The IR spectra for acetaminophen and phenacetin are shown below. Identify which spectrum is for acetaminophen and which is for phenacetin. Label the key bond/functional group signals on each spectrum: (e.g: N-H, C-O, O-H, C=C, C(sp²)-H, C(sp³)-H) OH acetaminophen phenacetin 1000 100 300 1500 1000 400
The Correct Answer and Explanation is:
To distinguish the IR spectra of acetaminophen and phenacetin, we must focus on the key functional groups and their characteristic absorption bands. Both acetaminophen and phenacetin are derivatives of paracetamol and contain similar functional groups (e.g., amides, aromatic rings), but they differ in their additional functional groups, which will be reflected in their IR spectra.
Key Functional Groups to Look For:
- Hydroxyl Group (OH):
- Acetaminophen contains a hydroxyl group attached to the benzene ring. This will show up as a broad absorption around 3200-3550 cm⁻¹ due to the O-H stretching vibration.
- Phenacetin also has a hydroxyl group, but the O-H stretch might be less pronounced due to differences in the rest of the structure.
- Amines (N-H):
- Both acetaminophen and phenacetin contain a nitrogen atom in their structures, but acetaminophen has an amide (NH group), while phenacetin contains an N-ethyl group. The N-H stretch in acetaminophen would typically appear around 3300 cm⁻¹, while in phenacetin, if present, the N-H stretch would be weaker.
- Carbonyl Group (C=O):
- Both compounds have an amide group, but acetaminophen contains a primary amide, and this will show a characteristic sharp absorption around 1650 cm⁻¹ for the C=O stretch.
- Phenacetin, containing an ether group in place of the primary amide, would not show the strong C=O stretch but instead show absorption around 1250-1350 cm⁻¹ due to the C-O stretch of the ether group.
- Aromatic C-H Stretching:
- Both acetaminophen and phenacetin have a benzene ring, and their C-H stretches will appear around 3000 cm⁻¹ (just below the O-H stretch, indicating aromatic C-H).
Identifying the Spectra:
- Acetaminophen’s spectrum will show a broad O-H stretch around 3200-3550 cm⁻¹, a strong C=O stretch around 1650 cm⁻¹, and aromatic C-H stretch just below 3000 cm⁻¹. The presence of a strong, broad N-H stretch can also be seen.
- Phenacetin’s spectrum will show a less prominent O-H stretch than acetaminophen, a weaker N-H stretch around 3300 cm⁻¹, and a strong C-O stretch in the ether region (1250-1350 cm⁻¹), with aromatic C-H stretch below 3000 cm⁻¹ as well.
These differences will allow you to match the spectra with their respective compounds.
