77PD-Quinone: Synthesis, Coho Salmon Toxicity Assessment, and Comparison with the Commercial Antidegradant 77PD

Since the emergence of reports demonstrating the toxicity of 6PPD-quinone towards coho salmon (Oncorhynchus kisutch), there has been a growing interest in assessing the toxicity of other para-phenylenediamine-based rubber additives, and their quinone transformation products. A technique for the multi-gram scale synthesis of 2,5-diaminoquinones was developed to prepare 2,5-bis((5-methylhexan-2-yl)amino)cyclohexa-2,5-diene-1,4-dione (77PD-quinone) for toxicity assessment. The toxicities of N,N'-bis(5-methyl-2-hexanyl)-1,4-benzenediamine (77PD) and 77PD-quinone were evaluated in coho salmon. Juvenile coho salmon were exposed to a geometric series of five test concentrations of 77PD or 77PD-quinone, a negative control (dilution water), and a solvent control (100 µL/L dimethylformamide) for 96 hours under flow-through conditions. 77PD was found to be toxic to coho salmon with a 96‑hour LC50 value of 24 µg/L active ingredient (a.i.), and the NOEC was 13 µg a.i./L. No mortality was observed for 77PD-quinone at the highest attainable dose level, 226 µg a.i./L, at which signs of test water saturation were observed.