Change in rate limiting step, and

Haloform reaction, 237, 296 Halogenation alkanes, 300, 323 alkenes, 179,186, 313 benzene, 138,316 ketones, 295 Hammett equation, 362 additional parameters, 374, 388, 395 derivation of, 362 deviations from, 375 empirical nature of, 395 implications of, 394 reaction pathway, and, 375 solvent effects and, 388 spectroscopic correlations, 392 standard reaction for, 362, 395 steric effects and, 361, 383 thermodynamic implications of, 394 Hammett plots, 359 change in rate-limiting step and, 383 change in reaction pathway and, 378... 

Figure 3 Hydrolysis of 2-methyl-4,6-dipheny I pyrylium salt over a pH-range exhibiting a change in rate-limiting step and mechanism. The line is calculated from k, = kfH OJKl + [H.ajkjk, + koHK7/ //3<97...

There is a non-linear dependence of the rate of aminolysis of cephalosporins upon hydroxide ion concentration (Page and Proctor, 1984) which is consistent with a change in rate-limiting step and hence formation of an intermediate as described in Section 11. 

Changing activation energies are, however, not always indicative for the presence of limitations. The approach of thermodynamic equilibrium in the case of exothermal reactions can cause this phenomenon, as for hydrogenation reactions [44]. Also, changes in rate determining steps and catalyst deactivation might be causes. The same holds for reaction orders. Table 3 gives the various observations that can be made when mass transfer affects the isothermal kinetic behavior of catalyst particles. 

It is essential to demonstrate that both the measured forward and reverse rate constants refer to the same rate-limiting step and that there is no change in rate-limiting step in the extrapolated range. Consider the quasi-symmetrical reaction of phenolate ions with 4-nitrophenyl acetate (Equation 13). 

The substituent parameters for the condition k, = 2 be predicted for quasi-symmetrical reactions. The decomposition steps of a quasi-symmetrical intermediate to reactants (, ) and to products therefore obey identical linear free energy relationships. A change in rate-limiting step will occur when the intermediate is perfectly symmetrical (and k. In the addition-elimination mechanisms of Scheme 9 the ratio k Jk2 becomes unity when entering and leaving groups are identical. 

The free energy relationship is diagnostic of a change in rate-limiting step (near pX = 2 and the simplest mechanism fitting these results is the Elcb mechanism of Scheme 26. 

Figure 11.6. Influence of temperature-induced changes in rate-limiting steps on observed S A/ain-Schaad exponents, / HD)obs nd observed H/D isotope effects. Sho A/n above each curve are variables a and (kj moh ) for the ratio of rate constants ka/k (defined in Fig. 11.4) expressed as The curves...

The reaction of isocyanic acid with amines exhibits a change in rate-limiting step as diagnosed by a hill-shaped free-energy correlation (Fig. 31). The overall mechanism for this reaction (Eqn. 130) [116] involves rate-limiting proton transfer for the weakly basic amines and addition for the strongly basic amines. 

Chamberlin AR, Mullholland RL, Kahn SD, Hehre WJ (1987) Modeling chemical reactivity. 7. The effect of a change in rate-limiting step on the stereoselectivity of electrophilic addition to allyhc alcohols and related chiral alkenes. J Am Chem Soc 109 672... 

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