GB744582A - Improvements in and relating to the production of unsaturated acids and esters - Google Patents

Abstract

a ,b -unsaturated acids or esters, either alone or in admixture with b ,g -unsaturated acids or esters, are prepared by heating at a temperature of at least 150 DEG C., a b -hydroxy acid or ester having at least one hydrogen on the a -carbon atom, and containing in solution 0.01 to 5 per cent by weight, based on the amount of b -hydroxy acid or ester employed, of a salt. Generally a ,b -unsaturated acids or esters are obtained but b ,g -compounds are also formed when the hydroxyl group is attached to a carbon atom to which no hydrogen is attached. The process may be applied to aliphatic p-hydroxy acids or esters such as ethyl b -hydroxy butyrate or to cyclic p-hydroxy acids or esters such as cis-methyl hexahydrosalicylate and dimethyl cyclohexane-2:5-diol-1:4-dicarboxylate. The reaction is preferably carried out in a hydroxylic solvent such as water, or an alcohol containing up to 4 carbon atoms and preferably the alcohol corresponding to the ester. The solvent diminishes the formation of the lactone, 4-carbomethoxy - 2 - hydroxy - 6 - oxabicyclo-[3:2:1] - octa - 7 - one which tends to form when dimethyl cyclohexane-2:5-diol-1:4-carboxylate is converted. In general, when a diester, acid ester, or monoester is reacted in the absence of a solvent or in the presence of water, an unsaturated monoester or free acid may be formed, and if a diester is reacted in an alcohol which does not correspond to the ester, a mixed ester may be obtained. The salt used must be sufficiently soluble under the reaction conditions to enable it to be present to the extent of at least 0.01 per cent by weight of the acid or ester employed, and salts which are not soluble to this extent such as silver iodide and barium sulphate do not facilitate the reaction. Specified salts are the chlorides, bromides and iodides of the alkali or alkaline earth metals, nickel or ferric chlorides, and sodium acetate or benzoate. The preferred salts are those which correspond to the acid or ester to be dehydrated for example sodium b -hydroxy butyrate in the conversion of methyl b -hydroxy butyrate. Such salts may be formed in situ by the addition of alkali metal hydroxide to the acid or ester. The reaction temperature is preferably about 200 DEG C., but is less than the critical temperature of the solvent, and pressure is used to maintain the solvent in liquid phase. In examples: (1) cis-methyl hexahydrosalicylate was heated alone and with sodium hexahydrosalicylate to give cyclohex-1-ene-1-carboxylic acid and methyl ester; (2) as in (1) but using potassium acetate; (3) ethyl b -hydroxy butyrate was heated alone and with sodium b -hydroxy butyrate to give a mixture of ethyl crotonate and crotonic acid; (4) dimethyl cyclohexane-2:5-diol-1:4-dicarboxylate was dissolved in methanol and heated in separate runs with ferric, nickel and sodium chlorides, and the iron and nickel salts of cyclohexane - 2:5 - diol - 1:4 - dicarboxylic acid to yield dimethyl cyclohexa-1:4-diene-1:4-dicarboxylate. Specification 740,388 is referred to.