Nucleophilicity in Acyl Transfers

When a Br nsted base functions catalytically by sharing an electron pair with a proton, it is acting as a general base catalyst, but when it shares the electron with an atom other than the proton it is (by definition) acting as a nucleophile. This other atom (electrophilic site) is usually carbon, but in organic chemistry it might also be, for example, phosphorus or silicon, whereas in inorganic chemistry it could be the central metal ion in a coordination complex. Here we consider nucleophilic reactions at unsaturated carbon, primarily at carbonyl carbon. Nucleophilic reactions of carboxylic acid derivatives have been well studied. These acyl transfer reactions can be represented by 

The most common manifestation of a structure-reactivity correlation in a reaction series of this type is a plot of log k for the reaction against pX of the conjugate acid of the nucleophile. Of course, this is identical with the graphical presentation 

They were able to infer p for the identity reaction in which Ar = Ar, and interpreted the results in terms of a More O Ferrall-Jencks diagram of the type described in Section 5.3. 

Sometimes Br nsted-type plots are curved. Many possible causes of such curvature have been discussed. 

The curvature may be an artifact of a selection of nucleophiles of mixed structural types chosen to display a wide range in pAo. Buncel et al. ° varied pK by changing the solvent composition over a limited range rather than by changing the structure. They studied the reaction between X-C6H4-CT and p-nitrophenyl acetate in 40-90 mol% dimethylsulfoxide—water mixtures with just three X substituents 

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In contrast to hydrol3dic reactions, where the nucleophile (water) is always in excess (55 mol/L), the concentration of the foreign nucleophile in acyl transfer reactions (such as another alcohol) is always hmited. As a result, trans- and interesterification reactions involving normal esters are generally reversible in contrast to the irreversible nature of a hydrolytic reaction. This leads to a slow reaction rate and can cause a severe depletion of the selectivity of the reaction for kinetic reasons (Sect. 2.1.1). 

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