Competitive Hydrogenation Reactions

Note that the common simple procedure of correlating total rate with total reactant concentration would lead to the rate increasing with decreasing concentration (i.e., a negative order). This effect would be rather suspect as a basis for design. In order to investigate this closer, data on the hydrogenation rates of A and B alone were measured, and they appeared to be zero order ret tions with rate constants  

B is more strongly adsorbed than A, and the ratio of equilibrium constants is 

Our problem is to explain all of these features with a consistent rate equation. 

Consider a simple chemisorption scheme with the surface reaction controlling. For A reacting alone, 

If the reaction product is weakly adsorbed, the total sites equation becomes 

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The usual procedure is to simply heat a mixture of the starting materials. A common side-reaction is the polyalkylation it can be suppressed by employing an excess of amine. In addition carbonyl substrates with a-hydrogens may undergo competitive aldol reactions the corresponding reaction products may then undergo a subsequent Leuckart-Wallach reaction. 

That the reaction with a lower rate constant is taking place preferentially and that the rate increases during the reaction are phenomena that can also occur with parallel reactions. As an example, Wauquier and Jungers (48), when studying competitive hydrogenation of a series of couples of aromatic hydrocarbons on Raney-nickel, have observed these phenomena for the couple tetraline-p-xylene (Table I). The experimental result was... 

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

In addition, also nonheme iron catalysts containing BPMEN 1 and TPA 2 as ligands are known to activate hydrogen peroxide for the epoxidation of olefins (Scheme 1) [20-26]. More recently, especially Que and coworkers were able to improve the catalyst productivity to nearly quantitative conversion of the alkene by using an acetonitrile/acetic acid solution [27-29]. The carboxylic acid is required to increase the efficiency of the reaction and the epoxide/diol product ratio. The competitive dihydroxylation reaction suggested the participation of different active species in these oxidations (Scheme 2). 

Competitive consecutive reactions are combinations of parallel and series reactions that include processes such as multiple halogenation and nitration reactions. For example, when a nitrating mixture of HN03 and H2S04 acts on an aromatic compound like benzene, N02 groups substitute for hydrogen atoms in the ring to form mono-, di-, and tri-substituted nitro compounds. 

Clearly there is a switch from MO to MIBK when hydrogen was made the carrier gas. The yield of isophorone dropped as at this temperature the hydrogenation intercepted the MO before it could undergo another aldol condensation. At 673 K the aldol condensation reaction became kinetically more competitive with the hydrogenation reaction. 

Although nitrobenzene, nitrosobenzene and azobenzene are often observed in nitrobenzene hydrogenation, we are aware of no studies of competitive reactions. In this paper we report on the competitive hydrogenations and their mechanistic implications. 

The competitive hydrogenation of azobenzene and nitrobenzene in a 0.5 1 molar mix was examined. A ratio of 0.5 1 was used as two nitrobenzene units are needed to produce a single azobenzene. The reaction profile is shown in Figure 4. In this reaction the concentrations of both nitrobenzene and azobenzene dropped simultaneously and coincided with an increase in aniline concentration. Aniline was produced at a rate of 3.5 mmol.mm g, which is five times slower than nitrobenzene hydrogenation in the absence of azobenzene and two-and-a-half times slower than azobenzene in the absence of nitrobenzene. No other by-products were observed with this reaction. 

The hydrogenation of nitrobenzene and nitrosobenzene are complex and a range of factors can influence by-product reactions, e.g. hydrogen availability, support acid/base properties (13,14). In this study we have examine competitive hydrogenation between nitrobenzene, nitrosobenzene and azobenzene. This methodology coupled with the use of deuterium has further elucidated the mechanism of these reactions. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

Benzaldehyde. The addition of less than stoichiometric quantities of benz-aldehyde to CoH (H2 atmosphere) did not result in hydrogen absorption. However, when this procedure was carried out with CoH containing added alkali (KOH, 2x cobalt concentration), hydrogen was taken up, 1.0 atom of hydrogen being absorbed per mole of substrate. Since benzyl alcohol was isolated in 66% yield, it is assumed that a portion of the product may have been formed via a competitive Cannizzaro reaction. Reinforcing this assumption is the observation of an apparent depletion of alkali during the run. 

It is true that completely independent experiments may be confused by subtle effects of inhibition by H2S, which is a byproduct in desulfurization reactions, but as discussed later, the magnitude of the inhibition at the levels produced in the experiments is not large enough to result in such drastic changes in the hydrogenation rates of biphenyl. A few well-chosen experiments in which competitive test reactions are conducted simultaneously can provide definitive information that can be used in setting reasonable limits on the ratios of the important rate constants. This is illustrated later. 

Structure-Reactivity of cycloalkenes. Comparisons of individual and competitive hydrogenation rates on Pt with related reactions. 

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