STRUCTURE-REACTIVITY CORRELATIONS IN THE REACTIONS OF HYDROCARBONS ONTRANSITION-METAL SURFACES .1. RING CONTRACTION OF CYCLOOCTENE, 1,3-CYCLOOCTADIENE, 1,5-CYCLOOCTADIENE, AND CYCLOOCTATETRAENE TO BENZENE ON A PLATINUM(111) SURFACE

The adsorption and subsequent reactions of the C-8 cyclic hydrocarbons cyclooctane (COA), cyclooctene (COE), 1,3-cyclooctadiene (1,3-COD), 1 ,5-cyclooctadiene (1,5-COD), and cyclooctatetraene (COT) have been stu died on a platinum(111) single crystal surface. On clean Pt(111), the majority of the adsorbed COA desorbs molecularly, whereas all of the u nsaturated hydrocarbons are dehydrogenated to COT and then converted t o benzene. In several cases, intermediates in the dehydrogenation path way can be identified spectroscopically; for example, COE is dehydroge nated to COT via the diene 1,3-COD. In all cases, whether added direct ly to the surface or formed via dehydrogenation reactions, COT is boun d to the surface initially in a tub-shaped eta4 fashion and is convert ed to a planar eta8 structure at higher temperatures. The conversion o f COT to benzene follows thereafter. Co-adsorption experiments with CO T and COT-d8 indicate that the majority of the benzene is formed via a n intramolecular rearrangement and not by complete fragmentation of CO T to acetylene followed by cyclotrimerization. We propose that, at 475 K, COT undergoes ring contraction to form bicyclo[4.2.0]-octa1,3,5-tr iene(BOT). The BOT is then transformed via a retro[2+2]cyclization to benzene, which desorbs, and acetylene, which is dehydrogenated first t o surface ethynyls and then to a surface carbon overlayer. The reactio ns of these cyclic C-8 hydrocarbons have also been studied in the pres ence of coadsorbed H-2: 1,5-COD and 1,3-COD hydrogenate to COE and iso merize to a distribution of cyclooctadienes. Interestingly, COT and CO E neither react with coadsorbed H-2 nor undergo deuterium exchange wit h coadsorbed D2. Correlations between the structures of these C-8 cycl ic hydrocarbons and the reactions they undergo are discussed.