Antioxidants and stabilizers, CXIII. Oxidation products of the antidegradant ethoxyquin

In connection with the study of the mechanism of antioxidant action of 6‐ethoxy‐2,2,4‐trimethyl‐1,2‐dihydroquinoline (ethoxyquin, I) and of its ecological responses in stabilized polymers we studied its oxidation with some selected agents and the properties of products thus obtained. The oxidation of I with silver oxide or lead dioxide proceeds by two main routes. One of them leads to 8‐(6‐ethoxy‐2,2,4‐trimethyl‐1,2‐dihydro‐1‐quinolinyl)‐6‐ethoxy‐2,2,4‐trimethyl‐1,2‐dihydroquinoline (IV), which is further oxidized to the blue compound 8‐(6‐ethoxy‐2,2,4‐trimethyl‐1,2‐dihydro‐1‐quinolinyl)‐2,2,4‐trimethyl‐6‐quinolone (IX). In the second route position 6 is attacked and 2,2,4‐trimethyl‐6‐quinolone (VII) is formed, which is stable under the conditions used, but is oxidized further with m‐chloroperbenzoic acid, giving rise to 2,2,4‐trimethyl‐6‐quinolone‐N‐oxide (VIII). The oxidation of ethoxyquin with potassium permanganate also gives rise to dimer IV and not to 1,1′‐bis(6‐ethoxy‐2,2,4‐trimethyl‐1,2‐dihydroquinoline) (III) reported in the literature. Potassium nitrosodisulfonate oxidizes I with formation of 6‐ethoxy‐2,2,4‐trimethyl‐8‐quinolone (X). The oxidation of ethoxyquin with m‐chloroperbenzoic acid gives rise to 6‐ethoxy‐2,2,4‐trimethyl‐1,2‐dihydroquinoline‐N‐oxide (V) and dimer IV. Nitroxide V was obtained in the crystalline state. In the presence of acids, and particularly on the surface of silica gel it decomposes to ethoxyquin and nitrone VIII. Nitroxide V is readily reduced to the starting ethoxyquin. The transitionally formed 6‐ethoxy‐1‐hydroxy‐2,2,4‐trimethyl‐1,2‐dihydroquinoline (XIV) readily disproportionates to I and V.