Mechanistic Investigation of the Oxidative Cleavage of the Carbon–Carbon Double Bond in α,β-Unsaturated Compounds by Hexachloroiridate(IV) in Acetate Buffer

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  • Contract grant sponsor: University Grants Commission, New Delhi, India.

ABSTRACT

The hexachloroiridate(IV) oxidation of α,β-unsaturated compounds such as acrylic acid, acrylamide, and acrylonitrile (CH2=CHX; X = –COOH, –CONH2, and –CN) was carried out in NaOAc-AcOH buffer medium. The reaction follows complex kinetics, being first order in [IrIV] and complex order in [CH2=CHX]. H+ ion has no effect on the reaction rate in the pH range 3.42–4.63. The pseudo–first-order rate constant decreases with a decrease in the dielectric constant and with a decrease of ionic strength of the medium. The oxidation rate follows the sequence: acrylonitrile > acrylamide > acrylic acid. A mechanism is proposed involving the formation of an unstable intermediate complex between the substrate and the oxidant which is transformed to the radical cation in a slow rate-determining step with the concomitant reduction of Ir(IV) to Ir(III). The radical cation subsequently decomposes to the aldehyde that appears as the ultimate product of the carbon–carbon double bond cleavage. The major product of oxidation was identified as HCHO by 1H NMR. Activation parameters for the slow rate-determining step and thermodynamic parameters associated with the equilibrium step of the proposed mechanism have been evaluated. The enthalpy of activation is linearly related to the entropy of activation, and this linear relationship confirms that the oxidation of all the α,β-unsaturated compounds follows a common mechanism.

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