Structure 3.2.9
Infrared (IR) Spectroscopy
Identifying functional groups by the vibrations of their covalent bonds.
The Fingerprint Region vs Functional Group Region
Fingerprint Region (400–1500 cm⁻¹)
Complex, unique pattern for every molecule (like a human fingerprint). Used for computer matching.
Functional Group Region (1500–4000 cm⁻¹)
Where specific bonds absorb. The area we analyze for identification.
Key Absorptions (Memorize These)
| Bond | Wavenumber (cm⁻¹) | Appearance | Found In |
|---|---|---|---|
| C=O | 1700 - 1750 | Strong, Sharp (The Sword) | Aldehydes, Ketones, Acids, Esters |
| O-H (Alcohol) | 3200 - 3600 | Strong, Broad (The Tongue) | Alcohols |
| O-H (Acid) | 2500 - 3000 | Very Broad, Messy (The Beard) | Carboxylic Acids |
| C-H | 2850 - 3090 | Strong, Sharp | Everything organic (Ignore usually) |