Acid chlorides are easily detected by their characteristic absorption near 1810 cm^–1. Acid anhydrides can be identified by a pair of absorptions in the carbonyl region, one at 1820 cm–1 and another at 1760 cm^–1. Note that each of these functional groups has a strong electron-withdrawing group attached to the carbonyl. The inductive withdrawal of electron density shortens the C=O bond, thereby raising its stretching frequency. Esters are detected by their absorption at 1735 cm^–1, a position somewhat higher than that for either aldehydes or ketones. Amides, by contrast, absorb near the low-wavenumber end of the carbonyl region, with the degree of substitution on nitrogen affecting the exact position of the IR band. That is, for amides, the delocalization of electron density (resonance) from nitrogen into the carbonyl lengthens the C=O bond and lowers its stretching frequency.

Problem 21-25

What kinds of functional groups might compounds have if they show the following IR absorptions?

(a) Absorption at 1735 cm^–1

(b) Absorption at 1810 cm^–1

(c) Absorptions at 2500 to 3300 cm^–1 and 1710 cm^–1

(d) Absorption at 1715 cm^–1

Problem 21-26

Propose structures for compounds that have the following formulas and IR absorptions:

(a) C₆H₁₂O₂, 1735 cm^–1

(b) C₄H₉NO, 1650 cm^–1

(c) C₄H₅ClO, 1780 cm^–1