BRAINCAKEInterpreting IR Spectra
To interpret an IR spectrum, match the main absorptions above 1500 cm−1 to bonds using the data sheet, then decide which functional group fits.
Step by step
1. Look for C=O: a strong, sharp absorption at 1680 to 1750 cm−1.
2. Look for O-H: a broad absorption at 3230 to 3550 cm−1 is an alcohol O-H. A very broad absorption at 2500 to 3000 cm−1 is a carboxylic acid O-H.
3. Check for other bonds such as N-H (3300 to 3500), C=C (1620 to 1680) or C≡N (2220 to 2260).
4. Do not rely on C-H (2850 to 3300), which almost every organic molecule shows.
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Key terms in this lesson
- aldehyde
- An organic compound containing the -CHO functional group at the end of a carbon chain.
- ketone
- An organic compound containing a C=O group bonded to two carbon atoms within the chain.
- carboxylic acid
- An organic compound containing the -COOH functional group.
More in Organic Analysis
- Identifying Functional Groups
- Test Tube Tests for Organic Compounds
- High Resolution Mass Spectrometry
- Precise Relative Molecular Mass
- Infrared Spectroscopy
- Bond Vibrations and IR Absorption
- The Fingerprint Region
- Infrared Radiation and the Greenhouse Effect
All 9 lessons in Organic Analysis · All AQA AS-level Chemistry topics