Infrared (IR) spectroscopy is particularly useful for identifying functional groups in organic molecules because each group exhibits characteristic absorption frequencies. These absorptions arise from the vibrational modes of specific bonds within the functional group. For example:
O–H stretching in alcohols and phenols appears as a broad peak around 3200–3600 cm⁻¹.
C=O stretching in carbonyl compounds (e.g., ketones, aldehydes, esters) typically occurs near 1700 cm⁻¹.
C–H stretching vibrations are found in the range of 2850–3300 cm⁻¹, depending on the hybridization of the carbon (sp³, sp², or sp).
By comparing the IR spectrum of an unknown compound to known absorption frequencies, chemists can identify the functional groups present in the molecule.
| Functional Group | Bond | Wavenumber (cm⁻¹) | Intensity | Notes |
|---|---|---|---|---|
| Alkanes | C-H | 2850-2960 | Strong | |
| Alkenes | C=C | 1620-1680 | Variable | |
| Alkenes | =C-H | 3010-3100 | Medium | |
| Alkynes | C≡C | 2100-2260 | Weak to medium | |
| Alkynes | ≡C-H | ~3300 | Strong, sharp | terminal alkynes |
| Aromatic Rings | C=C | 1450-1600 | Variable | Multiple peaks |
| Alcohols | O-H | 3200-3600 | Strong, broad | |
| Alcohols | C-O | 1000-1300 | Strong | various intensities and exact location dependant on if the alcohol is primary, secondary, or tertiary. |
| Carboxylic Acids | O-H | 2500-3300 | Strong, very broad | |
| Carboxylic Acids | C=O | 1700-1725 | Strong | |
| Aldehydes | C=O | 1720-1740 | Strong | |
| Aldehydes | C-H | 2700-2720 & 2800-2850 | Weak to medium | Two peaks |
| Ketones | C=O | 1705-1725 | Strong | |
| Esters | C=O | 1730-1750 | Strong | |
| Esters | C-O | 1000-1300 | Strong | Multiple peaks. |
| Amines | N-H | 3300-3500 | Medium | 1 or 2 peaks depending on primary or secondary amine. |
| Nitriles | C≡N | 2210-2260 | Medium | sharp |
| Amides | C=O | 1640-1690 | Strong | |
| Amides | N-H | 3100-3500 | variable |