EPR study of radical intermediates from the oxidation of 6-ethoxy-2,2,4-trimethyl- and 6-ethoxy-2,2,4,8-tetramethyl-1,2-dihydroquinoline

The EPR spectra of 1,2-dihydro-6-ethoxy-2,2,4-trimethyl- and 1,2-dihydro-6-ethoxy-2,2,4,8-tetramethyl-quinolin-1-yl radicals were observed in heptane solution. The hyperfine splittings showed that this class of radical is extensively delocalised with significant spin density at C(8). Both radicals decayed by second-order processes, the rate constants being 5 × 106 and 4 × 102 dm3 mol-1 s-1, respectively at 273 K. The latter reaction is much slower because the 8-methyl substituent blocks the formation of the 1,8′-dimer. Both radicals reacted with oxygen to give the corresponding nitroxides, although reaction was very slow for the 8-methyl derivative. A mechanism is proposed to rationalise product formation from 1,2-dihydro-6-ethoxy-2,2,4-trimethylquinoline when used as an antioxidant.