Final answer:
Dimethyl ether (option c) would not have characteristic functional group absorption in an IR spectrum as it lacks highly polar functional groups, unlike the other compounds listed.
Step-by-step explanation:
The question is asking which of the following compounds would not have a characteristic functional group absorption in an IR spectrum. The options given are butanal, benzoic acid, dimethyl ether, propanone, and 1-pentanol. Among these, dimethyl ether does not have a distinctive functional group that shows an absorbance in the IR spectrum because it lacks a highly polar group such as a carbonyl, hydroxyl, or triple bond, which would have characteristic IR absorptions.
An IR spectrum is used to identify different functional groups within a compound based on their unique absorption frequencies. For example, the carbonyl group found in substances such as propanone typically absorbs in a particular frequency range. Similarly, alcohols like 1-pentanol show characteristic hydroxyl group absorption, and aldehydes such as butanal exhibit specific signals for their carbonyl group. Benzoic acid also has distinguishable absorptions due to its carboxyl group. In contrast, dimethyl ether, being an ether, does not have a polar functional group that provides a strong, distinctive infrared absorbance signature.
In conclusion, the correct option is (c) dimethyl ether, as it would not have a characteristic functional group absorption in an IR spectrum.