Hydrogenation kinetic results

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

His researches and those of his pupils led to his formulation in the twenties of the concept of active catalytic centers and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To ex-... 

Kinetics studies of acid-catalysed chlorination by hypochlorous acid in aqueous acetic acid have been carried out, and the mechanism of the reactions depends upon the strength of the acetic acid an<( the reactivity of the aromatic. Different groups of workers have also obtained different kinetic results. Stanley and Shorter207 studied the chlorination of anisic acid by hypochlorous acid in 70 % aqueous acetic acid at 20 °C, and found the reaction rate to be apparently independent of the hydrogen ion concentration because added perchloric acid and sodium perchlorate of similar molar concentration (below 0.05 M, however) both produced similar and small rate increases. The kinetics were complicated, initial rates being proportional to aromatic concentration up to 0.01 M, but less so thereafter, and described by... 

Enantioselective hydrogenation of 2,3-butanedione and 3,4-hexanedione has been studied over cinchonidine - Pt/Al203 catalyst system in the presence or absence of achiral tertiary amines (quinuclidine, DABCO) using solvents such as toluene and ethanol. Kinetic results confirmed that (i) added achiral tertiary amines increase both the reaction rate and the enantioselectivity, (ii) both substrates have a strong poisoning effect, (iii) an accurate purification of the substrates is needed to get adequate kinetic data. The observed poisoning effect is attributed to the oligomers formed from diketones. 

The potential at which the Pt-H stretch appeared, and the correlation between its subsequent increase in intensity and the rise in the cathodic hydrogen evolution current, is extremely strong evidence that this form of Had9 is the intermediate in the H2-evolution reaction as studied by Schuldiner (1959). This resolved the paradox between the kinetic results and the electrochemical measurements since Bowden. Clearly, the on-top hydrogen is only present at extremely low coverage, presumably on active sites. The strongly and weakly bound hydrogen play no part in the reaction. 

The pyridine-catalysed lead tetraacetate oxidation of benzyl alcohols shows a first-order dependence in Pb(OAc)4, pyridine and benzyl alcohol concentration. An even larger primary hydrogen kinetic isotope effect of 5.26 and a Hammett p value of —1.7 led Baneijee and Shanker187 to propose that benzaldehyde is formed by the two concurrent pathways shown in Schemes 40 and 41. Scheme 40 describes the hydride transfer mechanism consistent with the negative p value. In the slow step of the reaction, labilization of the Pb—O bond resulting from the coordination of pyridine occurs as the Ca—H bond is broken. The loss of Pb(OAc)2 completes the reaction with transfer of +OAc to an anion. 

The kinetics of the ionic hydrogenation of isobutyraldehyde were studied using [CpMo(CO)3H] as the hydride and CF3C02H as the acid [41]. The apparent rate decreases as the reaction proceeds, since the acid is consumed. However, when the acidity is held constant by a buffered solution in the presence of excess metal hydride, the reaction is first-order in acid. The reaction is also first-order in metal hydride concentration. A mechanism consistent with these kinetics results is shown in Scheme 7.8. Pre-equilibrium protonation of the aldehyde is followed by rate-determining hydride transfer. 

Kinetic results show that the hydrogenation reaction rate exhibits a first-order dependence on both hydrogen concentration, [H2], and the total ruthenium concentration, [Ru]t and an inverse dependence on the nitrile concentration, [CN]. The catalytic mechanism proposed for polymer hydrogenation is illustrated in Scheme 19.5 and the main points of the mechanism are outlined below ... 

It was shown that the AB5/ABS composite tolerated the hydrogenation effects on metal particles, with no losses in hydrogenation kinetics. The results indicated that the compositions are suitable for metal hydride based hydrogen storage devices. 

The kinetic results show that as the temperature is raised to room temperature, ethyl on a Pt(lll) surface would only be a transient species, the more so in the presence of hydrogen. Backman and Masel (404) may have obtained a VEEL spectrum of ethyl on Pt(lll) by the back-reaction of the slow hydrogenation of ethylidyne at 298 K. 

Whilst, in principle, kinetic measurements should allow a differentiation between the two possible mechanisms, it must be noted that in catalytic hydrogenation reactions relatively few examples are sufficiently clear cut to allow this differentiation to be made. Thus, for example, it is quite commonly found that the experimentally observed orders of reaction are zero in the unsaturated substrate A and unity in hydrogen. Such results are readily interpreted by the adjacent-site mechanism by assuming A to be much more strongly adsorbed than hydrogen or by the Rideal— Eley type of mechanism. Clearly, kinetic measurements alone are insufficient for the establishment of mechanism. 

The kinetic results on the oxidation of secondary hydrogen (Figures 2 and 4) show good agreement with Reaction 8. 

Halans has studied the aluminum bromide catalyzed decomposition of phenyl azide in toluene solution at 0° in the presence of traces of hydrogen bromide.1 He has found that an equimolecular complex is formed between the catalyst and phenyl azide and that the catalyst is consumed in the reaction. On the basis of kinetic results, Scheme VI has been proposed for the decomposition mechanism. In connection herewith, Halans observed that whereas an equimolecular mixture of phenyl azide and hydrogen bromide does not decompose at 0°, a mixture of phenyl azide, aluminum bromide, and hydrogen bromide in the ratio 1/1/1 decomposes instantaneously at 0°. 

Agrawal and Wei (1984) isolated the metallochlorin and confirmed that the apparent fractional order kinetics resulted from a sequential mechanism much like HDS and HDN reactions. The hydrodemetallation of both nickel- and vanadyl-etioporphyrins on oxide CoMo/A1203 proceeded through two mechanistically different steps, an initial reversible hydrogenation followed by a terminal hydrogenolysis step... 

M 1)>CH3 (366 M 1)>C6H5 (66 M ) k7 varied in a similar manner, R = CH3CH2>C6H5 CH3>C6H5CH2, CFj. Thus the tendency for styrene and fluoroolefins to be hydrogenated could result from either the thermodynamic or kinetic difficulty with which the corresponding alkyl-cobalt carbonyls undergo carbonylation. 

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