Kinetics and mechanism of the oxidation of D-galactono-1,4-lactone by CrVI and CrV

Abstract

The oxidation of d-galactono-1,4-lactone by Cr VI yields d-lyxonic acid, carbon dioxide and Cr 3+ as final products when an excess of sugar acid over Cr VI is used. The redox reaction occurs through Cr VI → Cr III and Cr VI → Cr V → Cr III paths. The complete rate law for the Cr VI oxidation reaction is expressed by −d [Cr VI] \\dt = (k 0+k H [H +] ) [gal] [Cr VI] , where k 0 = (31±3) ×10 −4 M −1 s −1 and k H = (99±5) ×10 −4 M −2 s −1, at 40°C. Cr V is formed in a rapid step by reaction of the CO · − 2 radical with Cr VI. Cr V reacts with the substrate faster than does Cr VI. The Cr V oxidation follows the rate law : −d [Cr V] \\dt = ( k ′ 0 +k ′ H [H +] ) [gal] , where k ′ 0 = (15±2) ×10 −3 M −1 s −1 and k ′ H = (34±4) ×10 −3 M −2 s −1, at 40°C. The EPR spectra show that several intermediate [Cr (O) (gala) 2] − linkage isomers are formed in rapid pre-equilibria before the redox steps.