Ascorbate: antioxidant activity determined in a hydrocarbon phase using quaternary ammonium salts as phase-transfer agents

By shaking sodium ascorbate with tetra-n-butyl ammonium bromide in a two-phase system consisting of water and organic phases, such as 1,1,2,2-tetrachloroethane (TCE) or chlorobenzene, sufficient concentrations of the quaternary ammonium-ascorbate (quat-ascorbate) are transferred into the organic phase to be useful for antioxidant studies in non-polar solvents. The antioxidant activity of the quat-ascorbate was determined by measurement of the (inhibition) rate constant, kinh, for the inhibition of autoxidation of styrene initiated thermally by azobisisobutyrylnitrile (AIBN) at 30 °C. The kinh of ascorbate in styrene, found to be (9.63 ± 2.0) × 104 M−1 s−1, is very much lower than that found for ascor-byl-6-palmitate, kinh = (110 ± 11) × 104 M−1 s−1 under these conditions. The differences are discussed in terms of the relative effectiveness of hydrogen-atom transfer from the enolic hydroxyl at position 2 of the ascorbate ion compared with the more facile such transfer from position 3 of ascorbyl 6-palmitate in a nonpolar medium.