Which statement correctly describes the IR identification of O-H and C=O groups?

Which statement correctly describes the IR identification of O-H and C=O groups?

• A. O-H around 3200-3600 cm^-1; C=O around ~1700 cm^-1
• B. O-H around 3600 cm^-1; C=O around 1500 cm^-1
• C. O-H around 3200-3600 cm^-1; C=O around ~1700 cm^-1
• D. O-H around 2100 cm^-1; C=O around 2400 cm^-1

Answer: (C)

In IR spectroscopy, certain functional groups have characteristic vibrations that show up at specific frequencies. The O-H stretch from alcohols or phenols typically appears as a broad band in the 3200–3600 cm^-1 region, while the carbonyl C=O stretch shows up as a strong band near 1700 cm^-1. Saying O-H is around 3200–3600 cm^-1 and C=O is around ~1700 cm^-1 matches these well-known ranges, so it’s the best description for identifying these groups in IR spectra. Remember that hydrogen bonding can broaden and shift the O-H band, sometimes toward the lower end of that range, and carbonyl frequencies can vary with conjugation and other substituents. The other options place these bands in unlikely regions (for example, O-H at 2100 cm^-1 or C=O at 2400 cm^-1), which would not align with standard IR identifications.

In IR spectroscopy, certain functional groups have characteristic vibrations that show up at specific frequencies. The O-H stretch from alcohols or phenols typically appears as a broad band in the 3200–3600 cm^-1 region, while the carbonyl C=O stretch shows up as a strong band near 1700 cm^-1. Saying O-H is around 3200–3600 cm^-1 and C=O is around ~1700 cm^-1 matches these well-known ranges, so it’s the best description for identifying these groups in IR spectra. Remember that hydrogen bonding can broaden and shift the O-H band, sometimes toward the lower end of that range, and carbonyl frequencies can vary with conjugation and other substituents. The other options place these bands in unlikely regions (for example, O-H at 2100 cm^-1 or C=O at 2400 cm^-1), which would not align with standard IR identifications.