20.4 • Substituent Effects on Acidity
The listing of pKa values shown previously in Table 20.3 indicates that there are substantial differences in acidity from one carboxylic acid to another. For example, trifluoroacetic acid (Ka = 0.59) is 33,000 times as strong as acetic acid (Ka = 1.75 × 10–5). How can we account for such differences?
Because the dissociation of a carboxylic acid is an equilibrium process, any factor that stabilizes the carboxylate anion relative to undissociated carboxylic acid will drive the equilibrium toward increased dissociation and result in increased acidity. For instance, three electron-withdrawing fluorine atoms delocalize the negative charge in the trifluoroacetate anion, thereby stabilizing the ion and increasing the acidity of CF3CO2H. In the same way, glycolic acid (HOCH2CO2H; pKa = 3.83) is stronger than acetic acid because of the electron-withdrawing effect of the electronegative oxygen atom.
Because inductive effects operate through σ bonds and are dependent on distance, the effect of halogen substitution decreases as the substituent moves farther from the carboxyl. Thus, 2-chlorobutanoic acid has pKa = 2.86, 3-chlorobutanoic acid has pKa = 4.05, and 4-chlorobutanoic acid has pKa = 4.52, similar to that of butanoic acid itself.
Substituent effects on acidity are also found in substituted benzoic acids. We said during the discussion of electrophilic aromatic substitution in Section 16.4 that substituents on the aromatic ring strongly affect reactivity. Aromatic rings with electron-donating groups are activated toward further electrophilic substitution, and aromatic rings with electron-withdrawing groups are deactivated. Exactly the same effects can be observed on the acidity of substituted benzoic acids (Table 20.4).
|Y||Ka × 10–5||pKa|
As Table 20.4 shows, an electron-donating (activating) group such as methoxy decreases acidity by destabilizing the carboxylate anion, and an electron-withdrawing (deactivating) group such as nitro increases acidity by stabilizing the carboxylate anion.
Because it’s much easier to measure the acidity of a substituted benzoic acid than it is to determine the relative reactivity of an aromatic ring toward electrophilic substitution, the correlation between the two effects is useful for predicting reactivity. If we want to know the effect of a certain substituent on electrophilic reactivity, we can simply find the acidity of the corresponding benzoic acid. Worked Example 20.1 gives an illustration.
Predicting the Effect of a Substituent on the Reactivity of an Aromatic Ring toward Electrophilic Substitution
The pKa of p-(trifluoromethyl)benzoic acid is 3.6. Is the trifluoromethyl substituent an activating or deactivating group in electrophilic aromatic substitution?