Interpreting IR Spectra

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
• carboxylic acid: very broad O-H and C=O (and C-O)
• ester: C=O and C-O, no O-HIR can follow a reaction. When propan-1-ol is partially oxidised to propanal, the broad O-H absorption at 3200 to 3600 cm⁻¹ disappears and a C=O absorption appears. If the product were propanoic acid, the very broad O-H at 2500 to 3300 cm⁻¹ would appear as well.IR alone often cannot tell apart compounds with the same groups, such as propanal and propanone; other evidence (for example a mass spectrum) is then needed.Worked example: compound X, C₃H₆O₂, shows a very broad trough from 2500 to 3300 cm⁻¹ and a sharp trough at 1715 cm⁻¹. The very broad O-H and the C=O together show a carboxylic acid, so X is propanoic acid, CH₃CH₂COOH.