Pyrolysis and decomposition pathways of alphacypermethrin under non-oxidative conditions

The objective of the work is to understand the decomposition of alphacypermethrin which is one of the most common pyrethroid pesticides, and to examine the formation of pollutants formed during decomposition. This article reports the experimental results of the thermal decomposition of alphacypermethrin under non-oxidative conditions. The experiments were conducted in a tubular reactor at atmospheric pressure. The reaction variables considered were temperature (300-600 °C) and flow rate (27.8-18.2 cm³/min) which was adjusted to maintain a residence time of 5 s. The pesticide was slowly vaporised at an evaporation rate of 70 μg/min at a temperature of 185°C. The decomposition of alphacypermethrin started around 375°C and involved an unusual vinylcyclopropane rearrangement-cum-aromatisation reaction. At higher temperatures, alphacypermethrin was aromatised into 3-phenoxyphenyl nitrile acetic acid 3-methyl phenyl ester with the concomitant loss of hydrogen chloride molecules. The presence of hydrogen chloride gas was confirmed by FTJR spectroscopy. Other products detected and quantified by GC/MS were o-toluic acid, 3-phcnoxybcnzaldehydc, diphenyl ether, phenoxyphenyl acetonitrile, methyl benzonitrile, phenoxybenzonitrile and phenol. Previous studies carried out on permethrin in our laboratory showed that the process of aromatisation was around 20 kcal/mol lower in energy than the direct rupture of the O-CH₂ linkage for temperatures between 400-1000°C. The effect of the CN group in alphacypermethrin compared to permethrin was also investigated by density functional theory (OFT) calculations.