Key Absorptions (From Data Booklet)
| Bond | Wavenumber / cm⁻¹ | Appearance | Indicates |
|---|---|---|---|
| O-H (alcohol) | 3200-3550 | Broad | Alcohol |
| O-H (acid) | 2500-3300 | Very broad | Carboxylic acid |
| N-H | 3300-3500 | Medium, often 2 peaks | Amine / amide |
| C=O | 1700-1750 | Strong, sharp | Aldehyde, ketone, acid, ester |
| C-O | 1000-1300 | Strong | Alcohol, ester, ether |
| C=C | 1620-1680 | Medium | Alkene |
IR Regions
Diagnostic Region vs Fingerprint Region (< 1400 cm⁻¹)
Below ~\(1400\text{ cm}^{-1}\) is the fingerprint region: a complex pattern of bending and stretching vibrations unique to each molecule. It is used for computer database comparison rather than individual peak assignment.
Functional group diagnosis focuses on the functional group region (\(1500\text{--}4000\text{ cm}^{-1}\)).
Exam Tip
Carboxylic Acid Pattern: Broad O-H + Sharp C=O
A very broad absorption band at \(2500\text{--}3300\text{ cm}^{-1}\) (O–H acid) plus a sharp strong absorption at \(1700\text{--}1750\text{ cm}^{-1}\) (C=O carbonyl) is the unmistakable diagnostic signature of a carboxylic acid.
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