14.7
Spectroscopy of Carboxylic Acid Derivatives
Infrared spectroscopy is primarily used to determine the types of bonds and functional groups. In carboxylic acid derivatives, a typical carbonyl bond…
The IR spectra of all carboxylic acid derivatives exhibit a typical carbonyl stretching absorption between 1650 and 1850 cm−1.
For instance, esters show an absorption around 1740 cm−1, and acid halides exhibit absorption around 1800 cm−1.
In contrast, acid anhydrides exhibit two carbonyl absorptions due to the symmetrical and unsymmetrical vibrations from carbonyl stretching.
Interestingly, nitriles produce a strong characteristic absorption at a higher frequency due to the stretching of the polar carbon–nitrogen triple bond.
Conversely, amides exhibit a lower carbonyl absorption frequency due to the conjugation effect.
In 1H NMR, the highly deshielded acidic protons absorb far downfield around 10–12 ppm, whereas the protons of the α-carbon absorb around 2–2.5 ppm.
In 13C NMR, the carbonyl carbon of derivatives shows absorption around 160–185 ppm.
However, the nitrile carbon absorbs around 115–130 ppm due to sp hybridized carbon.
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Q1: What carbonyl absorption frequencies are observed in IR spectra of carboxylic acid derivatives?
Carboxylic acid derivatives exhibit carbonyl stretching absorption between 1650 and 1850 cm−1. Esters absorb around 1740 cm−1, while acid halides show higher absorption near 1800 cm−1. Acid anhydrides display two distinct carbonyl absorptions around 1760 and 1820 cm−1 due to symmetrical and unsymmetrical vibrations. Amides absorb at lower frequencies around 1660 cm−1 due to conjugation effects.
Q2: Why do amides show lower carbonyl absorption frequencies compared to other carboxylic acid derivatives?
Amides exhibit lower carbonyl absorption frequencies around 1660 cm−1 because conjugation delocalizes π electrons between the nitrogen and carbonyl carbon, inducing partial single bond character. This reduces the bond strength, requiring less energy to stretch and absorbing at lower frequencies than esters or acid halides.
Q3: How does the nitrile functional group appear in infrared spectroscopy?
Nitriles produce a strong characteristic absorption at higher frequency around 2200 cm−1 due to the stretching of the polar carbon–nitrogen triple bond. This distinct peak is significantly higher than carbonyl absorptions, making it easily identifiable in IR spectra and useful for distinguishing nitriles from other carboxylic acid derivatives.
Q4: What chemical shifts are observed for acidic protons and alpha-carbon protons in 1H NMR of carboxylic acid derivatives?
In 1H NMR spectra, highly deshielded acidic protons absorb far downfield around 10–12 ppm, while protons on the alpha-carbon adjacent to the carbonyl group absorb around 2–2.5 ppm. These distinct chemical shifts help identify the presence and position of functional groups in carboxylic acid derivatives.
Q5: Where do carbonyl carbons and nitrile carbons appear in 13C NMR spectroscopy?
In 13C NMR spectra, the carbonyl carbon of carboxylic acid derivatives shows absorption around 160–185 ppm. In contrast, the nitrile carbon absorbs at lower frequency around 115–130 ppm because it is sp hybridized, resulting in different electronic environment and shielding compared to sp2 carbonyl carbons.
Q6: Why do acid anhydrides exhibit two carbonyl absorptions in IR spectroscopy?
Acid anhydrides display two carbonyl absorption bands around 1760 and 1820 cm−1 because they contain two carbonyl groups that vibrate in symmetrical and unsymmetrical modes. These two distinct vibrations produce separate absorption peaks, distinguishing anhydrides from other carboxylic acid derivatives that show single carbonyl absorptions.
Q7: How does ring strain affect carbonyl absorption frequency in cyclic carboxylic acid derivatives?
Decreasing ring size in cyclic carboxylic acid derivatives increases ring strain, which increases the carbonyl absorption frequency in IR spectra. The increased strain tightens the carbonyl bond, requiring more energy to stretch and resulting in higher frequency absorption compared to acyclic derivatives.