21.10 • Spectroscopy of Carboxylic Acid Derivatives
All carbonyl-containing compounds have intense IR absorptions in the range 1650 to 1850 cm–1. As shown in Table 21.3 the exact position of the absorption provides information about the specific kind of carbonyl group. For comparison, the IR absorptions of aldehydes, ketones, and carboxylic acids are included in the table, along with values for carboxylic acid derivatives.
|Carbonyl type||Example||Absorption (cm–1)|
|Saturated acid chloride||Acetyl chloride||1810|
|Aromatic acid chloride||Benzoyl chloride||1770|
|Saturated acid anhydride||Acetic anhydride||1820, 1760|
|Saturated ester||Ethyl acetate||1735|
|Aromatic ester||Ethyl benzoate||1720|
|Saturated carboxylic acid||Acetic acid||1710|
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.
Nuclear Magnetic Resonance Spectroscopy
Hydrogens on the carbon next to a carbonyl group are slightly deshielded and absorb near 2 δ in the 1H NMR spectrum, although the exact identity of the carbonyl group can’t be determined. Figure 21.11 shows the 1H NMR spectrum of ethyl acetate.
Although 13C NMR is useful for determining the presence or absence of a carbonyl group in a molecule, the identity of the carbonyl group is difficult to determine. Aldehydes and ketones absorb near 200 δ, while the carbonyl carbon atoms of various acid derivatives absorb in the range 160 to 180 δ (Table 21.4).