Infrared spectroscopy: the carbonyl stretch of ketones and aldehydes typically occurs near which wavenumber?

• A. 3200-3500 cm^-1
• B. 1800-2000 cm^-1
• C. 1650-1750 cm^-1
• D. 2400-2600 cm^-1

Answer: (C)

In infrared spectroscopy, the C=O stretch in ketones and aldehydes is a strong, diagnostic absorption in the carbonyl region. For simple ketones and aldehydes, this carbonyl stretch typically appears around 1650–1750 cm^-1, with actual position near ~1710 cm^-1 for many aliphatic cases; conjugation or ring strain can shift it a bit higher or lower within that general window. The other ranges correspond to different features: O–H or N–H stretches show up around 3200–3500 cm^-1; a band around 1800–2000 cm^-1 is not the characteristic ketone/aldehyde carbonyl region; and 2400–2600 cm^-1 relates to CO2 or other overtones, not the C=O stretch.