20.9: Spectroscopy of Carboxylic Acids and Nitriles

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Infrared Spectroscopy

Carboxylic acids have two characteristic IR absorptions that make the –CO₂H group easily identifiable. The O–H bond of the carboxyl group gives rise to a very broad absorption over the range 2500 to 3300 cm^–1. The $C═O$ bond shows an absorption between 1710 and 1760 cm^–1. The exact position of $C═O$ absorption depends both on the structure of the molecule and on whether the acid is free (monomeric) or hydrogen-bonded (dimeric). Free carboxyl groups absorb at 1760 cm^–1, but the more commonly encountered dimeric carboxyl groups absorb in a broad band centered around 1710 cm^–1. As with other carbonyl-containing functional groups, conjugation with an alkene or benzene ring lowers the frequency of the $C═O$ stretch by 20 to 30 cm^–1.

Structures of free carboxylic acid (rare) and associated carboxylic acid (two molecules hydrogen-bonded together). I R peaks for free carboxyl group around 1760, associated carboxyl group at 1710 wavenumbers.

Both the broad O–H absorption and the $C═ O Figure 20.6.$

I R spectrum of butanoic acid shows broad peak at 3000 (hydroxyl group), and strong carbonyl peak at 1710 wavenumbers. — Figure 20.6: IR spectrum of butanoic acid, CH₃CH₂CH₂CO₂H.

Nitriles show an intense and easily recognizable $C≡N$ bond absorption near 2250 cm^–1 for saturated compounds and 2230 cm^–1 for aromatic and conjugated molecules. Few other functional groups absorb in this region, so IR spectroscopy is highly diagnostic for nitriles.

Cyclopentanecarboxylic acid and 4-hydroxycyclohexanone have the same formula (C₆H₁₀O₂), and both contain an –OH and a C═O group. How could you distinguish between them using IR spectroscopy?

Nuclear Magnetic Resonance Spectroscopy

Carboxyl carbon atoms absorb in the range 165 to 185 δ in the ¹³C NMR spectrum, with aromatic and α,β-unsaturated acids near the upfield end of the range (∼165 δ) and saturated aliphatic acids near the downfield end (∼185 δ). Nitrile carbons absorb in the range 115 to 130 δ.

In the ¹H NMR spectrum, the acidic –CO₂H proton normally absorbs as a singlet near 12 δ. The chemical shift of the carboxyl proton is concentration and solvent dependent, as these variables can change the extent of hydrogen-bonding in the sample. In some cases, the carboxyl-proton resonance is broadened to the point of being nearly undetectable. Traces of water in the sample can exacerbate the situation. As with alcohols (Section 17.11), the –CO₂H proton can be replaced by deuterium when D₂O is added to the sample tube, causing the absorption to disappear from the NMR spectrum. Figure 20.7 shows the ¹H NMR spectrum of phenylacetic acid. Note that the –CO₂H absorption occurs at 12.0 δ.

Figure 20.7: Proton NMR spectrum of phenylacetic acid, C₆H₅CH₂CO₂H.

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