Direct infrared spectroscopic evidence of allyl radical and definition of the initiation step in thermal decomposition of cycloalkanes.: A comparison with very low pressure pyrolysis of alkenes

Abstract

The very low pressure pyrolysis (VLPP) of both C 4–C 8 cycloalkanes and their corresponding isomeric 1-alkenes as well as 2-heptene (750–950°C, 10 −3 s, 10 −2–10 −3 Torr) was studied using the low temperature matrix infrared spectroscopy method (LTM/IR) to identify radicals and to evaluate reactions responsible for their generation. Apart from the pyrolysate of cyclobutane the reaction products of all other cycloalkanes exhibit absorption bands in the spectra due to the allyl radical (518, 808 and 985 cm 1̄) besides those of ethene, propene and the isomeric 1-alkenes. Moreover, in the case of cyclohexane the methyl radical and in the case of cyclopentane and of cycloheptane the ethyl radical were identified amongst cyclopropane and cyclopentane, respectively. The VLPP/LTM/IR spectra of 1-alkenes on their part show absorption bands due to both allyl and methyl (from C 4H 8, C 6H 12, C 8H 16) or ethyl (from C 5H 10, C 7,H 14) radicals. These results are considered to be direct evidence of the initiation step of the conventional radical chain process of cycloalkanes pyrolysis consisting in the generation of radicals from isomeric 1-alkenes which are considered primary products of the unimolecular thermal 1-alkenes conversion.