14.7 : Spectroscopy of Carboxylic Acid Derivatives

Infrared spectroscopy is primarily used to determine the types of bonds and functional groups. In carboxylic acid derivatives, a typical carbonyl bond absorption is observed around 1650–1850 cm⁻¹. For esters, the absorption is recorded at around 1740 cm⁻¹, while acid halides show the absorption at about 1800 cm⁻¹. Another acid derivative, the acid anhydrides, exhibit two carbonyl absorption around 1760 cm⁻¹ and 1820 cm⁻¹, arising from the symmetrical and unsymmetrical carbonyl vibration.

In the case of nitriles, the polar carbon–nitrogen triple bond stretching produces a strong absorption at a higher frequency, around 2200 cm⁻¹. In comparison, the carbonyl stretching of an amide occurs at a lower frequency, around 1660 cm⁻¹, which can be attributed to the conjugation effect. Conjugation in the carbonyl compounds delocalizes the π electrons, inducing a partial single bond character. Consequently, the bond requires less energy to stretch and absorbs at lower frequencies.

Interestingly, decreasing ring size in cyclic derivatives increases the ring strain, thereby increasing carbonyl absorption frequency.

The ¹H NMR spectra of carboxylic acid derivatives show a peak around 10–12 ppm for highly shielded acidic protons, and the peak for the protons of the α carbon is observed around 2–2.5 ppm. In the ¹³C NMR spectra, the carbonyl carbon shows a distinct peak around 160–185 ppm. The absorption of the nitrile carbon is observed at a lower frequency.