Final answer:
Infrared (IR) spectroscopy is used to identify functional groups in organic compounds based on their characteristic absorption frequencies.
Step-by-step explanation:
Infrared (IR) spectroscopy is a powerful technique used to identify functional groups in organic compounds. Each functional group has its own characteristic absorption frequency in an IR spectrum. By analyzing the absorption frequencies, we can determine which functional groups are present in a molecule.
a) Butanal contains a carbonyl group, which is characterized by a strong absorption around 1730 cm-1 in the IR spectrum. Therefore, it would have a characteristic functional group absorption.
b) Benzoic acid is a carboxylic acid and it also contains a carbonyl group. It would have a characteristic absorption around 1700-1750 cm-1.
c) Dimethyl ether does not have a carbonyl group. It contains only C-H bonds, which typically absorb in the range of 2800-3000 cm-1 in the IR spectrum.
d) Propanone, also known as acetone, has a carbonyl group and would exhibit a characteristic absorption around 1700-1750 cm-1.
e) 1-pentanol contains an OH (hydroxyl) group. Hydroxyl groups have a characteristic absorption around 3300 cm-1 in the IR spectrum.
Therefore, the compound that would not have a characteristic functional group absorption in an IR spectrum is dimethyl ether (C).