Decarboxylation REACTION INDEX

Another example of the use of transition state pKa values has been provided by Pollack (1978). From the rate constants for the decarboxylation of substituted a,a-dimethylbenzoylacetic acids ([37] — [38]) and their anions, he calculated pK for reaction of the acids (Table A6.2). The values vary significantly with the phenyl substituent (p = +1.7), much more so than the p/(a values of the substrate acids (p = +0.2). This difference is consistent with the proton being much closer to the phenyl group in the transition state than in the initial state, and it may even denote a relatively late transition state (Pollack, 1978). However, from the pKa values of the reactant acids (approximately 3.4), the transition states (approximately 4.4), and the enol product (11.8) (Pruszynski et al., 1986), the Leffler index... 

The correlation of the rates of decarboxylation of the difluoro diethyl lactone with solubdity parameter shows that the rate increases dramatically as the value of 8 increases. In fact, there was a factor of almost 500 difference in the rate constants depending on the solvent chosen. This is in agreement with the conclusion that the transition state for the reaction carried out in solution involves a substantial separation of charge. Furthermore, Figure 5.11 shows that the solubihty parameter can be a useful index for assessing the role of the solvent in a reaction of a greatly different type than those described earlier in this chapter. The entropy of activation for the reaction was reported... 

This index does not Hst generally used transformations such as cycloaddition(s), cyclocondensation, thermolysis, photolysis, photoisomerization, oxidation, reduction, decarboxylation, valence tautomerism, metalation, C-C-coupling reactions, metathesis, etc. 

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