The Benzene-photosensitized Isomerization of Butene-2 and Pentene-2

The benzene-photosensitized cis-trans isomerization of butene-2 has been reinvestigated, using 2537 Å as the light source. A comparative study with pentene-2 has also been carried out. In all experiments, pentene-2 behaved very similarly to butene-2. The quantum yield of the isomerization was estimated to be 0.27±0.03, irrespective of the olefin used. The following photostationary ratios of the cis to the trans isomer were obtained: 1.05±0.1 for butene-2, and 0.99±0.1 for pentene-2. The ratio of the initial rates of isomerization was, for both olefins, 1.15±0.15, in favor of the isomerization from the cis to the trans isomer. Both ratios are very close to unity; this finding is in agreement with our previous results (M. Tanaka, T. Terao and S. Sato, Bull. Chem. Soc. Japan, 38, 1645 (1965)).