Hydrocarbons, carbon transfer reaction

Here we shall briefly summarize the effects of individual poisons on various catalytic reactions taking place on automotive catalysts. There are three main catalytic processes oxidation of carbon monoxide and hydrocarbons and reduction of nitric oxide. Among secondary reactions there are undesirable ones which may produce small amounts of unregulated emissions, such as NH3, S03 (6), HCN (76, 77), or H2S under certain operating conditions. Among other secondary processes which are important for overall performance, in particular of three-way catalysts, there are water-gas shift, hydrocarbon-steam reforming, and oxygen transfer reactions. Specific information on the effect of poisons on these secondary processes is scarce. 

First, as discussed earlier in connection with the aluminum halide catalyzed rearrangements of hydrocarbons (Section II. A. 2), intermolecular hydride transfer reactions appear to be fairly unselective processes. Apparently, charge development in the transition states of these reactions is minimized a penta-coordinate carbon intermediate may be involved. As a result, the strong preference for the bridgehead positions exhibited by most ionic substitution reactions is partially overcome. 

The H" and HJ transfer reactions mentioned in Section 6.1.3 in conjunction with the fragment ion reactions in propane are the general reactions between a saturated hydrocarbon (R H2 ) and an olefinic ion (R ) containing fewer carbon atoms (R < R ), and will be discussed in some detail in Section 6.2.3. Here we discuss two types of reactions which are complementary to the above reactions, i.e. H and Hj transfer reactions between R H and R. These reactions have also been established as general reactions of alkane ions with olefins having fewer carbon atoms. 

The increasing importance of electron-transfer reactions with increasing aromatic hydrocarbon size is illustrated in the reaction of bromine with various aromatic compounds. With benzene (with a Lewis acid) and with naphthalene, electrophilic substitution occurs, and with anthracene, oxidative addition occurs (6) however, with graphite, only oxidation to the exclusion of carbon-bromine bond formation occurs, even at a stoichiometry of C8Br (II, 12). 

Alkenes and alkynes readily undergo addition reactions to the carbon-carbon multiple bonds. Additions of adds, such as HBr, proceed via a rate-determining step in which a proton is transferred to one of the alkene or alkyne carhon atoms. Addition reactions are difficult to carry out with aromatic hydrocarbons, but substitution reactions are easily accomplished in the presence of catalysts. 

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