13.7
IR and UV–Vis Spectroscopy of Carboxylic Acids
In IR spectroscopy of carboxylic acids, the C=O bond shows a characteristic band between 1710 and 1760 cm⁻¹, and the O–H bond exhibits a broad band be…
Carboxylic acids are easily detected via IR spectroscopy by identifying two characteristic absorptions formed from C=O and O–H stretching of the –COOH group.
In general, the –C=O group of a monomeric carboxylic acid absorbs at 1760 cm−1. However, dimerization due to hydrogen bonding lowers the stretching frequency to ∼1710 cm−1.
In conjugated systems, as resonance delocalization weakens the C–O double bond, the stretching frequency is further lowered to 1690 cm−1.
Additionally, the O–H bond stretching of the –COOH group generates a broad band at ∼2500–3500 cm−1, masking the C–H stretching peaks.
In UV–Vis spectroscopy, carboxylic acids absorb UV light in the range of 200–215 nm, which corresponds to an n→π∗ transition. It is a weak band with a molar absorptivity of <100.
However, the presence of an extensive conjugation shifts the absorption maxima to a higher wavelength with increased molar absorptivity of 25,000.
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Q1: What are the characteristic IR absorption bands for carboxylic acids?
Carboxylic acids display two distinctive IR absorptions from the –COOH group. The C=O stretch appears between 1710–1760 cm⁻¹, while the O–H stretch produces a broad band at 2500–3500 cm⁻¹. This broad O–H band masks the C–H stretching peaks in the spectrum, making these two features diagnostic for identifying carboxylic acids.
Q2: How does hydrogen bonding affect the C=O stretching frequency in carboxylic acids?
Monomeric carboxylic acids show C=O absorption at 1760 cm⁻¹, but dimerization through hydrogen bonding lowers this frequency to approximately 1710 cm⁻¹. This shift occurs because hydrogen bonding weakens the C=O double bond, reducing its stretching frequency and creating a characteristic lower-frequency band in IR spectra.
Q3: Why does conjugation lower the C=O stretching frequency in carboxylic acids?
In conjugated carboxylic acids, resonance delocalization weakens the C=O double bond character, lowering the C=O stretching frequency to approximately 1690 cm⁻¹. This further reduction compared to non-conjugated acids reflects the decreased bond strength caused by electron delocalization into the conjugated system.
Q4: What UV–Vis absorption characteristics distinguish carboxylic acids?
Carboxylic acids absorb UV light at 200–215 nm due to an n→π∗ transition, producing a weak band with molar absorptivity below 100. However, extensive conjugation shifts the absorption maximum to higher wavelengths with dramatically increased molar absorptivity reaching 25,000, enabling detection of conjugated carboxylic acids.
Q5: What is the difference between n→π∗ transitions in unconjugated versus conjugated carboxylic acids?
Unconjugated carboxylic acids exhibit weak n→π∗ transitions at 200–215 nm with low molar absorptivity. In highly conjugated carboxylic acids, this transition shifts to longer wavelengths with substantially higher molar absorptivity, allowing conjugated systems to absorb more strongly and be detected more readily in UV–Vis spectroscopy.
Q6: How can IR and UV–Vis spectroscopy be used together to identify carboxylic acids?
IR spectroscopy identifies the –COOH functional group through characteristic C=O and O–H stretches, while UV–Vis spectroscopy detects the n→π∗ transition at 200–215 nm. Combined, these techniques confirm carboxylic acid identity and reveal structural features like conjugation, which shifts UV absorption to higher wavelengths and increases molar absorptivity.
Q7: What spectroscopic features indicate a carboxylic acid is hydrogen-bonded or dimeric?
Dimeric carboxylic acids stabilized by hydrogen bonding show a C=O stretch at 1710 cm⁻¹ rather than the 1760 cm⁻¹ observed in monomeric forms. Additionally, small spikes or shoulder bands between 2500–2700 cm⁻¹ in the IR spectrum often indicate hydrogen-bonded carboxylic acids, reflecting the structural changes from dimerization.