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Multiple Choice
Which region in the IR spectrum could be used to distinguish between butanoic acid and 2-butanone?
A
The fingerprint region below
B
The broad O–H stretch region around
C
The C–H stretch region around
D
The C=O stretch region near
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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).
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.
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\(\textsuperscript{-1}\).
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.
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.