Exchange reactions hydrocarbon-hydrogen

Kaesz et al. have shown that simple, primary amines (e.g. MeNH2) will react with Ru3(CO)12 to form p-acetamido ligands at temperatures as low as -15°C. We find that simple primary, secondary and tertiary alkyl amines will react with Ru3(CO) 2 at temperatures of 70-150°C to undergo catalytic deuterium for hydrogen exchange reactions on the hydrocarbon groups and transalkylation (52). We have found that a... 

Following the Second World War, hydrogen very highly enriched in the isotope of mass 2 became available, and the mass spectrometer appeared as an analytical tool for the chemist the time was ripe for very detailed studies of catalyzed isotope exchange in hydrocarbons. The technique of continuously monitoring the reaction by means of a mass spectrometer linked directly to the reaction vessel has been used for many of the studies now to be described. The method by which the experimental data are treated is well known (84) it is reproduced briefly in the footnote (p. 136). 

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

The trans elimination can take place if the basic sites of the alumina attack the hydrogen from one side of the plane and the hydroxyl group is removed from the opposite side of the plane by the acidic sites of the alumina. This may be possible if the reaction occurs within the pores of molecular dimensions (46) or within the crevices of the aluminas. Crevice sites on silica-alumina catalyst have been proposed by Burwell and co-workers (57) on the basis of racemization and exchange reactions of hydrocarbons. 

It seems beyond debate that when an exchange reaction of a hydrocarbon (HC) with D2 is observed and the initial product distributions are binomial (random distribution of D atoms), single c-metal-carbon bonds are being formed. Nevertheless, this conclusion was puzzling in the period when virtually no homogeneous alkyl-metal complexes were known and the stability of alkyl-metal complexes was doubted for principal reasons (see, e.g., 169). However, it appeared that these complexes can be rather stable when one blocks a very fast and easy elimination of one of the H atoms in the jS-position, which step decomposes the alkyl-metal bond into an olefin and a bound hydrogen atom (170,171). On the other hand, this means that the transition... 

With the techniques available at that time, it was not possible to discover whether a single hydrogen atom is replaced by a deuterium atom or whether more extensive exchange occurs during the lifetime of the hydrocarbon molecule on the surface of the catalyst. This type of information became available only after the mass-spectrometric technique of following exchange reactions was introduced. 

Let us consider the reaction between a typical hydrocarbon, C Hm, and D2. As a result of the exchange reaction, two kinds of equilibria will be established. There will firstly be an equilibrium distribution between the total amount of deuterium in the hydrocarbon and the total amount of deuterium in the hydrogen. Secondly, the relative amounts of the different isotopic species of hydrocarbon will be in equilibrium, and the same will apply to the different isotopic species of the hydrogen. In other words, the following interconversion equilibria for the hydrocarbon will be established,... 

Similar results have been obtained for methane 12) and for ethane 19). The values quoted in Table II also illustrate the point that the distribution of deuterium between hydrogen and propane differs from the value expected for a random distribution. With the ratio of pressures used, the expected percentage for the mean deuterium content of the hydrocarbon would be 33.3, which is substantially less than the experimental value of 40.9 %. This type of deviation is also found with other hydrocarbons, but it does not affect the validity of using classical theory for the calculation of the interconversion equilibrium constants in studies of mechanism of exchange reactions. More accurate values for these equilibrium constants are necessary, however, if one is interested in the separation of isotopes by chemical processes. 

In the study of multiple exchange reactions, it is desirable to work with a high ratio of deuterium to hydrocarbon. This minimizes the influence of isotopic dilution of the deuterium on the rate of production of the more highly deuterated species during the early part of the reaction. The use of deuterium containing as small a percentage of hydrogen as possible is also important if true initial distributions of products are to be obtained. 

R H) is much faster than alkylation, so that alkylation products are also derived from the new alkanes and carbocations formed in the exchange reaction. Furthermore, the carbo-cations present are subject to rearrangement (Chapter 18), giving rise to new carbocations. Products result from all the hydrocarbons and carbocations present in the system. As expected from their relative stabilities, secondary alkyl cations alkylate alkanes more Teadily than tertiary alkyl cations (the r-butyl cation does not alkylate methane or ethane). Stable primary alkyl cations are not available, but alkylation has been achieved with complexes formed between CH3F or C2H5F and SbFs-212 The mechanism of alkylation can be formulated (similar to that shown in hydrogen exchange with super acids, 2-1) as... 

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