Homogeneous catalytic hydrogenation of sorbic acid with pentacyanocobaltate II

Abstract

Abstract The homogeneous hydrogenation of sodium sorbate, with pentacyanocobaltate II used as the catalyst, was followed manometrically at room temperature and atmospheric hydrogen pressure. One mole of hydrogen was absorbed by one mole of sorbate. UV and IR analyses demonstrated the reduction of sorbate to hexenoate. Gas liquid chromatography (GLC) of hexenoate indicated 82, 17, and 1% yields of 2‐,3‐, and 4‐hexenoates, respectively, with traces of sorbate. Increasing either the period of hydrogenation or the ratio of catalyst to sorbate, or both, did not cause the formation of caproate or change the ratio of the three isomers. Homogeneous catalytic hydrogenation in methanolic solutions showed higher selectivity since 2‐hexenoate was present in 96% yield. To standardize GLC columns, hexenoic acid isomers were synthesized by the Knoevenagel condensation. Trans‐ 2‐ hexenoic and trans‐ 3‐ hexe ‐ noic acids were prepared in 75 and 85% yields by a condensation of n ‐butanal and malonic acid in the presence of pyridine and triethanolamine, respectively. Trans‐ i‐ h.exea.oic acid was prepared in 70% yield by condensing diethyl malo‐nate with crotyl bromide. Physical properties, including UV and IR spectra, were determined for the purified synthetic acids and methyl esters. Although the absolute specificity of attack at the 4,5‐double bond of sorbic acid was not confirmed, the high degree of selectivity obtained does encourage further study of homogeneous catalysis on higher fatty acids.