Interpreting IR spectra: which band belongs to which group?
An IR spectrum mainly shows which functional groups are present. What matters are a few strong bands above 1500 cm⁻¹ and their shape.
Rules and values
- O–H alcohol 3200–3550 cm⁻¹, broad; O–H carboxylic acid 2500–3300 cm⁻¹, very broad
- N–H amine 3300–3500 cm⁻¹, narrower than O–H
- C–H: above 3000 aromatic, 2850–2960 alkyl, 2720 + 2820 aldehyde
- C≡N 2210–2260 cm⁻¹, sharp
- C=O: ester 1735–1750, aldehyde 1720–1740, ketone 1705–1725, acid 1700–1725; conjugation with a ring lowers it by about 20–30
- C–O 1000–1300 cm⁻¹ (alcohol, ether, ester, acid)
Your turn
Three problems on this topic. Type whole numbers, choose splitting and structures.
Task
Which band do you expect in the IR spectrum of benzyl acetate?
Typical band
Type whole numbers, e.g. 3. NMR shifts are typical values (±0.3 ppm), splitting follows the n+1 rule.
Worked example
An IR spectrum shows these bands (nothing else stands out). Which compound class does the substance belong to?
IR spectrum
- 2850–2960 cm⁻¹
- 1735–1750 cm⁻¹ (strong)
- 1000–1300 cm⁻¹ (strong)
Solution
- 1.2850–2960 cm⁻¹: C–H (alkyl)
- 2.1735–1750 cm⁻¹: C=O (ester), strong
- 3.1000–1300 cm⁻¹: C–O, strong
- 4.Decisive: 1735–1750 cm⁻¹ → ester.
- 5.For example isopropyl formate.
Typical mistake: Only looking at the C=O band. Aldehyde (2720/2820), acid (very broad O–H) and ester (C–O 1000–1300, C=O above 1735) are told apart by the extra bands.
Which band do you expect in the IR spectrum of methyl butanoate?
Solution
- 1.Methyl butanoate is an ester.
- 2.1735–1750 cm⁻¹: C=O (ester), strong
- 3.All bands: 2850–2960 · 1735–1750 · 1000–1300 cm⁻¹.
Typical mistake: Mixing up groups that look alike: O–H (broad, 3200–3550) and N–H (narrower, 3300–3500), ketone and ester C=O (ester higher, plus C–O).
Common questions
How do I tell an ester from a ketone in IR?
The ester C=O band is a little higher (from about 1735 cm⁻¹) and there are strong C–O bands between 1000 and 1300 cm⁻¹.