Identifying an Unknown Carbonyl Compound
An unknown liquid (boiling range 78–80 °C) is submitted for identification. Tests: it gives an orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNP); it does not give a silver mirror with Tollens' reagent; it gives a yellow precipitate with alkaline iodine (iodoform) test. Its IR spectrum shows a strong sharp absorption at about 1715 cm⁻¹ and no broad O–H absorption. Its ¹H NMR shows a singlet near δ 2.1 (3H) and a singlet near δ 2.5 (3H), with no other signals. A colleague suggests it is ethanol because the boiling point is close to ethanol's (78 °C).
1. Use the test results to identify the functional group and rule out the colleague's suggestion.
Read the worked answer
- 2,4-DNP positive → carbonyl compound (aldehyde or ketone).
- Tollens' negative → not an aldehyde; it is a ketone (or a hindered/aromatic carbonyl).
- Iodoform positive → contains a CH3CO– group (methyl ketone) or CH3CH(OH)–.
- IR 1715 cm⁻¹ with no O–H → carbonyl, no alcohol.
Ethanol is ruled out: it is an alcohol, would show a broad O–H stretch near 3300 cm⁻¹, would not give a 2,4-DNP precipitate (no carbonyl), and would not give a carbonyl stretch at 1715 cm⁻¹. The colleague matched only the boiling point, which is not diagnostic.
2. Interpret the NMR to propose a structure and explain how the data are consistent with it.
Read the worked answer
Two singlets, each integrating to 3H, with no coupling, means two isolated CH3 groups with no neighbouring protons - consistent with CH3–CO–CH3 (propanone/acetone).
- δ 2.1 (3H) and δ 2.5 (3H): the two equivalent methyl groups of a methyl ketone, slightly different environments reported; in reality acetone gives a single 6H singlet near δ 2.1, so the two resolved singlets suggest either an asymmetric methyl ketone such as CH3COCH2R without adjacent protons, or an impure/interpreted spectrum. The key structural conclusion is a CH3CO– unit with no adjacent CH protons.
- No O–H, no aldehyde proton near δ 9–10, consistent with a ketone.
Propanone is the best fit: it is a methyl ketone (iodoform positive), Tollens' negative, 2,4-DNP positive, carbonyl at ~1715 cm⁻¹, and boils near 56 °C - so the 78–80 °C boiling range actually argues against pure acetone and suggests a higher-boiling methyl ketone such as butan-2-one (bp ~80 °C, CH3COCH2CH3). Butan-2-one fits the boiling point better; its CH2 would couple, so the clean singlets favour a compound with no adjacent protons. The data are internally slightly inconsistent and the identification should be confirmed by mass spectrometry or comparison with an authentic sample.