Mechanistic aspects of osmium(VIII) catalysed oxidation of l-glutamic acid by copper(III) periodate complex in an aqueous alkaline medium

Abstract

The kinetics of oxidation of l-glutamic acid (GLU) by diperiodatocuprate(III) (DPC) has been investigated in the presence of osmium(VIII) as homogeneous catalyst in alkaline medium at a constant ionic strength of 0.11 mol dm⁻³ spectrophotometrically. The reaction exhibits 1:4 stoichiometry ([GLU]:[DPC]). The order of the reaction with respect to [DPC] was unity, while the order with respect to [GLU] was less than unity over the concentration range studied. The rate was increased with an increase in [OH⁻] and decreased with an increase in [IO₄ ⁻]. The order with respect to [Os(VIII)] was unity. The ionic strength and dielectic constant of the medium did not affect the rate significantly. The main product succinic acid was identified by spot tests, FT-IR, and LC–MS spectral studies. Based on the experimental results, the possible mechanism was proposed. The reaction constants involved in the different steps of the mechanism were evaluated. The activation parameters with respect to the slow step of the mechanism were computed and also thermodynamic quantities determined. Kinetic studies suggest that the active species of DPC and Os(VIII) are found to be [Cu(H₂IO₆ )(H₂O)₂] and [OsO₄(OH)₂]²⁻, respectively.

Graphical abstract

Appendix

According to Scheme 2:

$${\text{Rate}} = \frac{{ - {\text{d}}\left[ {\text{DPC}} \right]}}{{{\text{d}}t}} = k\left[ {\text{Complex}} \right]\left[ {\text{C}} \right],$$

$${\text{Rate}} = \frac{{kK_{1} K_{2} K_{3} \left[ {\text{DPC}} \right]\left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]\left[ {{\text{OH}}^{ - } } \right]\left[ {\text{GLU}} \right]}}{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right]}}.$$

(9)

Total concentration of DPC is given by

$$\left[ {\text{DPC}} \right]_{T} = \left[ {\text{DPC}} \right]_{f} + \left[ {{\text{Cu}}\left( {{\text{H}}_{ 3} {\text{IO}}_{ 6} } \right)} \right]^{2 - } + \left[ {{\text{Cu}}\left( {{\text{OH}}_{ 2} } \right)_{ 2} \left( {{\text{H}}_{ 2} {\text{IO}}_{ 6} } \right)} \right] + {\text{Complex}}\,{\text{C,}}$$

where T and f refer to total and free concentrations. Therefore,

$$\left[ {\text{DPC}} \right]_{T} = \left[ {\text{DPC}} \right]_{f} + K_{1} \left[ {\text{DPC}} \right]\left[ {{\text{OH}}^{ - } } \right] + \frac{{K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right]\left[ {\text{DPC}} \right]_{f} }}{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right]}} + \frac{{K_{1} K_{2} K_{3} \left[ {\text{GLU}} \right]\left[ {\text{DPC}} \right]_{f} \left[ {{\text{OH}}^{ - } } \right]}}{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right]}},$$

$$\left[ {\text{DPC}} \right]_{T} = \left[ {\text{DPC}} \right]_{f} \left\{ {1 + K_{1} \left[ {{\text{OH}}^{ - } } \right] + \frac{{K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right]}}{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right]}} + \frac{{K_{1} K_{2} K_{3} \left[ {\text{GLU}} \right]\left[ {{\text{OH}}^{ - } } \right]}}{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right]}}} \right\},$$

$$\left[ {\text{DPC}} \right]_{f} = \left\{ {\frac{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right]\left[ {\text{DPC}} \right]_{T} }}{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} K_{3} \left[ {\text{GLU}} \right]\left[ {{\text{OH}}^{ - } } \right]}}} \right\}.$$

(10)

Similarly

$$\left[ {\text{GLU}} \right]_{T} = \left[ {\text{GLU}} \right]_{f} + {\text{Complex C,}}$$

$$\left[ {\text{GLU}} \right]_{T} = \left[ {\text{GLU}} \right]_{f} \left\{ { 1+ K_{ 3} \left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]} \right\},$$

In view of low concentration of Os(VIII) used, the term K ₃[Os(VIII)] can be neglected. Hence

$$\left[ {\text{GLU}} \right]_{T} = \left[ {\text{GLU}} \right]_{f} .$$

(11)

Similarly

$$\left[ {{\text{OH}}^{ - } } \right]_{T} = \left[ {{\text{OH}}^{ - } } \right]_{f} .$$

(12)

The concentration of Os(VIII) is given by

$$\left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]_{T} = \left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]_{f} + \left[ {\text{C}} \right],$$

$$\left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]_{T} = \left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]_{f} + K_{3} \left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]\left[ {\text{GLU}} \right]_{f} ,$$

$$[{\text{Os}}({\text{VIII}})]_{T} = \left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]_{f} + \left\{ {1 + K_{3} \left[ {\text{GLU}} \right]} \right\},$$

$$\left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]_{f} = \frac{{\left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]_{T} }}{{1 + K_{3} \left[ {\text{GLU}} \right]}}.$$

(13)

Substituting Eqs. (9), (10), (11), and (12) in Eq. (8), we get

$$\frac{{{\text{d}}\left[ {\text{DPC}} \right]}}{{{\text{d}}t}} = k_{\text{obs}} = \frac{{kK_{1} K_{2} K_{3} \left[ {\text{GLU}} \right]\left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]}}{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} K_{3} \left[ {{\text{OH}}^{ - } } \right]\left[ {\text{GLU}} \right]}}.$$

(14)

$$k_{\text{obs}} = \frac{\text{Rate}}{{\left[ {\text{DPC}} \right]}} = \frac{{kK_{1} K_{2} K_{3} \left[ {\text{GLU}} \right]\left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{Os}}\left( {\text{VIII}} \right)} \right]}}{{\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{H}}_{ 3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} K_{3} \left[ {{\text{OH}}^{ - } } \right]\left[ {\text{GLU}} \right]}}.$$

(15)