Resonance in carboxylic acids

The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group. Resonance in carboxylate anions is more effective than resonance in carboxylic acids, acyl chlorides, anhydrides, thioesters, esters, and amides. 

Infrared spectroscopy can also be used to probe resonance in carboxylic acid derivatives. The dipolar resonance structure weakens the C=0 bond and causes a corresponding decrease in the carbonyl stretching frequency (Table 20-2). The IR data for carboxylic acids reported in Section 19-3 refer to the common dimeric form, in which hydrogen bonding reduces the stretching frequencies of both the 0-H and C=0 bonds to about 3000 and 1700 cm respectively. A special technique—vapor deposition at very low temperature—allows the IR spectra of carboxylic acid monomers to be measured, for direct comparison with the spectra of carboxylic acid derivatives. Monomeric acetic acid displays vc=o at 1780 cm similar to the value for carboxylic anhydrides, higher than that for esters, and lower than that of halides, consistent with the degree of resonance delocalization in carboxylic acids. 

The extent of resonance in carboxylic acid derivatives is also seen in their basicity (protonation at the carbonyl oxygen) and acidity (enolate formation). In all cases, protonation requires strong acid, but it gets easier as the electron-donating ability of the L group increases. Protonation is important in acid-catalyzed nucleophilic addition-elimination reactions. 

For many years, resonance in carboxylate ions was emphasized when explaining the acidity of carboxylic acids. Recently, however, it has been suggested that the inductive effect of the carbonyl group may be more important. It seems clear that, even though their relative contributions may be a matter of debate, both play major roles. 

The traditional explanation of the fact that carboxylic acids are much stronger acids than alcohols is that resonance stabilization of the conjugate base, which is more important than the charge-separation resonance in the acid, stabilizes RCOO-... 

This implies that substituent effects on 13C shielding in carboxylic acids and shielding in sulphonates are closely related, but —SO3 resonances appear to be almost twice as sensitive to substituent effects as -COO. ... 

Steric inhibition of resonance In benzoic acid, the phenyl ring and the carboxyl group are all in the same plane, and benzene is able to donate electrons by resonance overlap through parallel p orbitals. This stabilizes the starting acid (and destabilizes the carboxylate anion) and makes the acid weaker than it would be without resonance. 

Increasing lA concentration in copol5mierization results in lower viscosities (Fig. 1.14). lA content of copol5miers was determined using Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) and nuclear magnetic resonance (NMR) spectroscopies. The absoption bands at 1628 and 1730 cm due to C=0 stretching vibrations in carboxylic acid showed an increase... 

---