Hydrogenation IV Aromatic Compounds

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). 

Hydrogenation of other Doivcvciic aromatic compounds found in recycle solyents. These experiments were carried out at 400°C at 17 MPa for 2 h using a commercial 3%/15% CoMo cat yst. The first order rate constants are shown in Table IV it can be noticed that no values are reported for acenaphtiiylene and dibenzothiophene as hardly any of the starting compounds remained after the reaction and polymerisation was evident for both compounds. 

Table IV. First order rate constants for hydrogenation of some Dolyuvclic aromatic compounds with CoMo...

The contents of Sections II, III, and IV show that the activation of C—H bonds in alkanes by transition metal compounds has much in common with the activation of C—H bonds in aromatic compounds. It appears, therefore, to be more profitable at the present time to draw mechanistic parallels between alkanes and aromatic systems, as has been done here, than, say, between alkanes and molecular hydrogen, although, of course, much work has been done on hydrogen activation (59). 

The reactivities and selectivities of various polynuclear carbocyclic and iV-hetero-cyclic aromatic compounds in the hydrogenation over transition metals have been discussed by Sakanishi et al.264 and Minabe et al.261... 

The photodegradation reactions involve not only the photooxidation, but also the photoreduction. For example, Chu W. and Javert C.T. (1994) reported a photoreduction reaction for aromatic compounds in the presence of hydrogen sources in which high reaction quantum yields were observed. Many dyes are also noted for their ease of deco lour ization in the absence of oxygen when a suitable electron donor or hydrogen source is present. For example, in the scheme IV, formation of colourless leuco-form of anthraquinone dye is observed after photo-reduction that the main structural integrity of the dye molecule retains (Rys P. and Zollinger H., 1972). 

On the whole the effect of substituents on the relative stability of isomeric arenium ions (for details see Sect. IV, 1) is described in the same terms as those used to explain the influence of substituents on the orientation and relative rates of electrophilic aromatic substitution. However, the isomeric composition of electrophilic substitution products is often controlled by kinetic factors while the equilibrium composition of isomeric arenium ions formed in aromatic compound protonation is determined by thermodynamic equilibrium. Therefore, no quantitative agreement may be observed between the relative hydrogen substitution rates at different positions of this compound and the ratio of equilibrium concentrations of the respective arenium ions formed in protonating the same compound even under identical conditions (cf. Sect. IV, 7). 

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