Study on the pyrolytic mechanism of butanethiols using low pressure pyrolysis method

Low pressure pyrolysis of deuterium-labelled 1-butanethiol (1, 1-d2, 2, 2-d2, 4, 4, 4-d3), 2-butanethiol and 2-methyl-2-propanethiol was carried out at 10-2 Pa and below 1110K (Table 1), in order to investigate the following reaction mechanism proposed for pyrolysis of 1-butanethiol in our previous paper. n-C4H9SH-> CH3(CH2)2CH2[sm middot]+[sm middot]SH ->C2H5CH=CH2+H2S CH3(CH2)2CH2[sm middot]->C2H4+C2H5[sm middot] C2H5[sm middot]->C2H4+H Molecular weights of ethylene and 1-butene from deuterium-labelled 1-butanethiol were measured. The formers were 28, 29, 30 for 1, 1-d2, 28, 30 for 2, 2-d2 and 28, 29, 30for 4, 4, 4-d3 respectively (Table 2). This distribution can be accounted for by 1, 4hydrogen shift of butyl radical in this reaction. The latters were 58 for 1, 1-d2, 57for 2, 2-d2 and 59 for 4, 4, 4-d3 respectively. This distribution suggests that H2S elimination proceeds through withdrawal of mercapto group and β-hydrogen. 2-Butanethiol decomposed above 900 K (Fig.1) to yield propylene, butenes, methane and hydrogen sulfide (Figs.2 and 3). Compared with other experiments, which had been carried out at higher pressure than the present experiment, the distinctive feature of low pressure pyrolysis of 2-butanethiol consists in the formation of propylene. Estimating the kinetic feature of s-butyl radical based on the reported values, s-butyl radical, once produced, decomposes selectively to yield propylene and methyl radical under these conditions. Therefore, it is suggested that propylene and butenes observed reflect competition of two pathways, i. e. C-S bond cleavage and H2S elimination respectively. The RRK/2 calculation supported this suggestion (Fig.4). It is also noticed that the composition of butenes in this reaction is similar to that of butenes produced by pyrolysis of 2-chlorobutane. This similarity suggests that butenes are produced by the withdrawal of mercapto group and β-hydrogen of 2-butanethiol. 2-Methyl-2-propaneth iol decomposed above 850 K to yield 2-methylpropene and hydrogen sulfide stoichiometrically. Since 2-methylpropene can be formed through not only t-butyl radical but also H2 Selimination, no differentiation can be made experimentally. The RRKM calculation (Table 3) and the amount of hydrogen sulfide observed, however, seem to suggest that the main reaction is H2S elimination. It is concluded that butanethiols decom pose through two pathways. The ratio of two reactions seems to depend on molecular structure, because the proporti