Kinetic ambiguity

Kinetic ambiguity arises when different mechanisms lead to the same predicted rate equation, i.e. the mechanisms are kinetically equivalent (see Chapters 3 and 4). It is very widespread in reactions catalysed by acids and bases, but strategies have been developed to resolve some of the ambiguities. 

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Any two reaction schemes are considered to be kinet-ically equivalent if they imply the same orders of reaction for all chemical species involved. For example, assume that the rate expression for a particular scheme for a chemical reaction is given by d[P]/df = v = kik3[A][B]/(k2 + ksp]). For a second scheme, the expression is found to be ki k3 [A][B]/(k2 + [B]). Both expressions have the same general form and have the same dependence on reactant concentrations. Thus, the two schemes are kinetically equivalent. See Kinetic Ambiguity... 

MICROTUBULE ASSEMBLY KINETICS KINETIC ELECTROLYTE EFFECT IONIC STRENGTH KINETIC EQUIVALENCE KINETIC AMBIGUITY KINETIC HALDANE RELATIONSHIPS HALDANE RELATIONSHIPS... 

When we deal with the kinetics of hydride reactions we have to be aware that hydride thermodynamics cannot be properly formulated without taking into account the (relative) immobility of the metal component. This immobility can sometimes render the interpretation of the experimental reaction kinetics ambiguous. With this difficulty in mind, let us outline concepts which describe the kinetics of hydride formation and decomposition. An extensive account, including a first order phenomenological treatment, has been given by [P.S. Rudman (1983)]. The conceptual framework for a more rigorous discussion is found in, for example, [G. B. Stephenson (1988)]. 

Scheme 16 Kinetic ambiguity in hydrolysis reactions catalysed by a base or nucleophile...

The kinetic ambiguity registered between general acid and specific acid-general base mechanisms in carbonyl addition reactions with nucleophiles (Scheme 17) is a classic mechanistic problem. ... 

A frequently occurring problem in determining mechanisms is kinetic ambiguity which arises because of the lability of the proton. For example specific acid-catalysed hydrolysis of amides has the rate law (Eqn. 134) which could arise from either oxygen (Eqn. 135) or nitrogen (Eqn. 136) protonation. 

Iron(u), Rhodium(i), and Iridium(i).— Arguably iron(n) is square-planar in ferrous phthalocyanine in DMSO solution (c/. [PtClJ - in water ). The reaction of this compound with imidazole has been much studied, but kinetic ambiguities have prevented the unequivocal assignment of mechanism. Now the kinetic pattern for the analogous reactions with A-methylimidazole and with iV-n-butylimidazole, where there is no possibility of proton removal from nitrogen, has been shown to be very similar to that with imidazole. It is therefore now reasonable to rule out reaction of ferrous phthalocyanine with the deprotonated imidazole anion as a plausible pathway. ... 

S.9.3 Kinetic Ambiguity of the Assignment of Intermolecular General Acid Catalysis... 

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