Effect of surfactant micelles on homogeneous catalytic hydrogenation of 2,4‐hexadienoic acid with pentacyanocobaltate(III)‐hydride complex

Abstract

The influence of anionic (SLS), cationic (CTAB) and nonionic (NP 9E0) surfactant micelles on the homogeneous catalytic selective hydrogenation of 2,4-hexadienoic acid to 2-hexenoic acid in alkaline aqueous solutions of pentacyanocobaltate(llI)-hydride complex was studied at 1 atm pressure and 30 C. The results indicate that, compared to the reactions in the aqueous medium, (a) surfactant micelles present in concentrations in the neighborhood of their critical micelle concentrations (CMC) enhance both the net rate of formation of the pentacyanocobaltate(lll)-hydride complex and its equilibrium concentration by factors of 2 to 5 and 1.1 to 1.5, respectively; (b) the neutral NP 9E0 surfactant micelles enhance the rate of hydrogenation of 2,4-hexadienoic acid by pentacyanocobaltate(llI)hydride complex by a factor of 3, and, (c) the SLS and the CTAB surfactant micelles totally inhibit the hydrogenation of 2,4-hexadienoic acid. Probable mechanistic reasons are also advanced in order to explain the observed activation of the well known pentacyanocobaltate(lll)-hydride complex by surfactant miceiles.