Solvent effects on the radical scavenging activity of HP-136 benzofuranone: mechanistic and kinetic insights from quantum chemical calculations.

The radical-scavenging activity of Irganox HP-136 (5,7-di-tert-butyl-3-(3,4-dimethylphenyl)-3H-benzofuran-2-one), a representative benzofuranone antioxidant used in polymer stabilization, was systematically investigated using quantum chemical calculations. The reactivity of HP-136 toward a series of environmentally relevant radicals was evaluated, with particular emphasis on the highly reactive hydroxyl radical (HO˙). In the gas phase, HP-136 exhibited outstanding HO˙ scavenging activity, with an overall rate constant of 2.06 × 1010 M-1 s-1, predominantly governed by formal hydrogen transfer (FHT) at the C3-H and C12-H positions. In contrast, reactions with HOO˙ radicals were several orders of magnitude slower. In benzene, HP-136 showed high reactivity toward the tert-butoxyl radical (TBO˙), with a calculated rate constant of 3.50 × 105 M-1 s-1, in good agreement with the experimental value (1.35 × 105 M-1 s-1). HP-136 exhibited relatively low scavenging activity toward HOO˙ and 2-cyanoprop-2-yl (AI˙) radicals in both lipid and polar media. The reactivity toward TBO˙ and cumyloxyl (CMO˙) radicals was moderate in benzene and DMSO, with overall rate constants ranging from 103 to 105 M-1 s-1. The results further revealed that the sequential proton loss-electron transfer (SPLET) mechanism is not rate-determining in the DPPH˙, and AI˙ scavenging reactions of HP-136 in DMSO, although it contributes significantly to the antiradical activity toward TBO˙ and CMO˙ radicals in polar aprotic media.