Platinum ions hydrocarbon reactions

Immeasurable ions are formed when hydrocarbons in an air stream are oxidized in the presence of a heated platinum filament (3,10). The experimental evidence suggests that these ions are produced by the chemical oxidation reactions catalyzed by the platinum surface (10). That this process is one of chemionization is supported by the fact that the extent of ionization observed depended greatly on the molecular structure of the particular hydrocarbon being oxidized. This is in contrast to the ionization measured in flames, where the extent of ionization for saturated hydrocarbons, for example, does not appear to depend in any way on the molecular structure but only on the number of carbon atoms (9). 

Reactions of atomic carbon, produced by nuclear reactions, with a number of hydrocarbons have been studied by Wolfgang and his collaborators (69). To minimize radiation induced secondary reactions which occur when use is made of C14, a technique has been developed using short-lived C11 produced by a neutron exchange reaction between a platinum foil and a C12 ion beam from a heavy ion accelerator. Part of the scattered Cu atoms has been allowed to penetrate through the thin brass foil wall of a brass vessel and come in contact with the compound wrhose reaction is studied. Products have been analyzed by gas chromatography using a technique of simultaneous mass and radioactivity determination. 

Nevertheless, the metal-catalyzed isomerization reaction is of interest from the point of view of understanding the nature of hydrocarbon transformations on metal surfaces. It has been suggested that carbonium-ion-like intermediates are involved in alkane isomerization reactions on platinum (23), and a specific mechanism has been proposed by Anderson and Avery (24). 

Catalysts which promote carbonium ion reactions have long been used for hydrocarbon isomerization (2, 6). Although such catalysts promote aromatics isomerization (3), in the case of ethylbenzene, the predominant reactions are disproportionation and dealkylation (5). Pitts, Connor, and Leum (7) demonstrated that hydrogenated intermediates were required to isomerize both ethylbenzene and cumene over platinum-... 

This equation is in good agreement with experimental results. An analysis of this dependence results in the following ratio of constants k -.k2. k-i = 100 14 0.6 (for cyclohexane at 100 °C), i.e in effect, the uncharged complex S2PtCl2 is the most active. This may be an indication of the importance of the electrophilic properties of the platinum complex in the reaction with hydrocarbons, although an appreciable value of the rate constant of such a moiety as PtCl, in its reaction with alkanes in aqueous solution, does not fit the usual concepts of electrophiles very well. It has been shown that the particular Pt(II) complexes obtained, which should form positive ions upon solution in water, PtCI(H20)j and [Pt(H20)4] react more slowly with alkanes than PtCl2. This demonstrates a more complicated nature of interaction of the bivalent platinum and the hydrocarbon molecule than that of the interaction of an electrophile with a nucleophile. 

Normally intramolecular elimination of alkane from alkyl(hydride) complexes occurs readily and is favoured thermodynamically. There is interest, however, in the possibility of carrying out the reverse reaction, the addition of a C—H bond to an unsaturated transition metal centre. Alkanes are susceptible to electrophilic attack, for example by Lewis or Br nsted acids which convert linear alkanes into their branched isomers via carbonium ion intermediates. Linear and cyclic alkanes can be converted into aromatic hydrocarbons and hydrogen over metal surfaces such as platinum. These reactions are carried out on a large scale industrially in the reforming of petroleum. 

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