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 p ractical pK values, pK'(I), of the corresponding hydroquinone (TBQH(2) ) were obtained at given values of I. By use of an empirical Debye-Huc kel (D-H) type approximation, the values of pK' were extrapolated to I = 0 for estimating the thermodynamical pK(a) values for acid dissocia tion of TBQH(2) (pK(a,1) = 10.7 +/- 0.1; pK(a,2) ca. 13.6 +/- 0.2). Op tical absorption spectra and molar extinction coefficients of the tran sient 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-prop anol (2 M) and acetone (0.1 M). Oxidation of TBQH(2) by OH. to eventua lly give TBQ(.-) has been studied (k(15) = 8 x 10(9) M(-1) s(-1) for f ormation of the intermediate [(HO)TBQH(2)](.) in aqueous solution at 2 2 degrees C). The equilibrium for formation of TBQ(.-) from TBQ and TB QH(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 appa rent semiquinone formation constant at pH i, K'(f,i)(I), was determine d 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 b een obtained by combining the two-electron reduction potentials of TBQ and the semiquinone formation constants. Pulse-radiolytic equilibrati on studies of TBQ with O-2(.-) and of TBQ(2-) with DMAP(.) (DMAP(.) = 4-(dimethylamino)phenoxyl) afforded the redetermination of the thermod ynamical standard potential for one-electron reduction of O-2 (E degre es = -0.140 +/- 0.012 V vs NHE for 1 M O-2 in aqueous solution at 22 d egrees 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 NH E 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 subst ituent and solvent effects as well as in view of the Marcus theory of electron transfer. Another objective of the present study is to contri bute to a better understanding of the properties of the synthetic anti oxidant 2- or 3-tert-butyl-4-hydroxyanisole (BHA).