Proton transfers in the transition state

Because a relates the sensitivity to structural changes that the proton-transfer process exhibits to that exhibited by dissociation of the acid, it is frequently assumed that the value of a can be used as an indicator of transition-state structure. The closer a approaches unity, the greater is the degree of proton transfer in the transition state. There are limits to the generality of this interpretaton, however. ... 

At the time the mechanism shown in [60] was proposed it was not known whether it was even chemically reasonable. Bimolecular general acid catalysis involving proton transfer in the transition state [equation (45)] had never been observed in the hydrolysis of glycosides or simple acetals. Bronsted and Wynne-Jones (1929) had... 

It should be noted that, for the limiting case when = 1, the degree of proton transfer in the transition state is complete and equation (55) reduces to the form of equation (49), i.e. the A-l mechanism is seen to be a limiting case of the A-SB2 mechanism. 

Since (16) is a rate-equilibrium relationship [equivalent to the relationship (2) discussed earlier], a is considered to reflect the degree of proton transfer in the transition state and hence is a measure of selectivity. Values of a close to 0 are associated with exothermic reactions in which the degree of proton transfer in the transition state is as yet small. Similarly, values of a close to 1 are associated with endothermic reactions in which the degree of proton transfer in the transition state is almost complete. 

The preceding discussion has suggested that a (or 0) may be considered a measure of transition state structure even if the expected reactivity-selectivity relationship is not observed. There is, however, strong evidence to suggest that the Br nsted coefficient does not always reflect the degree of proton transfer in the transition state. 

It has been common practice to equate the value of )3 with the degree of proton transfer in the transition state /3 values close to 0 are taken to be indicative of reactant-like transition states and those close to 1 of product-like transition states. Any value outside these limits is inconsistent with this practice. Early investigators were only able to follow reactions within a limited rate constant range. With the development of fast reaction techniques (Eigen, 1964 Caldin, 1964) the predicted (Br nsted and Pedersen, 1923) curvature of the plots was fully established (cf. Bell and Lidwell, 1940). Pronounced curvature is in fact seen for fast proton transfers in DMSO (see p. 156). 

Interpretations of the slopes, /, in terms of the extent of proton transfer in the transition state have been attempted (Kollmeyer and... 

The Role of Proton Transfer in the Transition State for Glycosylation Reactions [ 30... 

More recent experiments have given results other than the linear dependence of rate on Hq- Although the interpretation of these experiments is in dispute, there seems to be general agreement that the extent of proton transfer in the transition state and the residual interaction of the proton with A is responsible for the results (7). 

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