decomposition may be effected at atmospheric, superatmospheric or, advantageously, subatmospheric pressures.  If the ketene/crotonaldehyde reaction is effected in a solvent which is suitable for the thermal decomposition, the solution obtained may be directly subjected to thermal decomposition after evaporation of excess crotonaldehyde, if any, and, if desired, in the presence of an alkaline catalyst.  Sorbic acid may be isolated, for example, by filtration of the distillate with an entraining agent. After conversion or distillation, more volatile solvents may be added to the mixture of sorbic acid and diluent to improve separation.  Water may be used for this if the solvents are miscible with it.