What indicates the presence of a carbonyl group when using 2,4-dinitrophenylhydrazine?

The presence of a carbonyl group in a compound is indicated by the formation of an orange-yellow precipitate when using 2,4-dinitrophenylhydrazine. This reagent reacts specifically with carbonyl compounds, such as aldehydes and ketones, to form a hydrazone derivative. This reaction typically results in the formation of a brightly colored precipitate, which is characteristic of the carbonyl function.

The orange-yellow color of the precipitate is attributed to the intense coloration of the dinitrophenylhydrazone products that are formed during this reaction. This color change serves as a visual confirmation of the presence of a carbonyl group, making it a useful qualitative test in organic chemistry for identifying aldehydes and ketones. Other options do not reflect the standard outcomes of chemical tests for carbonyls with this reagent.