Which of the following compounds contains a $CO$ (carbonyl) group as its main functional group?

• A. ${\mathrm{CH}}_3\mathrm{OH}$ (methanol)
• B. ${\mathrm{CH}}_3\mathrm{NH}{2}_{}$ (methylamine)
• C. ${\mathrm{CH}}_3\mathrm{COOH}$ (acetic acid)
• D. ${\mathrm{CH}}_3\mathrm{CHO}$ (acetaldehyde)

Answer: (D)

Verified step by step guidance

1. Identify the carbonyl group, which is characterized by a carbon atom double-bonded to an oxygen atom, represented as \(\mathrm{C=O}\).
2. Examine each compound's structure to determine if it contains the \(\mathrm{C=O}\) group as the main functional group:
3. 1. Methanol (\(\mathrm{CH_3OH}\)) contains an \(\mathrm{-OH}\) group (hydroxyl), so it is an alcohol, not a carbonyl compound.
4. 2. Methylamine (\(\mathrm{CH_3NH_2}\)) contains an amino group (\(\mathrm{-NH_2}\)), so it is an amine, not a carbonyl compound.
5. 3. Acetic acid (\(\mathrm{CH_3COOH}\)) contains a carboxyl group, which includes a carbonyl (\(\mathrm{C=O}\)) bonded to a hydroxyl (\(\mathrm{-OH}\)), so it does have a carbonyl group as part of the carboxylic acid functional group.
6. 4. Acetaldehyde (\(\mathrm{CH_3CHO}\)) contains a carbonyl group (\(\mathrm{C=O}\)) bonded to a hydrogen and a methyl group, making it an aldehyde with the carbonyl as the main functional group.
7. Conclude that the compound with the carbonyl group as the main functional group is acetaldehyde (\(\mathrm{CH_3CHO}\)).