Structures of butyl ions formed by electron impact ionization of isomeric butyl halides and alkanes

Abstract

Using a pulsed ion cyclotron resonance (ICR) spectrometer, it is demonstrated that at pressures of about 10/sup -6/ Torr and at observation times ranging from 10/sup -3/ to 0.5 s, isobutane, neopentane, 2,2-dimethylbutane, isobutyl halides, and tert-butyl halides form C/sub 4/H/sub 9//sup +/ ions having the tertiary structure. In n-alkanes, 2-methylbutane, 3-methylpentane, n-butyl halides, and sec-butyl halides, both sec-C/sub 4/H/sub 9//sup +/ and t-C/sub 4/H/sub 9//sup +/ ions are observed, the sec-C/sub 4/H/sub 9//sup +/ ions surviving without rearrangement for at least 0.1 s. However, in the case of the halides, a collision-induced isomerization of the sec-C/sub 4/H/sub 9//sup +/ to the t-C/sub 4/H/sub 9//sup +/ ions occurs. The efficiency of this process increases with the basicity of the alkyl halide. Radiolysis experiments carried out at atmospheric pressures indicate, in agreement with ICR and solution experiments, that at times as short as 10/sup -10/ s the majority of the i-C/sub 4/H/sub 9//sup +/ ions from isobutyl bromide rearrange to the t-C/sub 4/H/sub 9//sup +/ structure. On the other hand, in the radiolysis of both n-hexane and 3-methylpentane, the abundance of t-C/sub 4/H/sub 9//sup +/ relative to sec-C/sub 4/H/sub 9//sup +/ is substantially smaller than that observed in the ICR experiments,more » and decreases with decreasing collision interval. It is suggested that about 90% of the i-C/sub 4/H/sub 9//sup +/ can rearrange to t-C/sub 4/H/sub 9//sup +/ by simple 1,2-hydride shift without involving secondary or protonated methylcyclopropane structures as intermediates. 4 figures, 2 tables.« less