13.8
NMR and Mass Spectroscopy of Carboxylic Acids
In ¹H NMR spectroscopy, acidic protons (–COOH) of carboxylic acids are highly deshielded and absorb far downfield, at around 9–12 ppm. The chemical sh…
In the 1H NMR spectra of carboxylic acids, the highly deshielded –COOH protons appear far downfield—in the range of 9 to 12 ppm.
In contrast, the shielded α protons appear upfield—between 2 to 2.5 ppm, while the β protons appear further upfield.
In deuterated solvents, the exchange of the acidic –COOH proton with deuterium results in the disappearance of the corresponding proton signal, which serves as an identification of carboxylic acids.
The 13C NMR spectra of carboxylic acids show a strongly deshielded –C=O carbon peak at ~160 to 180 ppm, while the shielded α carbons absorb at ~20 to 40 ppm.
The mass spectra of aliphatic carboxylic acids display a significantly small molecular ion peak, along with the base peak corresponding to a fragment formed by McLafferty rearrangement.
The second strongest peak is generated by a resonance-stabilized cation, formed by the loss of an alkyl radical.
Aromatic carboxylic acids show intense molecular ion peaks followed by other fragmentation peaks, signifying loss of –OH and –C=O groups.
View the full transcript and gain access to JoVE Core videos
Q1: Where do carboxylic acid protons appear in 1H NMR spectra?
The acidic –COOH protons of carboxylic acids are highly deshielded and appear far downfield at 9 to 12 ppm in 1H NMR spectra. In contrast, the shielded α protons appear between 2 to 2.5 ppm, while β protons absorb further upfield. The exact chemical shift depends on concentration and solvent used.
Q2: How can you identify carboxylic acids using deuterium oxide in NMR?
Carboxylic acids are easily identified by dissolving them in deuterium oxide, which causes rapid exchange of the acidic –COOH protons with deuterium. This exchange results in the disappearance of the corresponding proton signal in the 1H NMR spectrum, confirming the presence of a carboxylic acid functional group.
Q3: What chemical shift range characterizes carbonyl carbons in 13C NMR of carboxylic acids?
In 13C NMR spectroscopy, the carbonyl carbons of carboxylic acids absorb at approximately 160 to 180 ppm, appearing strongly deshielded. This range is lower than aldehydes and ketones due to the high shielding effect of the carboxylate oxygen's unshared pair of electrons. The α carbons absorb at 20 to 40 ppm.
Q4: What is the McLafferty rearrangement in mass spectra of carboxylic acids?
The McLafferty rearrangement of aliphatic carboxylic acids produces the base peak, an even-numbered mass fragment ion formed by loss of an alkene. This fragmentation is characteristic of carboxylic acids and appears as the strongest peak in their mass spectra, alongside a smaller molecular ion peak and a second prominent peak from resonance-stabilized cations.
Q5: How do aromatic carboxylic acids differ from aliphatic ones in mass spectrometry?
Aromatic carboxylic acids display intense molecular ion peaks followed by significant fragmentation peaks representing loss of –OH and –C=O groups. This contrasts with aliphatic carboxylic acids, which show significantly small molecular ion peaks and prominent base peaks from McLafferty rearrangement, reflecting the greater stability of aromatic molecular ions.
Q6: What fragmentation pattern results from loss of an alkyl radical in carboxylic acid mass spectra?
Loss of an alkyl radical from carboxylic acids generates the second strongest peak in their mass spectra, a resonance-stabilized cation with odd mass. This fragmentation is particularly prominent in aliphatic carboxylic acids and represents a major decomposition pathway alongside the McLafferty rearrangement.
Q7: Why are carboxylic acid carbonyl carbons more shielded than those in aldehydes and ketones?
Carboxylic acid carbonyl carbons absorb at lower chemical shifts (higher field) than aldehydes and ketones due to the high shielding effect of the carboxylate oxygen's unshared pair of electrons. This electron density shields the carbonyl carbon, reducing its deshielding and causing it to appear upfield relative to other carbonyl-containing compounds.