In infrared spectroscopy, a strong absorption near 1700 cm^-1 indicates which functional group?

• A. Alcohol groups
• B. Carbonyl groups (C=O)
• C. Carboxylic acids
• D. Alkanes

Answer: (B)

In infrared spectroscopy, a strong, sharp absorption around 1700 cm^-1 is the telltale signature of a carbonyl group (C=O). The C=O bond is highly polar and its stretching vibration is very intense, giving a strong band in this region. The exact position can vary slightly: unconjugated carbonyls often appear near 1700–1750 cm^-1, while conjugation or ring strain can shift the band toward a lower frequency (around 1680–1700 cm^-1). This absorption is characteristic of carbonyl-containing compounds such as ketones, aldehydes, esters, amides, and carboxylic acids. Other functional groups don’t produce a strong band near 1700 cm^-1—alcohols show broad O–H stretches well above 3000 cm^-1, and alkanes show C–H stretches near 2900 cm^-1 with no sharp 1700 cm^-1 feature. So the strong absorption at that region identifies a carbonyl group.