Dichloroacetic acid, basicity

The relative strengths of weakly basic solvents are evaluated from the extent of protonation of hexamethylbenzene by trifluoro-methanesulfonic acid (TFMSA) in those solvents or from the effect of added base on the same protonation in solution in trifluoroacetic acid (TFA), the weakest base investigated. The basicity TFA < di-fluoroacetic acid < dichloroacetic acid (DCA) < chloroacetic acid < acetic acid parallels the nucleophilicity. 2-Nitropropane appears to be a significantly stronger base than DC A by the first approach, although in the second type of measurement, the two have essentially equal basicity. The discrepancy is due to an interaction, possible for hydroxylic solvents such as DC A, with the anion of TFMSA. This anion stabilization is a determining factor of carbocationic reactivity in chemical reactions, including solvolysis. A distinction is made between carbocation stability, determined by structure, and persistence (existence at equilibrium, e.g., in superacids), determined by environment, that is, by anion stabilization. 

The kinetics and mechanism of the decarboxylation of 5- (p-substituted phenyl )-2-oxazolecarboxylic acids in neutral, acidic, and basic media have been studied by Tanaka.392 In quinoline or in dichloroacetic acid as a nonaqueous solvent, the reaction shows first-order kinetics.392 Studies on the decarboxylation of oxazolium (and other related azolium) carboxylates indicate that both 2- and 5-acids decarboxylate through their zwitterionic tautomers.393 Heating an oxazole-2-carboxylic acid at or above its melting point may also result in decarboxylation.29 394... 

Figure 2.5 The Br0nsted plot for the general-base catalysis of the hydrolysis of ethyl dichloroacetate. The logarithms of the second-order constants obtained from the plot of Figure 2.4 are plotted against the pAT s of the conjugate acid of the catalytic base. The slope is the /3 value. Note that the points for amine bases ( ) fall on the same line as those for oxyanion bases (O), showing that the catalysis depends primarily on the basic strength of the base and not on its chemical nature.

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