Reactivities Of D-Mannitol And Related Alcohols Toward Chromium(Vi) And Chromium(Iv)

The kinetics of the reactions between Cr(VI) and a homologous series of reductants, formed by CH3OH and polyalcohols of the type CH2OH(CHOH)(n-2)CH2OH with n = 2-6, have been studied in perchloric acid aqueous solutions. The rate law was: nu = k(4)[Cr(VI)][alcohol][H+](2). The activation parameters were in the range DeltaH(not equal)degrees = 37.4-43.1 kJ mol(-1) and DeltaS(not equal)degrees: = (-168) - (-139) J K-1 mol(-1). The reactions were inhibited by the Cr(IV)-trapping agent Mn(II), and this effect was used to obtain the rate constants for the 2-electron reduction of the intermediate Cr(IV) by the alcohols (k(2) = 52.2-166 M-1 s(-1)). A mechanism is proposed according to which Cr(VI) is reduced by the alcohol of n carbon atoms through n(n + 1)/2 different reaction pathways involving chromate monoesters and diesters as intermediates, whereas Cr(IV) is reduced through n reaction pathways by direct hydride-ion transfer.