The mechanism of hydrogenation by aqueous chromium(II) ion of the carbon–carbon double bond of olefinic compounds with polar substituents
The kinetics of the reaction of Cr2+ with maleic acid, fumaric acid, methylmaleic acid, chloromaleic acid, dichloromaleic acid, and methylfumaric acid have been investigated over a wide range of chloromaleic > maleic ≃ methylmaleic > dichloromaleic > fumaric and maleic > dichloromaleic ≃ ligand is in excess. In excess of Cr2+ the rate law is as shown below and k3 follows the trend: Rate =k3[Cr2+]2[L] chloromaleic > maleic > dichloromaleic > methylmaleic > methylfumaric. With excess ligand, L, the rate law has two terms (below) and the two rate constants, k′3 and k′2 follow the order: Rate =k′2[Cr2+][L]+k′3[Cr2+][L]2 chloromaleic > maleic ≃ methylmaleic > dichloromaleic > fumaric and maleic > dichloromaleic ≃ methylmaleic > chloromaleic respectively. The kinetic data are supplemented by stoicheiometric data, by determinations of product distribution, and by spectroscopic data, and they are discussed in terms of a model involving at least partial attack by Cr2+ directly on the CC double bonds.