Which molecule would you expect tohave the lowest wavenumber for the C=O π bond IR band?

• A.
• B.
• C.
• D.

Answer: (C)

Verified step by step guidance

1. Identify the functional groups present in each molecule: ester, aldehyde, amide, and acyl chloride.
2. Understand that the C=O bond in different functional groups absorbs IR radiation at different wavenumbers due to variations in bond strength and resonance effects.
3. Recognize that resonance and electron-withdrawing effects can influence the C=O bond strength. For example, amides have resonance that delocalizes the lone pair on nitrogen, reducing the C=O bond strength.
4. Compare the typical IR absorption ranges for the C=O bond in each functional group: esters (1735-1750 cm⁻¹), aldehydes (1720-1740 cm⁻¹), amides (1640-1690 cm⁻¹), and acyl chlorides (1750-1820 cm⁻¹).
5. Conclude that the amide group, due to its resonance stabilization, will have the lowest wavenumber for the C=O bond IR band among the given options.