Equilibria and Rates of Redox Reactions Involving the 2-tert-Butyl-1,4-benzosemiquinone Radical in Aqueous Solution: An Investigation by Potentiometry, ESR, and Pulse Radiolysis

Article abstract of DOI:10.1021/j100004a023

Midpoint potentials, E_m, for two-electron reduction of 2-tert-butyl-1,4-benzoquinone (TBQ) were measured in aqueous solution as a function of pH at 22 deg C and given ionic strength I.From E_m(pH), the practical pK values, pK'(I), of the corresponding hydroquinone (TBQH2) were obtained at given values of I.By use of an empirical Debye-Hueckel (D-H) type approximation, the values of pK' were extrapolated to I = 0 for estimating the thermodynamical pK_a values for acid dissociation of TBQH2 (pK_a,1 = 10.7 +/- 0.1; pK_a,2 ca. 13.6 +/- 0.2).Optical absorption spectra and molar extinction coefficients of the transient semiquinone radical (TBQH^. and TBQ^.-) were obtained by pulse-radiolytic one-electron reduction of TBQ in aqueous solution.The pK_a of TBQH^. is 4.3 +/- 0.1 at 22 deg C in the presence of 2-propanol (2 M) and acetone (0.1 M).Oxidation of TBQH2 by OH^. to eventually give TBQ^.- has been studied (k₁₅ = 8 x 1E9 M^-1 s^-1 for formation of the intermediate <(HO)TBQH2>^. in aqueous soluiton at 22 deg C).The equilibrium for formation of TBQ^.- from TBQ and TBQH2 was investigated by ESR (aqueous solution, pH 6.8-9.4, I = 0.12 M).Hyperfine couplings and the g value of TBQ^.- are given.The apparent semiquinone formation constant at pH i, K'_f,i(I), was determined as a function of pH (log K'_f,7 = -8.8 +/- 0.1, pH 7) at I = 0.12 M.The pH-independent thermodynamical formation constants, at I = 0, of TBQH^. from TBQ and TBQH2 (log K₁₇ = -14.4 +/- 0.3) and of TBQ^.- from TBQ and TBQ^2- (log K₁₈ = 1.3 +/- 0.4) were determined by use of the above pK_a values and D-H type approximations.Various other one-electron-transfer reactions involving TBQH^. or TBQ^.- have been investigated in detail.The standard potentials, E^o (at I = 0), for one-electron reduction of TBQ and of TBQH^. (or TBQ^.-) have been obtained by combining the two-electron reduction potentials of TBQ and the semiquinone formation constants.Pulse-radiolytic equilibration studies of TBQ with O2^.- and of TBQ^2- with DMAP^. (DMAP^. = 4-(dimethylamino)phenoxyl) afforded the redetermination of the thermodynamical standard potential for one-electron reduction of O2 (E^o = -0.140 +/- 0.012 V vs NHE for 1 M O2 in aqueous solution at 22 deg C and I = 0) and of the midpoint potential of the couple DMAP^./DMAP^-, respectively, at pH 13.5 (E_m,13.5 = 0.10 +/- 0.02 V vs NHE at 22 deg C and I = 0.5 M).The respective rate and equilibrium constants are given.All these results are discussed in terms of substituent and solvent effects as well as in view of the Marcus theory of electron transfer.Another objective of the present study is to contribute to a better understanding of the properties of the synthetic, antioxidant 2- or 3-tert-butyl-4-hydroxyanisole (BHA).