Spectroscopy of Carboxylic Acid Derivatives

The IR spectra of all carboxylic acid derivatives exhibit a typical carbonyl stretching absorption between 1650 and 1850 cm⁻¹.

For instance, esters show an absorption around 1740 cm⁻¹, and acid halides exhibit absorption around 1800 cm⁻¹.

In contrast,  acid anhydrides exhibit two carbonyl absorptions due to the symmetrical and unsymmetrical vibrations from carbonyl stretching.

Interestingly, nitriles produce a strong characteristic absorption at a higher frequency due to the stretching of the polar carbon–nitrogen triple bond.

Conversely, amides exhibit a lower carbonyl absorption frequency due to the conjugation effect.

In ¹H NMR, the highly deshielded acidic protons absorb far downfield around 10–12 ppm, whereas the protons of the α-carbon absorb around 2–2.5 ppm.

 In ¹³C NMR, the carbonyl carbon of derivatives shows absorption around 160–185 ppm.

However, the nitrile carbon absorbs around 115–130 ppm due to sp hybridized carbon.