12.8 Infrared Spectra of Some Common Functional Groups

Infrared spectroscopy identifies functional groups in organic compounds by detecting how different bonds absorb infrared light. Each functional group vibrates at characteristic frequencies, producing a unique absorption pattern that acts like a fingerprint. This section covers the key IR absorptions you need to recognize for common functional groups, including hydrocarbons, alcohols, and carbonyl compounds.

Infrared Spectroscopy and Functional Group Identification

How IR spectroscopy works

Infrared spectroscopy measures how molecules absorb infrared radiation. When a molecule absorbs IR light, its bonds vibrate more intensely through stretching (bonds getting longer and shorter) or bending (bond angles changing). Different bonds absorb at different frequencies, measured in wavenumbers ($cm^{-1}$).

The strength of an absorption depends on the change in dipole moment during the vibration. Polar bonds like $O-H$ and $C=O$ produce strong absorptions because their vibrations create large dipole changes. Symmetric, nonpolar bonds (like the $C \equiv C$ in an internal alkyne) produce weak or even absent signals.

Fourier transform infrared spectroscopy (FTIR) is the modern technique used to collect IR spectra quickly and with high resolution.

IR absorptions of common functional groups

• Alkanes ($C-H$)
  • Strong absorption at 2850–3000 $cm^{-1}$ from C–H bond stretching
  • Bending vibrations at 1450–1470 $cm^{-1}$ and 1370–1390 $cm^{-1}$ (e.g., propane)
  • These peaks appear in almost every organic molecule, so they're not very diagnostic on their own
• Alkenes ($C=C$)
  • Weak absorption around 1600–1680 $cm^{-1}$ from C=C stretching
  • Out-of-plane C–H bending at 800–1000 $cm^{-1}$ (e.g., cis-2-butene)
  • Also look for $=C-H$ stretching just above 3000 $cm^{-1}$ (3020–3100 $cm^{-1}$), which distinguishes alkene C–H from alkane C–H below 3000 $cm^{-1}$
• Alkynes ($C \equiv C$)
  • Weak absorption at 2100–2260 $cm^{-1}$ from C≡C stretching
  • Terminal alkynes show a sharp, strong $\equiv C-H$ stretch near 3300 $cm^{-1}$
  • Internal alkynes (like 2-butyne) lack that 3300 $cm^{-1}$ peak, and the C≡C stretch may be very weak or absent due to the symmetric bond having little dipole change
• Alcohols ($O-H$)
  • Strong, broad absorption at 3200–3600 $cm^{-1}$ from O–H stretching. The broadness comes from hydrogen bonding between alcohol molecules.
  • C–O stretching at 1050–1150 $cm^{-1}$ (e.g., ethanol)

Functional group analysis strategy

When you're interpreting an IR spectrum, work through it systematically:

1. Check the 3200–3600 $cm^{-1}$ region first. A strong, broad peak here points to an $O-H$ group (alcohol or carboxylic acid). A sharp peak near 3300 $cm^{-1}$ suggests a terminal alkyne $\equiv C-H$.
2. Look at the 2850–3000 $cm^{-1}$ region. C–H stretches appear here in nearly all organic compounds. Peaks just above 3000 $cm^{-1}$ suggest $sp^2$ C–H bonds (alkenes or aromatics).
3. Scan the 2000–2500 $cm^{-1}$ region. This is relatively quiet for most molecules. A peak at 2100–2260 $cm^{-1}$ indicates a triple bond ($C \equiv C$ or $C \equiv N$).
4. Examine the 1600–1800 $cm^{-1}$ region. Strong absorptions here usually mean a $C=O$ group. Weaker absorptions near 1600–1680 $cm^{-1}$ suggest $C=C$.
5. Use the absence of peaks too. No broad O–H stretch? The compound isn't an alcohol or carboxylic acid. No carbonyl peak near 1700 $cm^{-1}$? Rule out aldehydes, ketones, acids, and esters.

Carbonyl Compounds and Their IR Absorptions

The carbonyl group ($C=O$) produces one of the strongest and most recognizable absorptions in IR spectroscopy, typically appearing as an intense peak between 1650–1800 $cm^{-1}$. The exact position of this peak tells you which type of carbonyl compound you're dealing with.

Aldehydes ($R-CHO$)

• Strong $C=O$ stretch at 1720–1740 $cm^{-1}$
• Two weak C–H stretches at 2700–2850 $cm^{-1}$ (sometimes called "Fermi doublet"). These twin peaks are the key feature that distinguishes aldehydes from ketones. Look for them in propanal or butanal spectra.

Ketones ($R-CO-R'$)

• Strong $C=O$ stretch at 1705–1725 $cm^{-1}$ (slightly lower than aldehydes)
• No C–H stretches in the 2700–2850 $cm^{-1}$ region, which is how you tell them apart from aldehydes (e.g., 2-butanone)

Carboxylic acids ($R-COOH$)

• Very broad $O-H$ stretch at 2500–3300 $cm^{-1}$. This is broader than an alcohol O–H and often overlaps the C–H stretching region, giving the spectrum a distinctive "messy" look in that area.
• Strong $C=O$ stretch at 1700–1730 $cm^{-1}$
• C–O stretch at 1210–1320 $cm^{-1}$ (e.g., acetic acid)

Esters ($R-COO-R'$)

• Strong $C=O$ stretch at 1735–1750 $cm^{-1}$ (the highest carbonyl frequency among these four groups)
• C–O stretches at 1000–1300 $cm^{-1}$ (often two bands)
• No broad O–H absorption, which distinguishes esters from carboxylic acids (e.g., ethyl acetate)

A useful trend to remember: electron-withdrawing effects and ring strain push the $C=O$ frequency higher, while conjugation (like a $C=C$ next to the $C=O$) lowers it. That's why ester carbonyls absorb at higher frequencies than ketones.