Conjugation and Hybridization

The aliphatic acids discussed above do not provide any insight into p-π or π-π conjugation effects, since the sp³-hybridized α-carbon insulates the carboxyl group from such interactions. Conjugation may be studied by using α,β-unsaturated and aromatic carboxylic acids. The two parent compounds of these classes, acrylic acid (CH₂=CHCO₂H) and benzoic acid (C₆H₅CO₂H), are both slightly stronger than acetic acid and have similar pK_a's of 4.26 and 4.20 respectively. Since their influence is probably a combination of inductive and resonance effects, it would be helpful to evaluate one of these alone. The following four compounds represent acetic acid derivatives in which a methyl hydrogen has been replaced with a methyl group, a vinyl group, a phenyl group and a chlorine atom respectively. In each compound a methylene group insulates the substituent from the carboxyl group, prohibiting conjugative interactions. As noted above, the methyl substituent is weakly electron donating and the chlorine exerts a strong electron withdrawing influence. Comparatively, the vinyl and phenyl groups have an electron withdrawing inductive effect roughly 25% that of chlorine. From this we may conclude that resonance electron donation to the carboxyl function in acrylic acid and benzoic acid substantially dilutes the inductive effect of the sp² substituent groups.

The increased electronegativity of sp² and sp hybridized carbon compared with sp³ carbon was noted earlier. This increase is particularly dramatic for triply bonded substituents, as seen in the acidity of 2-propynoic acid, HC≡CCO₂H, and 3-butynoic acid, HC≡CCH₂CO₂H, having respective pK_a's of 1.90 and 3.30. Conjugative electron donation in 2-propynoic acid is very small, compared with acrylic acid, reflecting the poor electron donating character of the triple bond. 
Another aspect of conjugation concerns the ability of a double bond, triple bond or aromatic ring to transmit the influence of a remote substituent to the carboxyl group. The compounds in the following table provide information bearing on this issue. The top row consists of β-substituted acrylic acid derivatives. Methyl and phenyl substituents exert a weakening effect; whereas chlorine strengthens the acid. Comparing these relationships with similar substituent effects in equivalent saturated acids (previous table) leads to some interesting differences.
      • A β-chlorine substituent exerts the same acidity strengthening effect regardless of unsaturation in the connecting chain.
      • A β-methyl group decreases the acidity of the unsaturated acid ten fold over that of the saturated analog. 
      • A β-phenyl group increases the acidity of the saturated acid, but decreases that of the unsaturated acid by roughly the same degree.
These observations may be interpreted in several ways. First, the inductive electron withdrawal by chlorine through a C–C sigma-bond is about the same as through a pi-bond. Second, The inductive electron donation by a methyl group occurs to a significant degree by hyperconjugation or conjugated hyperconjugation. Finally, the curious inversion of the phenyl influence may be attributed to an exclusive inductive electron withdrawal down the saturated connecting group, overpowered by a conjugative donation through the unsaturated chain.

The substituted benzoic acids in the above table exhibit many of the same effects noted for the acrylic acid derivatives. It must, however, be noted that the meta and para-substituent locations in these compounds are further removed from the carboxyl group, both in distance and number of connecting bonds, than in the acrylic acid examples. This will reduce the magnitude of any inductive effects. The para-location permits conjugative interaction of the substituent with the carboxyl function; the meta location does not. For comparison purposes remember that benzoic acid itself has a pK_a = 4.2. 
A para-methyl substituent appears to have double the electron donating effect of a meta-methyl group, again suggesting that conjugative hyperconjugation may be important. The meta-chlorobenzoic acid isomer is significantly more acidic than the para-isomer, largely because it is closer to the carboxyl function, and in part due to resonance electron donation by the para-chlorine. The two methoxybenzoic acids are particularly informative, inasmuch as the meta isomer has a slightly increased acidity, whereas the para-isomer is significantly weakened. Oxygen has a much larger electronegativity than carbon, but it is an excellent p-π electron donor to sp² carbon functions. For the meta isomer, the inductive effect is somewhat stronger than the resonance donation, but the para-isomer is able to donate an oxygen electron pair directly into the electrophilic carboxyl function. Both the meta and para-nitro substituent withdraw electrons from the benzene ring by a combination of inductive and resonance action, and the corresponding acids are greatly strengthened. A quantitative treatment of meta and para-substituent effects on the properties and reactions of benzoyl derivatives has been developed by L.P. Hammett