20.8 Spectroscopy of Carboxylic Acids and Nitriles

Spectroscopic Identification of Carboxylic Acids

Carboxylic Acid IR Absorptions

The IR spectrum of a carboxylic acid is one of the most recognizable in organic chemistry because two features overlap in a distinctive way.

• Broad O–H stretch: 2500–3300 cm$^{-1}$. This absorption is unusually broad compared to a typical alcohol O–H stretch. The broadening comes from extensive hydrogen bonding between carboxylic acid molecules, which often exist as dimers in solution and the neat liquid. The breadth of this band frequently overlaps with the C–H stretches in the same region, giving the baseline a characteristic "hump."
• Strong C=O stretch: 1700–1730 cm$^{-1}$. This is a sharp, intense peak. Conjugation with an aromatic ring or alkene lowers the frequency toward 1700 cm$^{-1}$, while simple aliphatic acids (like propanoic acid) absorb closer to 1710–1725 cm$^{-1}$.

Seeing both the broad O–H and the strong C=O together is the hallmark of a carboxylic acid. An alcohol gives a broad O–H but no carbonyl peak; a ketone gives a carbonyl peak but no broad O–H. The combination is what makes the assignment definitive.

NMR Spectra of Carboxylic Acids

$^{1}$H NMR

The acidic proton (COOH) appears as a broad singlet between 10–13 ppm, far downfield of most other protons. This signal is broad because the proton rapidly exchanges with trace water or other acidic protons in solution. In some spectra, the peak may be so broad it nearly disappears into the baseline, so don't rely on it alone.

To distinguish carboxylic acids from other carbonyls in $^{1}$H NMR:

• Aldehydes show a sharp singlet at 9–10 ppm (the CHO proton), which is not exchangeable and is typically much sharper than the COOH signal.
• Ketones and esters have no proton signal in this downfield region at all.

$^{13}$C NMR

The carboxylic acid carbonyl carbon resonates at 170–185 ppm. This range overlaps somewhat with esters (~165–180 ppm), but carboxylic acids tend to appear at the higher end. Aldehydes and ketones absorb further downfield (190–210 ppm), so $^{13}$C NMR is useful for distinguishing these functional groups from one another.

Spectroscopic Identification of Nitriles

IR Absorption of Nitriles

The C$\equiv$N triple bond produces a sharp, medium-intensity absorption at 2200–2260 cm$^{-1}$. This region of the IR spectrum is relatively "quiet" for most organic molecules, which makes the nitrile peak easy to spot. Few other functional groups absorb here; the main one to watch for is an alkyne C$\equiv$C stretch, which appears nearby at 2100–2260 cm$^{-1}$ but is often weaker and sometimes absent entirely in symmetrical alkynes.

A couple of points to keep in mind:

• The nitrile stretch position is fairly insensitive to substituents, so conjugated and unconjugated nitriles absorb in nearly the same place.
• If you see a peak in the 2200–2260 cm$^{-1}$ region without a broad O–H or N–H stretch nearby, a nitrile is a strong possibility.

NMR of Nitriles

$^{13}$C NMR

The nitrile carbon (C$\equiv$N) resonates at 110–125 ppm. This is well separated from carbonyl carbons (>165 ppm) and from most sp$^{2}$ and sp$^{3}$ carbons, making it a reliable diagnostic signal.

$^{1}$H NMR

Nitriles have no proton on the functional group itself, so there's no single "nitrile proton" peak to look for. Instead, look at the $\alpha$-protons (the hydrogens on the carbon directly attached to C$\equiv$N). These typically appear around 2.0–3.0 ppm, shifted slightly downfield by the electron-withdrawing nitrile group. The deshielding effect is milder than what you'd see next to a carbonyl or a nitro group.

Combining Techniques

In practice, you'll rarely rely on a single spectrum. A good approach for an unknown that might be a carboxylic acid or nitrile:

1. Check the IR first for the broad O–H + C=O combination (carboxylic acid) or the sharp C$\equiv$N stretch near 2220 cm$^{-1}$ (nitrile).
2. Confirm with $^{13}$C NMR: look for a signal at 170–185 ppm (carboxylic acid) or 110–125 ppm (nitrile).
3. Use $^{1}$H NMR to check for the broad, exchangeable COOH peak (10–13 ppm) or the absence of any functional-group proton (nitrile).
4. Mass spectrometry can supply the molecular weight and fragmentation pattern to further narrow down the structure.