Patent GB-678568-A (GB)
Title: Process for the preparation of carboxylic acids from mercaptans and the acids so prepared
IPC: C07C51/295
Filed: 1949-11-18
Abstract: Carboxylic acids are produced from primary mercaptans containing at least three carbon atoms in the molecule, the carboxylic acids having the same number of carbon atoms in the molecule as the mercaptan, by contacting the mercaptan with a molten caustic alkali in the presence of a substance containing a heavy metal as catalyst, the latter being defined as any metal other than the alkali and alkaline earth metals, e.g. Fe, Ni, Ru, Rh, Pd, Mn, Cr, V, Mo, Ge, Cb, Sb, Re, Os, Ir, Pt, W, Ta, La, Ce, Pb, Cu, Ag, Co, Ga, Au and Zn.  Suitable compounds of heavy metals which may be used include the sulphides, oxides, tellurides, selenides, phosphides, and salts, especially of oxy acids of heavy metals, e.g. chromates, tungstates, vanadates or molybdates. Mixtures or alloys of the metals may also be used. Preferred catalysts are the oxides and sulphides of Mo, Cr, W, Ni, Fe and Co. The -CH2-SH group in the mercaptan may be p  linked to a cyclic, alicyclic, or aromatic group.  Specified mercaptans are propyl, butyl, isobutyl, pentyl, 3-methylbutyl, hexyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 2,2-dimethylbutyl, heptyl, 3-ethyl pentyl, octyl, nonyl, 2-propylhexyl, 2,3-dimethylheptyl, decyl, hendecyl, dodecyl, tridecyl, 3-propyldecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl and heneicosyl mercaptans, and their branched chain isomers.  The above compounds may be substituted by straight chain, cyclic and/or heterocyclic radicals, and by substituent groups or atoms, e.g. alkoxy, halogen, nitro, amino, or alkyl amino.  More than one mercapto group may be present. Thus, polycarboxylic acids may be produced by the process.  The hydrocarbon radical linked to the mercapto group may be saturated or unsaturated, e.g. it may contain one or more olefinic and/or acetylenic linkages.  Unsaturated carboxylic acids may thus be produced.  As the caustic alkali sodium or potassium hydroxide or mixtures thereof are preferably employed although lithium, rubidium and caesium hydroxides may also be employed.  The preferred temperature range is 185-485 DEG  C. and particularly 200-300 DEG  C. The process may be carried out batchwise, or in a continuous or intermittent manner, and a large excess of the caustic alkali may be used.  The carboxylic acid is obtained in the form of its salt with the caustic alkali and the free acid may be recovered by acidification or in the form of an insoluble soap, e.g. a calcium, zinc or aluminium soap, by dissolving the hydroxide containing the product with water and adding a suitable compound to precipitate the soap.  In an example, lauric acid is obtained by adding dodecyl mercaptan continuously to a molten mixture of sodium hydroxide and potassium hydroxide containing a suspension of nickel sulphide-tungsten sulphide whilst keeping the temperature at 250 DEG C., cooling the product, dissolving in water, and acidifying the solution with hydrochloric acid.  Other specified acids that may be produced are butyric, valeric, trimethyl acetic, caproic, heptylic, caprylic, pelargonic, capric, hendecanoic, tridecanoic, myristic acid, pentadecanoic, palmitic, margaric, stearic, nonadecanoic and arachidic acids; also dicarboxylic acids such as adipic and tetradecanedioic acid; and unsaturated acids such as 4-tetradecenoic acid and oleic acid which may be obtained from the unsaturated mercaptans.  The Specification as open to inspection under Sect. 91 states that organic sulphides may be employed as well as mercaptans. This subject-matter does not appear in the Specification as accepted.