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Organic Chemistry

21.10 Spectroscopy of Carboxylic Acid Derivatives

Organic Chemistry21.10 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.

Problem 21-25
What kinds of functional groups might compounds have if they show the following IR absorptions?
Absorption at 1735 cm–1
Absorption at 1810 cm–1
Absorptions at 2500 to 3300 cm–1 and 1710 cm–1
Absorption at 1715 cm–1
Problem 21-26
Propose structures for compounds that have the following formulas and IR absorptions:
C6H12O2, 1735 cm–1
C4H9NO, 1650 cm–1
C4H5ClO, 1780 cm–1

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.

The proton NMR spectrum of ethyl acetate shows peaks at 1.23 (triplet), 2.01 (singlet), and 4.10 (quartet). The methylene protons are deshielded due to the presence of an adjacent O atom.
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
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