19.14 Spectroscopy of Aldehydes and Ketones

Spectroscopic Analysis of Aldehydes and Ketones

Interpretation of IR spectra

The carbonyl group produces one of the most recognizable absorptions in all of IR spectroscopy. Because the C=O bond is both strong and polar, it gives a sharp, intense peak that's hard to miss.

• Aldehydes show a strong C=O stretch at 1720–1740 cm$^{-1}$ (e.g., formaldehyde, acetaldehyde)
• Ketones absorb at a slightly lower frequency, around 1705–1720 cm$^{-1}$ (e.g., acetone, cyclohexanone)

The frequency difference comes down to electronic effects. Ketones have two electron-donating alkyl groups on the carbonyl carbon, which increases electron density in the C=O bond and lowers the stretching frequency slightly.

Aldehydes have one additional diagnostic feature: two weak C–H stretches between 2700–2850 cm$^{-1}$ from the aldehyde proton. These often appear as a pair of bands (sometimes called "Fermi doublet") and are unique to aldehydes. If you see a carbonyl stretch plus those two bands in the 2700–2850 region, you're almost certainly looking at an aldehyde.

Analysis of NMR spectra

NMR is where you can really pin down the structure of an aldehyde or ketone.

$^1$H NMR:

• The aldehyde proton (R–CHO) resonates far downfield at 9–10 ppm. This is a distinctive signal that immediately tells you an aldehyde is present.
• That aldehyde proton can couple to neighboring protons on the $\alpha$-carbon, though the coupling constant is small ($J \approx 2{-}3$ Hz). In simple cases like acetaldehyde (no $\alpha$-neighbors besides the methyl), you'll see it as a singlet or a slightly split peak.
• $\alpha$-Protons (on the carbon directly next to C=O) are deshielded by the electron-withdrawing carbonyl and typically appear around 2.0–2.5 ppm, noticeably downfield from ordinary alkyl protons.

$^{13}$C NMR:

• Aldehyde carbonyls resonate around 190–205 ppm
• Ketone carbonyls appear at 195–220 ppm

These are the most downfield signals in a typical $^{13}$C spectrum, making them easy to spot.

Substituent effects matter here too. Electron-withdrawing groups (halogens, nitro groups) shift signals further downfield, while electron-donating groups (alkyl, alkoxy) cause slight upfield shifts. For example, 4-chlorobenzaldehyde has a more downfield carbonyl signal than 4-methoxybenzaldehyde.

Mass spectrometry for isomer identification

Mass spectrometry reveals fragmentation patterns that help distinguish between isomeric aldehydes and ketones.

$\alpha$-Cleavage is the most common fragmentation:

1. The bond between the carbonyl carbon and an $\alpha$-carbon breaks
2. This produces an acylium ion ($RCO^+$), which is resonance-stabilized and gives a prominent peak
3. For ketones, cleavage can occur on either side of the carbonyl, so you may see two different acylium ions (e.g., 2-pentanone gives fragments at m/z = 43 from $CH_3CO^+$ and m/z = 71 from $CH_3CH_2CH_2CO^+$)

McLafferty rearrangement occurs when a $\gamma$-hydrogen is available (at least 3 carbons away from the carbonyl):

1. The $\gamma$-hydrogen transfers to the carbonyl oxygen through a six-membered cyclic transition state

2. The bond between the $\alpha$- and $\beta$-carbons breaks

3. This produces an enol radical cation and a neutral alkene that leaves the molecule

For example, pentanal (MW = 86) undergoes McLafferty rearrangement to give an enol fragment at m/z = 44.

Molecular ion peak ($M^+$): Usually observable for aldehydes and ketones, though aldehyde $M^+$ peaks tend to be weaker than ketone $M^+$ peaks. Aromatic carbonyl compounds like benzaldehyde and acetophenone give relatively strong molecular ion peaks due to the stability of the aromatic ring system.

Isotope peaks (M+1, M+2) help determine molecular formulas. The M+1 peak intensity reflects the number of carbons (each $^{13}$C contributes about 1.1% per carbon), while a significant M+2 peak suggests the presence of elements like chlorine or bromine.