Acidity of substituted benzoic acids

A considerable body of data is available on the acidity of substituted benzoic acids Ben zoic acid Itself is a somewhat stronger acid than acetic acid Its carboxyl group is attached to an sp hybridized carbon and ionizes to a greater extent than one that is attached to an sp hybridized carbon Remember carbon becomes more electron withdrawing as its s character increases... 

A related comparison of remote substituent effects on acidities is provided by the data in Table 6-20. Here, the acidities of/ -substituted benzoic acids complexed to chromium tricarbonyl are related to the acidity of the parent compound. 

Acidities of />-Substituted Benzoic Acid Chromium Tricarbonyl Organometallics Relative to Benzoic Acid Chromium TricarbonyP... 

Table 12-13 Effect of Choice of Geometry on Acidities of / -Substituted Benzoic Acids Relative to Benzoic Acid/ 6-31G Model... 

Extensive efforts have been made to characterize nucleophile and electrophile strengths. Hammett first correlated411 the acidities of substituted benzoic acids (1)... 

The acidity of substituted benzoic acids is dependent on the relative ability of the substituent to donate or withdraw electron density. The more strongly a group withdraws electrons, the greater is the acidity. Thus the order of increasing acid strength is 4-hydroxybenzoic acid < benzene-1,4-dicarboxylic acid < 4-acetyl-benzoic acid < 4-cyanobenzoic acid... 

Recall from Chapter 18 that substituents on a benzene ring either donate or withdraw electron density, depending on the balance of their inductive and resonance effects. These same effects also determine the acidity of substituted benzoic acids. There are two rules to keep in mind. 

Figure 19.8 illustrates how common electron-donating and electron-withdrawing groups affect both the rate of reaction of a benzene ring towards electrophiles and the acidity of substituted benzoic acids. 

For calculated gas-phase acidities of substituted benzoic acids, see Wiberg, K. B. J. Org. Chem. 2002, 67, 4787. 

Another type of proximity effect arises when the placement of a substituent interferes with the orbital overlap required for resonance stabilization. This can be seen clearly in the acidity of substituted benzoic acids and in the basicity of substituted anilines. 

Comparison of the gas phase acidity of substituted benzoic acids with p/f values in aqueous solutions reveals some interesting comparisons. 

As early as 1979, Catalan and Macias demonstrated linear relationships between the gas-phase acidities of monosubstituted phenols and benzoic acids and a variety of molecular indices calculated by Intermediate Neglect of Differential Overlap (INDO) and ab initio methods [171]. Among these indices the calculated energy difference AE = E(AH) -E(A ) provided especially good linear relationships with the acidities. La Manna, Tschinke, and Paoloni reported a linear relationship between the gas-phase acidities of substituted benzoic acids and the highest occupied molecular orbital (HOMO) energies of the corresponding anions [227]. 

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