21.10 Spectroscopy of Carboxylic Acid Derivatives

Spectroscopic Analysis of Carboxylic Acid Derivatives

Carboxylic acid derivatives each leave a distinct spectroscopic fingerprint. IR spectroscopy reveals where the carbonyl absorbs, NMR shows characteristic chemical shifts for nearby hydrogens and the carbonyl carbon itself, and mass spectrometry provides molecular weight and fragmentation clues. Combining these techniques lets you confidently distinguish between acids, esters, amides, anhydrides, and acid chlorides.

Carbonyl Identification through Infrared

The $C=O$ stretch is one of the strongest, most recognizable absorptions in IR spectroscopy, appearing in the 1630–1850 cm$^{-1}$ region. The exact frequency tells you which type of derivative you're looking at, because the electronic environment around the carbonyl shifts the absorption up or down.

The general principle: anything that donates electron density into the $C=O$ bond (through resonance) lowers the stretching frequency, while electron-withdrawing groups raise it. More double-bond character means a stiffer bond and a higher frequency.

Here's how each derivative compares:

• Acid chlorides absorb at the highest frequencies, around 1785–1815 cm$^{-1}$. The electronegative chlorine withdraws electron density, increasing the $C=O$ bond's force constant. Examples: acetyl chloride, benzoyl chloride.
• Anhydrides are distinctive because they show two carbonyl absorptions due to symmetric (in-phase) and asymmetric (out-of-phase) stretching of the two $C=O$ groups. The symmetric stretch appears around 1800–1830 cm$^{-1}$, and the asymmetric stretch around 1740–1775 cm$^{-1}$. If you see two carbonyl peaks, think anhydride. Examples: acetic anhydride, phthalic anhydride.
• Esters absorb around 1735–1750 cm$^{-1}$. The oxygen's lone pair donates into the carbonyl through resonance, but oxygen is electronegative enough that the frequency stays relatively high. Examples: ethyl acetate, methyl benzoate.
• Carboxylic acids absorb around 1700–1725 cm$^{-1}$. The key diagnostic feature is the broad $O-H$ stretch at 2500–3300 cm$^{-1}$, which is unmistakable because extensive hydrogen bonding makes it very wide and often overlaps the $C-H$ stretches. Examples: acetic acid, benzoic acid.
• Amides absorb at the lowest frequencies, around 1640–1690 cm$^{-1}$. Nitrogen is less electronegative than oxygen, so its lone pair donates more effectively into the carbonyl, giving the $C=O$ bond more single-bond character. Primary and secondary amides also show $N-H$ stretches around 3200–3400 cm$^{-1}$. Examples: acetamide, benzamide.

NMR Detection of Carbonyl-Adjacent Hydrogens

Hydrogens near a carbonyl group are deshielded by its electron-withdrawing effect, so they appear at higher chemical shifts (further downfield) than typical alkyl hydrogens. Beyond the $\alpha$-hydrogens, each derivative class has additional diagnostic signals in $^1H$ NMR.

• Carboxylic acids: The acidic $O-H$ proton appears as a broad singlet at 10–13 ppm. This signal is broad because the proton undergoes rapid exchange. The $\alpha$-hydrogens appear at 2.0–2.5 ppm.
• Esters: The $\alpha$-hydrogens appear at 2.0–2.5 ppm (similar to acids). The hydrogens on the alkoxy group ($OCH_3$ or $OCH_2$) appear further downfield at 3.7–4.2 ppm because of the deshielding effect of the directly attached oxygen. This alkoxy signal is a reliable way to identify an ester.
• Amides: $N-H$ protons appear as broad signals at 5–9 ppm. The broadness comes from hydrogen bonding and exchange, and the wide chemical shift range reflects how sensitive these protons are to concentration, solvent, and temperature. The $\alpha$-hydrogens appear at 2.0–2.5 ppm.
• Anhydrides and acid chlorides: The $\alpha$-hydrogens appear at 2.0–2.5 ppm. These derivatives lack the distinctive $O-H$ or $N-H$ signals, so you'll rely more heavily on IR and $^{13}C$ NMR to tell them apart.

Carbonyl Types in $^{13}C$ NMR

The carbonyl carbon appears far downfield (roughly 160–185 ppm) because it's bonded to an electronegative oxygen. The ranges overlap somewhat between derivative classes, so $^{13}C$ NMR is most useful when combined with IR and $^1H$ data.

• Carboxylic acids: carbonyl carbon at 170–185 ppm
• Acid chlorides: carbonyl carbon at 170–185 ppm (similar range to acids)
• Esters: carbonyl carbon at 165–175 ppm; the alkoxy carbon also appears at 50–70 ppm, which helps confirm an ester
• Amides: carbonyl carbon at 160–180 ppm
• Anhydrides: carbonyl carbons at 160–175 ppm

Because several of these ranges overlap, you can't usually identify the derivative from $^{13}C$ NMR alone. The carbonyl carbon shift narrows your options, but the IR carbonyl frequency and $^1H$ NMR signals are what clinch the identification.

Putting It All Together: Structural Elucidation

When you're given an unknown carboxylic acid derivative, work through the data systematically:

1. Start with IR. Locate the carbonyl stretch and note its frequency. Check for additional diagnostic absorptions (broad $O-H$ for acids, two $C=O$ peaks for anhydrides, $N-H$ for amides).
2. Check $^1H$ NMR. Look for signals in the 10–13 ppm range (acid $O-H$), 5–9 ppm range (amide $N-H$), or 3.7–4.2 ppm range (ester alkoxy). The $\alpha$-hydrogen region (2.0–2.5 ppm) confirms a carbonyl is present but doesn't distinguish between types.
3. Use $^{13}C$ NMR to confirm the carbonyl carbon's presence and narrow the range. Look for an alkoxy carbon (50–70 ppm) if you suspect an ester.
4. Use mass spectrometry for molecular weight and fragmentation patterns. Common fragmentations include loss of $OH$ (mass 17) or $OCH_3$ (mass 31) for acids and methyl esters, loss of $Cl$ (mass 35/37) for acid chlorides, and loss of $RCO_2H$ for anhydrides.

No single technique gives you the full answer. The power is in combining them.