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

Question

BlackTom AI · Accepted Answer

To tackle this question, I’ll evaluate each functional group’s typical infrared (IR) absorption region and compare it to the given 1630-1800 cm-1 window.

Option a: alcohol. Alcohols show a broad O–H stretching band centered around 3200–3550 cm-1, which is well above the 1630–1800 cm-1 region. They may couple with C–C vibrations, but there is no characteristic strong absorption in the 1630–1800 cm-1 range for alcohols. Therefore, this option is not consistent with the specified region.

Option b: amide. Amides have a very diagnostic carbonyl stretch (the amide I band) around 1640–1690 cm-1, arising from the C=O stretch of the amide linkage, and it is often the dominant peak in that region. Additionally, amides may show an amide II band near 1500–1560 cm-1, but the hallmark absorption in the 1630–1800 cm-1 window is the amide I band, which aligns with the given region. This makes amide a strong match for the stated region.

Option c: amine. Amines exhibit N–H bending and stretching features, typically showing N–H stretches around 3300–3500 cm-1 (for primary and secondary amines) and bending modes near 1600–1650 cm-1, but the most diagnostic, strong absorptions in the high-frequency region are not centered in 1630–1800 cm-1. The 1630–1800 cm-1 range is not the characteristic fingerprint for amines, so this option is unlikely.

Option d: alkene. Alkenes show C=C stretching vibrations around 1620–1680 cm-1, which can fall near the lower end of the provided window. However, this region is not as distinctive or exclusive to alkenes and often overlaps with other carbonyl and conjugated systems. Since the question asks for a characteristic absorption in that region, the presence of a carbonyl-containing amide band (amide I) provides a clearer, more diagnostic assignment than a generic alkene C=C stretch. Thus, this option is less correct than the amide.

In summary, comparing the typical IR regions, the 1630–1800 cm-1 window is best associated with the amide carbonyl stretch (amide I) rather than alcohol O–H, amine N–H, or alkene C=C stretches, making the amide option the most consistent with the given region.

CHM1052 - MUM S2 2025 CHM1052 2022 practice exam 1

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

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