Competitive hydrogenation

Most problems concerned with olefin hydrogenation involve the competitive hydrogenation of a double bond in the presence of another double bond or other function. There Is usually a way of achieving the desired selectivity. 

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 photoelectrochemical reduction of carbon dioxide, organic solvents and their mixtures with water have also been used. The use of organic solvents has the advantages103 that (1) competitive hydrogen formation can be suppressed and (2) the increased solubility of C02 in nonaqueous solutions28 30 has similar effects to the use of higher C02 pressures. 

V. A. Russell, M. C. Etter, M. D. Ward, Guanidinium para-substituted benzenesulfonates competitive hydrogen bonding in layered structures and the design of nonlinear optical materials , Chem. Mater. 1994, 6,1206-1217. 

The mechanism for the hydrogenation probably involves Rh-jt interaction with the unsaturated systems. The selectivity of competitive hydrogenation experiments has established the relative order of the interaction forces [3] ... 

P-Cyclodextrin was modified by attaching 2-(diphenylphosphinoethyl)-thio- (127) and 2-bis(diphenylphosphinoethyl)amino- (126) moieties at the C-6 position [8-11]. The resulting macroligands were reacted with [ RhCl(NBD) 2] to provide the corresponding cationic rhodium-bisphosphine complexes. These catalysts showed pronounced selectivity due to complexation of the substrate by the CD unit adjacent to the catalyticaUy active metal center. For example, in competitive hydrogenation of similarly substituted terminal olefins (Scheme 10.4), 4-phenyl-but-l-ene was... 

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. 

The appearance of turbidity indicates saturation of alkyl halide. In this way both sodium thiosulfate and 2-bromopropane are nearly in a one-phase system, thus shortening significantly the heating period. Furthermore, the competitive hydrogen bromide elimination and the ensuing acid-promoted decomposition of thiosulfate into sulfur and sulfur dioxide are minimized, the checkers added 300 ml. of water over a period of 90 minutes. 

The competitive hydrogenation of pairs of cycloalkenes over metal catalysts has been studied by a number or workers [234,235,239,241] in an attempt to establish the mechanism of hydrogenation. The results of these studies have been discussed in detail in a recent review [239] and will not be discussed further here. 

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