GB774687A - Process for the production of -e-hydroxycaproic acid esters - Google Patents

Abstract

e -Hydroxycaproic acid esters which may have hydrocarbon substituents in the acid radical are obtained by oxidizing cyclohexane, or one of its homologues, catalytically and, if desired continuously, with air, distilling off the volatile components of the reaction products, and completely esterifying the residue at an elevated temperature, and if desired under super-atmospheric pressure, in the absence of an acid esterification catalyst, with a C1 to C4 aliphatic or C4 cycloaliphatic alcohol, and then isolating the e -hydroxycaproic acid ester or correspondingly substituted e -hydroxycaproic acid ester, from the esterified residue. Isolation of the ester may be effected by fractional distillation, solvent extraction, or extractive distillation of the esterified residue. The solvent extraction is preferably effected by a continuous two-solvent extraction process one of the solvents being a polar solvent such as water or aqueous methanol in which the e -hydroxycaproic acid ester dissolves and the other being a non-polar solvent, e.g. a hydrocarbon such as cyclohexane or a light benzine fraction in which the dicarboxylic acid diester and the other compounds present dissolve. The solvent of higher specific gravity may be passed into the top of a vertical column and the one of lower specific gravity into the bottom of the column, the mixture to be separated being passed in midway. It is desirable to distil the ester mixture off from the higher boiling impurities before subjecting the mixture to the solvent extraction. In the extractive distillation the mixture is subjected to distillation while the vapours of the volatile part are washed by a liquid, e.g. a high boiling hydrocarbon, passed in counterflow, e.g. from the top of the apparatus used. Increased yields of the hydroxyacid ester may be obtained by carrying out the oxidation of the cyclohexane rapidly, i.e. over a period less than 30 minutes and then subjecting the product to brief distillation over a period of less than 10 mins. The volatile components of the oxidation product may be recycled to the oxidizer along with fresh cyclohexane. In examples: (1) cyclohexane is oxidized continuously with air and the products worked up by brief distillation and the residue heated in an autoclave for 4 hours at 200 DEG C. with methanol. The unreacted methanol is distilled off and the residue fractionally distilled to recover adipic acid dimethyl ester and e -hydroxycaproic acid methyl ester; (2) as in (1) except that butanol is used and the corresponding butyl esters are recovered; (3) as in (1) except that the esterified mixture is injected at the mid-point of an extraction column into which cyclohexane is passed in from below and water is passed in from above. The aqueous phase is extracted with benzene and after distilling off the benzene the residue is fractionated to yeld e -hydroxycaproic acid methyl ester. The cyclohexane phase contains the adipic acid diester and other esters; (4) cyclohexane is oxidized continuously as in (1) and the volatile components from the brief distillation, comprising cyclohexane, water, cyclohexanone, cyclohexanol, and small amounts of volatile acids are separated into two phases and the non-aqueous phase is fed back continuously to the oxidation process until the composition of the mixture fed back no longer changes. The residual acids are then heated with methanol and the product fractionally distilled as in (1) to recover the e -hydroxy caproic acid methyl ester and the adipic acid dimethyl ester. Reference is also made to the use of ethanol, propanol and cyclohexanol as the esterifying alcohol and to the use of methyl cyclohexane to prepare the corresponding methyl-substituted e -hydroxycaproic acid ester.