Rate-determining mechanism

The carbonyl-carbon kinetic isotope effect (KIE) and the substituent effects for the reaction of lithium pinacolone enolate (112) with benzaldehyde (equation 31) were analyzed by Yamataka, Mishima and coworkers ° and the results were compared with those for other lithium reagents such as MeLi, PhLi and AllLi. Ab initio (HF/6-31-I-G ) calculations were carried out to estimate the equilibrium isotope effect (EIE) on the addition to benzaldehyde. In general, a carbonyl addition reaction (equation 32) proceeds by way of either a direct one-step polar nucleophilic attack (PL) or a two-step process involving electron transfer (ET) and a radical ion intermediate. The carbonyl-carbon KIE was of primary nature for the PL or the radical coupling (RC) rate-determining ET mechanism, while it was considered to be less important for the ET rate-determining mechanism. The reaction of 112 with benzaldehyde gave a small positive KIE = 1.019),... 

Before considering some analytical and numerical functions found to describe the kinetics of ion exchange it is useful to consider the various rate controlling steps in more qualitative terms. Figure 6.1 represents schematically the three fundamental rate determining mechanisms put forward as controlling the overall rate of exchange of say ion B in solution with beads of resin RA in a well stirred suspension. 

The rate-determining mechanism involved in initial as well as final stages of solid--state sintering is, according to current ideas, boundary or lattice diffusion of vacancies from sources at the boundary. The main arguments in favour of the diffusion mechanism are as follows ... 

When r = 1, the same form of equation is obtained from several different rate-determining mechanisms. For external diffusion, Eq. (31) becomes... 

Flo. 2. The calculated profiles of the potential energy surfaces (at the DFT-B3LYP level) for the insertion reaction (rf-CsFUCotNOXCHa) + phosphine - (r)5-C5H5)Co(phos-phine)[N(0)CH3] on the assumption (a) of an intramolecular rate-determining mechanism, or (b) of an associative mechanism (S),... 

Notably, for standard chromatography, the assumption that adsorption is the only rate-determining mechanism is unrealistic, because this step is generally much faster than other mass transfer-related effects (Guiochon ef al., 2006 Ruthven, 1984). However, in special cases this simplification may be justified. 

Rate-determining mechanism depends also on saturation index and pH. Rate-determining step for precipitation of salt minerals such as BaS04 from supersaturated aqueous solution is volume diffusion in aqueous solution, but surface reaction controls the precipitation when degree of supersaturation is small (Nielsen 1958). 

Ns Symbol Name of the function g(a) f(a) Rate-determining mechanism... 

Low conversion - prior to the onset of the autoacceleration phenomenon known as the gel or Norrish-Trommsdotlf effecP and characterized by highly mobile propagating species. Center of mass and/or segmental diffusion are the rate-determining mechanisms for chain end movement. Initiator efficiencies are high and approximately constant. 

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