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

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 degrees C and given ionic strength I. From E(m)(pH), the practical pK values, pK'(I), of the corresponding hydroquinone (TBQH(2)) were obtained at given values of I. By use of an empirical Debye-Huckel (D-H) type approximation, the values of pK' were extrapolated to I = 0 for estimating the thermodynamical pK(a) values for acid dissociation of TBQH(2) (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 degrees C in the presence of 2-propanol (2 M) and acetone (0.1 M). Oxidation of TBQH(2) by OH. to eventually give TBQ(.-) has been studied (k(15) = 8 x 10(9) M(-1) s(-1) for formation of the intermediate [(HO)TBQH(2)](.) in aqueous solution at 22 degrees C). The equilibrium for formation of TBQ(.-) from TBQ and TBQH(2) 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 TBQH(2) (log K-17 = -14.4 +/- 0.3) and of TBQ(.-) from TBQ and TBQ(2-) (log K-18 = 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 degrees (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 O-2(.-) and of TBQ(2-) with DMAP(.) (DMAP(.) = 4-(dimethylamino)phenoxyl) afforded the redetermination of the thermodynamical standard potential for one-electron reduction of O-2 (E degrees = -0.140 +/- 0.012 V vs NHE for 1 M O-2 in aqueous solution at 22 degrees 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 degrees 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).