Interpreting IR Spectra

OCR AS-level ChemistryAnalytical TechniquesLesson 4 of 10

To interpret an IR spectrum, look for a small number of key absorptions rather than trying to explain every trough.

Method
1. Is there a strong, sharp C=O trough at 1630 to 1820 cm⁻¹?
2. Is there a broad O-H trough? Alcohol O-H is at 3200 to 3600 cm⁻¹; carboxylic acid O-H is very broad at 2500 to 3300 cm⁻¹.
3. Is there a C-O trough at 1000 to 1300 cm⁻¹?
4. Combine the evidence:
• alcohol: broad O-H and C-O, no C=O
• aldehyde or ketone: C=O, no O-H

More in Analytical Techniques

• Infrared Spectroscopy
• Bond Vibrations and IR Absorption
• Characteristic IR Absorptions
• IR in Breathalysers and Pollution Monitoring
• Mass Spectrometry of Organic Compounds
• The Molecular Ion Peak
• The M+1 Peak
• Fragmentation

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