Which functional group has a characteristic absorption in the 1630-1800 cm-1 region?

Question

BlackTom AI · Accepted Answer

Carbonyl stretching vibrations are strong and diagnostic: the amide C=O (the amide I band) typically absorbs in the 1630–1700 cm⁻¹ region (often quoted ~1630–1690 cm⁻¹), so an amide is the correct choice.

1) Relevant knowledge points and concepts
- Infrared (IR) spectroscopy: bond stretching frequencies depend on bond strength and reduced mass; C=O stretches are intense and fall in the 1600–1800 cm⁻¹ region.
- Amide-specific bands: the amide I band (mainly C=O stretch) appears near 1630–1690 cm⁻¹; the amide II band (N–H bend + C–N stretch) appears near ~1540 cm⁻¹.
- Other functional groups have characteristic, different regions (O–H and N–H stretches are higher frequency, C=C stretches are weaker and often lower intensity).

2) Step-by-step reasoning
- Identify the given spectral window: 1630–1800 cm⁻¹ falls squarely into the carbonyl stretching region.
- Determine which answer choices contain a carbonyl: among the options, “amide” contains a C=O; alkene, amine, and alcohol do not have a carbonyl.
- Recall that amide C=O stretching (amide I) commonly lies around 1630–1690 cm⁻¹; conjugation or hydrogen bonding can shift and broaden it but keeps it within the 1630–1800 cm⁻¹ window.

3) Intermediate numerical/placement steps
- Typical ranges: amide I ≈ 1630–1690 cm⁻¹; ketone/aldehyde ≈ 1700–1750 cm⁻¹; ester ≈ 1735–1750 cm⁻¹. The provided window (1630–1800) therefore includes the amide I band comfortably.

4) Why the correct answer is correct
- The amide C=O stretch is a strong, characteristic absorption within the specified region; it is routinely used to identify amide linkages (e.g., peptides, amides) in IR spectra.

5) Why the other options are incorrect
- b. Alkene: C=C stretches are weaker and typically near ~1620–1680 cm⁻¹ but are less diagnostic and often lower intensity; the canonical diagnostic in this region for a strong band is a carbonyl, not an alkene.
- c. Amine: N–H stretches occur around 3300–3500 cm⁻¹; N–H bends are lower but not the strong 1630–1800 cm⁻¹ carbonyl band.
- d. Alcohol: O–H stretches are broad around 3200–3600 cm⁻¹, not 1630–1800 cm⁻¹.

6) Conclusion
The correct answer is a. amide, which matches the provided answer.

Which functional group has a characteristic absorption in the 1630-1800 cm-1 region?Single choice

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