Which region in the IR spectrum could be used to distinguish between the following structures?

Question

BlackTom AI · Accepted Answer

Amide C=O stretches appear in the mid-IR near 1630–1700 cm^-1 (the amide I band), so the presence of a CONH2 group versus a CH2NH2 group is best distinguished in the 1600–1650 cm^-1 region.

1) Relevant knowledge points and concepts
- Functional groups give characteristic IR absorptions: a carbonyl (C=O) in an amide gives a strong amide I band (~1630–1690 cm^-1, often ~1650 cm^-1 for conjugated/aromatic amides). 
- Primary amines (NH2) show N–H stretches around 3300–3500 cm^-1 and N–H bends near 1600–1650 cm^-1 can be weaker, but a strong, sharp C=O stretch is diagnostic for an amide.
- Alkyl/aromatic C–H stretches are 2850–3100 cm^-1 and C–C aromatic stretches appear around 1450–1600 cm^-1.

2) Step-by-step reasoning
- Identify the two structures: (A) benzamide (Ar–C(=O)–NH2) contains a carbonyl directly attached to an aromatic ring; (B) benzylamine (Ar–CH2–NH2) lacks a carbonyl and instead has a methylene next to an amine.
- The key differentiator is the presence vs absence of the C=O group. The strongest, most diagnostic band for that is the amide C=O stretch (amide I) near 1650 cm^-1.
- Therefore the IR region around 1600–1650 cm^-1 will show a strong C=O absorption for benzamide and no comparable C=O band for benzylamine.

3) (Calculations) No numeric calculation required; mapping of typical band positions suffices.

4) Why the correct answer is correct
- Option d (1600–1650 cm^-1) corresponds to the amide C=O stretch region; benzamide gives a strong band here while benzylamine does not. This provides a clear distinguishing feature.

5) Why other options are incorrect (brief)
- a (2850–3000 cm^-1): both compounds have C–H stretches in this region (aromatic and CH2), so not distinguishing.
- b (1800–1950 cm^-1): corresponds to highly conjugated/strained carbonyls or cumulenes (e.g., acid chlorides, anhydrides) not relevant here.
- c (1450–1550 cm^-1): falls in CH bending/aromatic ring region; both compounds show bands here and it won’t uniquely identify the carbonyl.

6) Conclusion
- The distinguishing IR region is 1600–1650 cm^-1, confirming answer d.

Which region in the IR spectrum could be used to distinguish between the following structures?单项选择题

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