Homogeneous reactions rate-determining step

Most catalytic cycles are characterized by the fact that, prior to the rate-determining step [18], intermediates are coupled by equilibria in the catalytic cycle. For that reason Michaelis-Menten kinetics, which originally were published in the field of enzyme catalysis at the start of the last century, are of fundamental importance for homogeneous catalysis. As shown in the reaction sequence of Scheme 10.1, the active catalyst first reacts with the substrate in a pre-equilibrium to give the catalyst-substrate complex [20]. In the rate-determining step, this complex finally reacts to form the product, releasing the catalyst... 

When k eCp/kc <1, the rate-determining step is the forward homogeneous electron transfer reaction, meaning that the system behaves as just analyzed above. When, conversely, k ed /kc 1, the rate-determining step is the follow-up reaction, while the homogeneous electron transfer plays the role of a preequilibrium. The governing kinetic parameter is then... 

The zero slope found for transesterification (series 45) can be explained in accordance with the general view on acid-catalyzed reactions of organic acids and esters. The first step is the protonation of the acid or ester, which is followed by interaction with the alcohol (or water in ester hydrolysis). The absence of any observable influence of the alcohol structure on rate indicates that the rate-determining step must be the protonation of the ester. This is in contrast to the homogeneous reaction, in which this step is usually very rapid. The parallel dehydration of the alcohols exhibited a large structure effect on rate (Case 7 from Table II), confirming the independence of the two reaction routes. 

The activated Ba(OH)2 was used as a basic catalyst for the Claisen-Schmidt (CS) condensation of a variety of ketones and aromatic aldehydes (288). The reactions were performed in ethanol as solvent at reflux temperature. Excellent yields of the condensation products were obtained (80-100%) within 1 h in a batch reactor. Reaction rates and yields were generally higher than those reported for alkali metal hydroxides as catalysts. Neither the Cannizaro reaction nor self-aldol condensation of the ketone was observed, a result that was attributed to the catalyst s being more nucleophilic than basic. Thus, better selectivity to the condensation product was observed than in homogeneous catalysis under similar conditions. It was found that the reaction takes place on the catalyst surface, and when the reactants were small ketones, the rate-determining step was found to be the surface reaction, whereas with sterically hindered ketones the adsorption process was rate determining. 

For the same reaction under similar conditions and on supported H3P04, another equation was applied [287]. It is based on Taft s mechanism for homogeneously catalysed olefin hydration [291,292] [see scheme (B), p. 327] according to which water enters into the reaction scheme in a fast step which follows the rate-determining step and therefore appears in the rate expression in a negative term, viz. 

Homogeneous kinetics is used instead of diffusion kinetics to express the dependence of intraspur GH, on solute concentration. The rate-determining step for H2 formation is not the combination of reducing species, but first-order disappearance of "excited water." Two physical models of "excited water" are considered. In one model, the HsO + OH radical pair is assumed to undergo geminate recombination in a first-order process with H3O combination to form H2 as a concomitant process. In this model, solute decreases GH, by reaction with HsO. In the other model, "excited water" yields freely diffusing H3O + OH radicals in a first-order process and solute decreases GH, by reaction with "excited water." The dependence of intraspur GH, on solute concentration indicates th,o = 10 9 — 10 10 sec. 

Here (3 is the cathodic symmetry factor of the rate-determining step and v is a positive integral number indicating how many times the RDS is occurring in the global electron-transfer reaction (mostly v=l). The parameter r takes into account a homogeneous chemical reaction, the rate of which is not dependent on the potential, as RDS when the RDS is a charge-transfer step, r=l applies, and for a chemical RDS, r=0. 

The interaction of a prochiral molecule with a chiral homogeneous catalyst results in the formation of diastereomeric intermediates and transition states. High enantioselectivity is obtained if for the rate-determining step out of the many possible diastereomeric transition states, one is energetically favored. In such a situation the reaction follows mainly this path. Other possible pathways that cause dilution of optical purity are avoided. A fundamental point to note is that diastereomers, unlike enantiomers, need not have identical energies. The presence of a C2 axis of symmetry in these ligands makes some of the possible diastereomeric transition states structurally and energetically equivalent. As the... 

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