An IR spectrum shows a strong, broad absorption around $3300–3500cm-1$ and a sharp absorption near $1700cm-1$. Which functional group is most likely present?

• A. Aldehyde
• B. Amine
• C. Carboxylic acid
• D. Alcohol

Answer: (C)

Verified step by step guidance

1. Identify the significance of the absorption around 3300–3500 cm\textsuperscript{−1}: This region typically corresponds to O–H or N–H stretching vibrations. A broad and strong absorption here is characteristic of hydrogen-bonded O–H groups, such as those found in alcohols or carboxylic acids.
2. Examine the sharp absorption near 1700 cm\textsuperscript{−1}: This is commonly associated with the C=O (carbonyl) stretching vibration, which is a strong and sharp peak in IR spectra.
3. Combine the two observations: The presence of both a broad O–H stretch and a sharp C=O stretch suggests a functional group containing both these features simultaneously.
4. Recall that carboxylic acids have both a hydroxyl group (O–H) and a carbonyl group (C=O) in the same functional group, leading to the characteristic broad O–H stretch and sharp C=O stretch in their IR spectra.
5. Conclude that the functional group most likely present is a carboxylic acid, as it uniquely shows both a broad, strong O–H absorption around 3300–3500 cm\textsuperscript{−1} and a sharp C=O absorption near 1700 cm\textsuperscript{−1}.