Structural information unsaturated hydrocarbons

Perhaps the next simplest molecular adsorbates for which quantitative structural information exists are the unsaturated C2 hydrocarbons, notably acetylene (ethyne, HC CH) and ethylene (ethene, H2C=CH2), adsorbed on a number of metal surfaces (especially, Cu, Ni and Pd), and also on Si(100), studied by LEED, SEXAFS, and PhD. In some systems adsorption of ethylene is accompanied by a surface reaction. In particular, on both Pt(lll) [74] and Rh(lll) [75] ethylene is converted to an ethylidyne species, H3C—C—, which bonds to these surfaces through the C atom with the —C axis essentially perpendicular to the surface, in three-fold coordinated hollow sites. In addition, ethylene adsorbed on Ni(l 11) at low temperature dehydrogenates to produce adsorbed acetylene as the surface is warmed towards room temperature this particular system actually provided the first example of the... 

A trimer of norbornadiene, m.p. 201°, was obtained from the reaction of the olefin with Ni[P(CaH5)3]4 as the catalyst 58). Except that it is unsaturated, no further structural information is presently available on this hydrocarbon. Rhodium on carbon also catalyzes norbornadiene dimerization and trimerization (45). Three dimers, (XXIII), (XXIX), and (XXX) or (XXXI) are formed in the ratio of 4 84 12, in 70-80% yield at 90% conversion (45). In addition, 20-30% of a crystalline trimer, m.p. 67-68°, was isolated. On the basis of NMR and IR measurements it could have either structure (LIV) or (LV). 

Information on atomic structure and the periodic table, chemical reactions, organic compounds, saturated and unsaturated hydrocarbons, and metabolic systems... 

Transition States in Acid-Cataiyzed Reactions on Zeolites. A considerable number of experimental attempts were carried out to elucidate the structure of intermediates in the acid-catalyzed reactions. This information may be used to deduce the nature of elusive transition states, which are present on the catalytic surface in concentrations far below detection limit. For example, adsorption of alcohols and unsaturated hydrocarbons on acidic zeolites gives rise to the MAS NMR signal attributed to alkoxy species (135). Transition of these surface complexes to the corresponding carbenium ions, an important step in their further transformations, was shown to have low activation energy. In the synthesis of methyl-tetrabuthyl ether on zeolite Beta, features due to secondary and tertiary carbon atoms in alkoxy species have been observed. This observation along... 

The effect of cholesterol on the thermotropic phase behavior of PC bilayer also varies significantly with the structure, particularly the degree of unsaturation, of the hydrocarbon chains, with more highly unsaturated PCs exhibiting a reduced miscibility with cholesterol and other sterols. Moreover, the structure of the lipid polar headgroup is also important in determining the effect of cholesterol on the host lipid, as is the structure of the sterol molecule itself. For more information on the application of DSC to the biologically important area of lipid-sterol interactions, the reader is referred to recent reviews (23-25). 

The lowest level structure descriptor is simply the count of atoms. However, only in alkanes are the properties even roughly proportional to the number of atoms. More information-rich structure indicators are needed. In the earliest topological methods, numerical indexes were developed simply by counting the number of edges or pairs of adjacent edges. Wiener,Platt,and others " developed structure indexes that produced interesting correlations to some properties of saturated hydrocarbons. As impressive as this work is, it could not be readily or usefully extended to unsaturated compounds or to heteroatom-... 

Both compounds were shown to be acetates when activity was lost on reduction with LiAlILt and regained after treatment of the products with acetyl chloride. Hydrogenation and further reduction to a hydrocarbon skeleton of both compounds afforded identical products. MS spectra were indicative of doubly and triply unsaturated C14 acetates and only the latter component gave a product, 3-ketobutan-l-yl acetate, on ozonolysis. Finally, NMR spectra were obtained, and the information was used to derive structures (332 a) and (333) for the sex pheromones. These structures were confirmed by synthesis. 

---