The hydrogenation of olefines - II. The hydrogenation of butene-2 and iso butene

Abstract

The work on the photosensitized hydrogenation of simple olefines described in part I has been extended to some butenes. The identity of the radicals produced in the primary reaction may be determined by analysis of the octane produced by their combination. The results obtained here agree with previous investigations in that the hydrogenation of iso butene yields tert .-butyl radicals, but disagree in that sec .-butyl radicals are found to be the principal product from butene-2. Disproportionation to combination ratios have been obtained for these radicals by the methods previously described. Vapour-phase chromatography has been used to separate the products of the secondary reactions occurring in these systems and analysis of the fractions attempted by mass spectrometry. The variation of the rate of formation of some of the products with pressure has investigated and from this information hot butyl radicals has been found both to decompose in the system thus C 4 H* 9 →CH 3 + C 3 H 6 , and to abstract from the parent olefine, C 4 H* 9 + C 4 H 8 → C 4 H 10 + C 4 H 7 . Addition of a hydrogen atom to the symmetrical olefine butene-2 can yield only a sec .-butyl radical in the absence of rearrangement. The reaction has previously been studied both at room temperature, and at 300°C (Moore & Wall 1949). The results were remarkable in that only very small traces of 3, 4-dimethyl hexane were detected, while in this work it is one of the major products. The same authors, however, found a considerable amount of 3, 4-dimethyl hexane in the hydrogenation products of butene-1. A few experiments carried out here with butene-1 using v. p. c. analysis indicate that the products from either olefine are similar both with respect to molecular weight distribution and relative quantities. The mercury-photosensitized hydrogenation of iso -butene has been studied by Moore & Wall (1949) who found considerable amounts of 2, 2, 3, 3-tetramethyl butane and iso -butane as well as traces of other hydrocarbons.