Edwards and Ritchie correlations

5 Acid-Base Related Effects—Brensted Relationships 

A final linear free energy relationship that we cover is one that is still extensively used in modern research—almost as much as Hammett plots. It is called a Bronsted plot, and it relates acidity or basicity to other kinds of reactions. The reason that this LFER is still actively used is in part because this relationship is essential to an understanding of acid-base catalysis in enzymology. However, the same relationship can be used to study nucleophilicity and leaving group ability. We describe all three uses below, but we leave a more detailed analysis of the use of this relationship in studies of acid-base catalysis to Chapter 9. 

Some of the earliest correlations between nucleophilicity and reactivity focused solely on the pKn of the conjugate acid of the nucleophile. If one keeps the electronic and steric properties of a series of nucleophiles similar, and uses the same solvent for the analyses, reactivity does nicely correlate with the basicity of the nucleophile Kt, see Chapter 5). An LFER as defined in Eq. 8.48 can be used to correlate the data. This structure-function relationship is called a Brensted relationship. 

ATP Hydrolysis—How /Ilg and Pnm Values Have Given Insight into Transition State Structures 

Shown below is the Bronsted plot for the reaction of ATP, and the complex between Mg and ATP , as a function of thep/C. of primary alcohol nucleophiles (closed and open circles, respectively). The values of )3nuc are 0.07 and 0.06, respectively, reflecting that the nucleo-philicity of the alcohols has little effect on the rate of the reaction. This indicates very little bond formation to the nucleophiles in the transition state. 

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