21.10 Spectroscopy of Carboxylic Acid Derivatives

21.10 • Spectroscopy of Carboxylic Acid Derivatives

Infrared Spectroscopy

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.

Table 21.3 Infrared Absorptions of Some Carbonyl Compounds

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 amide	Acetamide	1690
Aromatic amide	Benzamide	1675
N-Substituted amide	N-Methylacetamide	1680
N,N-Disubstituted amide	N,N-Dimethylacetamide	1650
Saturated aldehyde	Acetaldehyde	1730
Saturated ketone	Acetone	1715
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.

Figure 21.11 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).

Table 21.4 ¹³C NMR Absorptions of Some Carbonyl Compounds

Compound	Absorption (δ)
Acetic acid	177.3
Ethyl acetate	170.7
Acetyl chloride	170.3
Acetamide	172.6
Acetic anhydride	166.9
Acetone	205.6
Acetaldehyde	201.0