Benzene hydrogenation rate equation

This involves rate-determining proton transfer, equation (37) in principle it should show general acid catalysis, but in practice this usually cannot be seen as the catalyzing acid is simply H30+ . A typical example would be aromatic hydrogen exchange, such as the detritiation of tritiated benzene shown in equation (38) 147... 

Initial rate data on hydrogenation of benzene over kieselguhr at 173 C and 1 atm were taken by Kehoe Butt (J appl Chem Biotech, p 23, 1972). Check the rate equation,... 

Rate (Xi, X2) is the rate expression for benzene hydrogenation that depends on X, and X2. For example, the following rate equation could be used if the constants and were known at the reaction temperature ... 

If one is allowed this latitude, then it is possible to try exploring a complex set of kinetics, such as the hydrogenation of benzene on Ni or Pt. This reaction generally is known to be described by the rate equation ... 

Although much work has been done on the hydrogenation of aromatics (especially benzene and its substituted compounds) there is still no consensus of opinion on the kinetic behaviour and mechanisms. Table 3.12 from Zmcevic and Rusic (1988) shows the wide range of suggested rate equations in the literature for benzene hydrogenation on nickel catalyst. 

It should be noted that the same rate equation can also be obtained through entirely different reaction mechanisms. A similar rate equation was previously foimd to hold for the hydrogenation of ethylene (3) and benzene... 

The problem whether the periodical operation of a technical reactor with input concentrations which change periodically provides higher selectivities and yields than stationary operating was examined with the example of benzene oxidation into malein anhydride [103], A rather complex example was the oxide-hydrogenation of isobutyric aldehyde to methacrolein [100], Based on dynamic experiments a reaction scheme is proposed and estimation of kinetic parameters of the main reaction using an Eley-Rideal type rate equation was carried out. The examples revealed that the wave-front analysis provides valuable qualitative and quantitative kinetic information of heterogeneous catalytic reactions. 

It follows from the above that, in the reactions of fairly unreactive aromatics, the formation of Cl+ (either from HOC1 or H2OCl+) will be relatively fast compared with the subsequent reaction of this ion with the aromatic so that the kinetics will be governed mainly by the third term in equation (94). Hence de la Mare et al.204 found the rate of chlorination of benzene and toluene by acidified hypochlorous acid to depend on the concentration and nature of the aromatic and to increase with hydrogen ion concentration though (as in the case of positive... 

The effect of the medium on the rates and routes of liquid-phase oxidation reactions was investigated. The rate constants for chain propagation and termination upon dilution of methyl ethyl ketone with a nonpolar solvent—benzene— were shown to be consistent with the Kirkwood equation relating the constants for bimolecular reactions with the dielectric constant of the medium. The effect of solvents capable of forming hydrogen bonds with peroxy radicals appears to be more complicated. The rate constants for chain propagation and termination in aqueous methyl ethyl ketone solutions appear to be lower because of the lower reactivity of solvated R02. .. HOH radicals than of free RO radicals. The routes of oxidation reactions are a function of the competition between two R02 reaction routes. In the presence of water the reaction selectivity markedly increases, and acetic acid becomes the only oxidation product. 

In a more detailed study of the hydrogenation using [Ru(OAc)2(PPh3)2] (9) in benzene, methanol and methanol containing p-toluenesulfonic acid, the rate passed through a maximum at low acid concentration. This was attributed to the stepwise protonation of the acetate ligands, leading to cationic, methanol-solvated complexes (equation 17). 

In experiments of major importance, first published in 1950, Melander found that in the nitration and bromination of a number of benzene derivatives the tritium isotope effect (kHlkT) is not 10-20 as is to be expected if carbon-hydrogen bond breaking occurs in the rate-determining step, but rather is less than 1.3. The direct displacement mechanism was thus ruled out, and the two-step mechanism of Equation 7.70 with the first step rate-determining was implicated.157... 

Were reversal of Equation 9.99 to compete with the subsequent removal of a hydrogen atom from 40, the rate of the hydrogen removal step would enter the overall rate expression the reaction would then show an isotope effect. (See Section 7.4, p. 385.) In some instances, for example, when the benzoyl radical attacks benzene, the initial addition is apparently reversible,175 and an isotope effect is found.176... 

Recently, the use of sulfolane solvent allowed better kinetic control of the oxidation chain, with an increase of the selectivity to 80% or greater, at ca 8% benzene conversion. The by-products were catechol (7%), hydroquinone (4%), 1,4-benzo-quinone (1%) and tar (5%) [53, 54]. According to these authors, a rather stable complex, formed by hydrogen bonding with sulfolane, promoted desorption and hindered the re-adsorption of phenol, protecting it from consecutive oxidation (Equation 18.7). Actually, the rate of oxidation of phenol in the presence of sulfolane was only 1.6 times that of benzene, while it was 10 times higher in the presence of acetone. 

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