2-メチル-2-ペンテン, 3-メチル-2-ペンテン, 2-メチル-1-ペンテン, 2-メチル-1-ブテンおよび2-メチル-2-ブテンの熱分解反応

抄録

Thermal decomposition of 2-methyl-2-pentene [I], 3-methyl-2-pentene[II], 2-methyl-1-pentene [III], 2-methyl-1-butene [IV] and 2-methyl-2-butene [V] was studied in a flow apparatus under an atmospheric pressure, at temperatures ranging from 500 to 750°C, residence times from 0.05 to 40sec and nitrogen/alkene mole ratio of 15. The main reaction products are methane and isoprene from [I] and [II], ethylene and isobutene from [III], methane, isobutene and isoprene from [IV] and [V]. The reaction products from each of the alkenes examined are compared with those from the corresponding linear alkenes, i.e. 2-pentene, 1-pentene, 1-butene and 3-butene. It is found that the structural difference between two kinds of products mentioned above is attributed only to the existence of the methyl groups in reactants. The overall reaction rates obey the first order rate equation with rate constants being several times larger than those of the corresponding linear alkenes. The main reaction can be explained by a free radical chain process, while a molecular process is also expected in the decomposition of 2-methyl-1-pentene.
Based on the product distribution, the ratio of addition to hydrogen atom abstraction as well as the relative rate of addition to the two positions of alkenes at 650°C are estimated. The ratios of addition to abstraction range from 3 to 15 for reactions of hydrogen atom with a series of alkenes, and from 0.1 to 1 for reactions of methyl radical. The ratios of addition to the less substituted olefinic carbon atom to the more substituted one range from 3 to 4.