Bronsted Relation Interpretation

The Bronsted relation has proved to be a useful equation for correlating rate and equilibrium results for proton transfer reactions. However, following the analysis by Leffler and Grunwald [73] in 1963 considerable effort has been made to go further than this and understand why the relation should hold, and also to attach some significance to the values of a and 3 in terms of the structure of the transition state for proton transfer. An alternative approach from that to be discussed here interprets the Bronsted relation from molecular potential energy diagrams [74]. 

If it is accepted that reactions catalyzed by acids or bases involve at some stage a slow acid-base reaction, then the Bronsted relation (cf. Sec. II.4) assumes a reasonable aspect. The constants used to express the strengths of the catalyzing species are usually defined with reference to an equilibrium with some standard acid-base system such as the solvent, but they could in principle be defined in terms of the (hypothetical) protolytic equilibrium between the catalyst and the substrate. The Bronsted relation then amounts to a parallelism between the rates and equilibrium constants of a series of similar reactions. The general form of the relation can in fact be inferred without any reference to a molecular interpretation. Suppose that we have any acid-base equilibrium... 

The right-hand side of Table II shows that certain catalysts exhibit large positive deviations from the Bronsted relation. This can be interpreted if we remember that both the carboxylic acids and the phenols undergo some structural rearrangement on ionization. The carboxylate... 

Equation 8.18 and find relation 8.21 between activation free energy, AG, and standard free energy change, AG°. This equation is equivalent to the Bronsted catalysis law as was shown in Section 3.3 (Equations 3.49 and 3.53), and we may conclude that an interpretation of a as a measure of the position of the transition state is consistent with the Hammond postulate. 

No definite assignment of these two bands was made. Lavalley and coworkers, however, report a correlation between the frequency of CO interacting with a Bronsted center and the shift suffered by the related O - H stretch, which helps in proposing an interpretation for the hidden species [70]. From this correlation, it results that the corresponding CO frequency of 2173 cm would imply a shift of ca. 200cm . This means that the 0-H stretch of the acidic hydroxyls before interaction with CO would fall, roughly speaking, around 3680 and 3620 cm respectively. 

The kinetics of the reaction between peroxomonosulphate and thiocyanate were earlier interpreted in terms of the intermediate of species such as HS(0)CN, (SCN)a, and HO(SO)CN. No definite proof of this is available, but the detection of similar intermediates, RSO2CN and R(SO)CN, in reactions between alkyl thiocyanates and peroxomonosulphate lends support to the intermediacy of HS(0)CN and HO(SO)CN in the peroxomonosulphate-thiocyanate reaction.The kinetics of decomposition of the iV-nitroso-hydroxylamine-A -sulphonate anion to sulphate and nitrous oxide have already been described under nitrogen. The mechanism of substitution at sulphur(iv) in sulphinyl compounds, RS(0)Y, is still a topic of unresolved discussion, despite much effort in this area. The question awaiting answer is whether the mechanism is simple Ssl or whether there is a four-co-ordinate intermediate of significant lifetime. > Though the determination of Bronsted coefficients did not provide definite mechanistic evidence relating to substitution at... 

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