Which region in the IR spectrum could be used to distinguish between butanoic acid and 2-butanone?

• A. The fingerprint region below $1500 \mathrm{cm}{-}^1$
• B. The broad O–H stretch region around $2500-3300 \mathrm{cm}{-}^1$
• C. The C–H stretch region around $2850-2960 \mathrm{cm}{-}^1$
• D. The C=O stretch region near $1700 \mathrm{cm}{-}^1$

Answer: (B)

Verified step by step guidance

1. Identify the functional groups present in butanoic acid and 2-butanone. Butanoic acid contains a carboxylic acid group (-COOH), while 2-butanone contains a ketone group (C=O).
2. Recall that both compounds have a carbonyl (C=O) stretch, which appears near 1700 cm\textsuperscript{-1} in the IR spectrum. This means the C=O stretch region alone cannot distinguish between them.
3. Understand that the key difference lies in the presence of the O–H group in butanoic acid, which is absent in 2-butanone. The O–H stretch in carboxylic acids is broad and appears in the region around 2500–3300 cm\textsuperscript{-1}.
4. Note that the C–H stretch region (2850–2960 cm\textsuperscript{-1}) is common to many organic compounds and is not distinctive enough to differentiate these two molecules.
5. Conclude that the broad O–H stretch region around 2500–3300 cm\textsuperscript{-1} is the most reliable IR region to distinguish butanoic acid from 2-butanone.