Mechanistic study on the oxidation of l-phenylalanine by copper(III) in aqueous alkaline medium: a kinetic approach

Abstract

Oxidation of the amino acid l-phenylalanine by diperiodatocuprate(III) in alkaline medium at constant ionic strength of 0.25 mol dm⁻³ was studied spectrophotometrically at different temperatures (298–313 K). The reaction between diperiodatocuprate(III) and l-phenylalanine in alkaline medium exhibits 1:2 stoichiometry. Intervention of free radicals was observed in the reaction. Based on the observed orders and experimental evidence, a mechanism involving monoperiodatocuprate(III) as the reactive oxidant species has been proposed, proceeding through the formation of a complex and reaction of the intermediate of l-phenylalanine with monoperiodatocuprate(III) to give the products. The products were identified by spot test, infrared (IR), and gas chromatography-mass spectrometry (GC-MS). The reaction constants involved in the different steps of the mechanism were calculated. The activation parameters with respect to the slow step of the mechanism were computed and are discussed. The thermodynamic quantities were determined for different equilibrium steps. The isokinetic temperature was also calculated and found to be 331 K.

Graphical Abstract

Appendix

According to Scheme 1

$$ {\text{Rate}} = {\frac{{ - \text{d}[{\text{DPC}}]}}{{{\text{d}}t}}} = k[C] = {\frac{{kK_{1} K_{2} K_{3} \left[ {{\sc{l}}{\text{-PA}}}\right]\left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{Cu}}({\text{OH}})_{2} \left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)_{2} } \right]^{3 - } }}{{\left[ {{\text{H}}_{2} {\text{IO}}_{6}^{3 - } } \right]}}}, $$

(7)

$$ \begin{aligned} \left[ {\text{DPC}} \right]_{\text{T}} & = \left[ {\text{DPC}} \right]_{\text{f}} + \left[ {{\text{Cu}}({\text{OH}})_{2} \left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)\left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)} \right]^{4 - } + \left[ {{\text{Cu}}({\text{OH}})_{2} \left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)} \right]^{ - } + {\text{Complex}}\;({\text{C}}) \\ & = \left[ {\text{DPC}} \right]_{\text{f}} \left[ {{\frac{{\left[ {{\text{H}}_{2} {\text{IO}}_{6}^{3 - } } \right] + K_{1} \left[ {{\text{H}}_{2} {\text{IO}}_{6}^{3 - } } \right]\left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} K_{3} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\sc{l}}{\text{-PA}}}\right]}}{{\left[ {{\text{H}}_{2} {\text{IO}}_{6}^{3 - } } \right]}}}} \right] \\ \end{aligned}, $$

(8)

where [DPC]_T and [DPC]_f refer to total and free DPC concentrations, respectively. The free [DPC] is given by

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

(9)

Similarly, total [OH⁻] can be calculated as

$$ \left[ {{\text{OH}}^{ - } } \right]_{\text{T}} = \left[ {{\text{OH}}^{ - } } \right]_{\text{f}} + \left[ {{\text{Cu}}({\text{OH}})_{2} \left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)\left( {{\text{H}}_{2} {\text{IO}}_{6} } \right)} \right]^{4 - } + \left[ {{\text{Cu}}({\text{OH}})_{2} \left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)} \right]^{ - } + [{\text{C]}} ,$$

(10)

$$ \begin{aligned} & = \left[ {{\text{OH}}^{ - } } \right]_{\text{f}} + K_{1} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{Cu}}({\text{OH}})_{2} \left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)_{2} } \right]^{3 - } + {\frac{{K_{1} K_{2} \left[ {{\text{Cu}}\left( {\text{OH}} \right)_{2} \left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)_{2} } \right]^{3 - } \left[ {{\text{OH}}^{ - } } \right]}}{{\left[ {{\text{H}}_{2} {\text{IO}}_{6}^{3 - } } \right]}}} \\ & \quad + {\frac{{K_{1} K_{2} K_{3} \left[ {{\text{Cu}}\left( {\text{OH}} \right)_{2} \left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)_{2} } \right]^{3 - } \left[ {{\text{OH}}^{ - } } \right]\left[ {{\sc{l}}{\text{-PA}}}\right]}}{{\left[ {{\text{H}}_{2} {\text{IO}}_{6}^{3 - } } \right]}}} \\ \end{aligned}. $$

(11)

In view of the low concentrations of DPC used, the second, third, and fourth term in the above equation are neglected; therefore,

$$ \left[ {\text{OH}} \right]_{\text{T}} = \left[ {\text{OH}} \right]_{\text{f}}, $$

(12)

similarly

$$ \left[ {{\sc{l}}{\text{-PA}}}\right]_{\text{T}} = \left[ {{\sc{l}}{\text{-PA}}}\right]_{\text{f}} .$$

(13)

Substituting Eqs. 9, 10, and 12 into Eq. 7, we get

$$ {\text{Rate}} = {\frac{{kK_{1} K_{2} K_{3} \left[ {{\sc{l}}{\text{-PA}}}\right]\left[ {{\text{OH}}^{ - } } \right]\left[ {\text{DPC}} \right]}}{{\left[ {{\text{H}}_{2} {\text{IO}}_{6}^{3 - } } \right] + K_{1} \left[ {{\text{H}}_{2} {\text{IO}}_{6}^{3 - } } \right]\left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} K_{3} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\sc{l}}{\text{-PA}}}\right]}}}. $$