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

Answer: (A)

Verified step by step guidance

1. Identify the significance of the broad absorption around \$3300 - 3500\ \text{cm}^{-1}$. This region typically corresponds to O-H or N-H stretching vibrations, which are characteristic of alcohols, carboxylic acids, and amines.
2. Note that the absorption is described as 'strong' and 'broad'. A broad, strong peak in this region is often due to hydrogen bonding, which is especially prominent in carboxylic acids and alcohols.
3. Examine the sharp absorption near \$1700\ \text{cm}^{-1}$. This is characteristic of a carbonyl (C=O) stretch, which is found in aldehydes, ketones, carboxylic acids, and esters.
4. Combine the two observations: a broad O-H stretch and a sharp C=O stretch. This combination is distinctive of carboxylic acids, which have both an O-H group and a carbonyl group in the same molecule.
5. Conclude that the functional group most likely present is a carboxylic acid, because amines do not have a carbonyl stretch near \$1700\ \text{cm}^{-1}$, aldehydes have a carbonyl but not a broad O-H stretch, and alcohols have the broad O-H stretch but lack the carbonyl absorption.