Which of the following compounds does not undergo oxidation to form a carbonyl group?

The correct answer is ketones because they do not undergo oxidation to form a carbonyl group. Ketones already contain a carbonyl functional group (C=O) as part of their structure, which consists of a carbon atom double-bonded to an oxygen atom, flanked by two other carbon atoms. Therefore, there is no further oxidation process available to convert a ketone into another compound with a carbonyl group, as it already features it.

In contrast, aldehydes, primary alcohols, and secondary alcohols can all undergo oxidation. For instance, primary alcohols can oxidize to form aldehydes, and with further oxidation, aldehydes can transform into carboxylic acids. Secondary alcohols can be oxidized to form ketones. Aldehydes can also be further oxidized under specific conditions to form carboxylic acids. However, once a compound has achieved the ketone structure, it is effectively at its maximum oxidation state in relation to carbonyl functionalities.