Analysing an Unknown Organic Compound from Combined Data
A colourless liquid of molecular formula C₄H₈O₂ is analysed. It is neutral to litmus. It reacts with sodium carbonate solution to release carbon dioxide. When warmed with ethanol and a few drops of concentrated sulfuric acid, it forms a sweet-smelling liquid. Its ¹H NMR spectrum shows three signals: a triplet, a quartet, and a singlet, with relative areas 3:2:3. Its infrared spectrum has a strong sharp absorption near 1710 cm⁻¹ and a broad absorption around 2500–3300 cm⁻¹. A student proposes the structure is an ester because of the sweet smell formed in the reaction.
1. Use the infrared and carbonate evidence to identify the functional group present, and evaluate the student's ester proposal.
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The broad absorption at 2500–3300 cm⁻¹ is characteristic of the O–H stretch of a carboxylic acid (hydrogen-bonded, very broad). The sharp strong band near 1710 cm⁻¹ is a C=O stretch, consistent with a carboxylic acid (or ester/ketone, but combined with the broad O–H it points to acid). The reaction with sodium carbonate to release CO₂ confirms an acidic proton: RCOOH + NaHCO₃/Na₂CO₃ → RCOO⁻Na⁺ + CO₂ + H₂O. Esters do not liberate CO₂ from carbonate.
The student's reasoning is inverted. The sweet-smelling liquid formed when the unknown is warmed with ethanol and acid is the ester produced by Fischer esterification of the carboxylic acid. The ester is the product, not the starting compound. So the proposal that the unknown is an ester is wrong; the unknown is a carboxylic acid, and the smell is evidence for the reaction, not the structure.
2. Use the NMR data to distinguish between the possible carboxylic acid isomers of C₄H₈O₂, explaining your reasoning.
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Possible monocarboxylic acids with C₄H₈O₂: butanoic acid (CH₃CH₂CH₂COOH), 2-methylpropanoic acid ((CH₃)₂CHCOOH).
Butanoic acid has four distinct proton environments: CH₃ (3H), CH₂ (2H), CH₂ next to COOH (2H), and COOH (1H). That would give four signals, not three. 2-Methylpropanoic acid has three environments: two equivalent CH₃ groups (6H, doublet or singlet depending on coupling), the CH (1H, multiplet), and COOH (1H). That gives three signals, but the areas would be roughly 6:1:1, not 3:2:3.
Neither acid fits 3:2:3. The 3:2:3 pattern with a triplet and quartet is the classic ethyl group: CH₃ (3H, triplet) coupled to CH₂ (2H, quartet). The remaining 3H as a singlet suggests a CH₃ not coupled to neighbouring protons, such as CH₃–C(=O)– or CH₃–O–. With a carboxylic acid confirmed, the structure consistent with all data is actually an ester, ethyl ethanoate, CH₃COOCH₂CH₃, which gives CH₃CO (3H singlet), OCH₂ (2H quartet) and CH₃ (3H triplet). This creates a conflict with the carbonate test.
3. Resolve the conflict between the carbonate test and the NMR data, and state the most likely conclusion with a caveat.
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There is a genuine inconsistency. The carbonate test and broad O–H IR indicate a carboxylic acid, but the 3:2:3 triplet/quartet/singlet pattern fits ethyl ethanoate far better than any C₄H₈O₂ acid. Butanoic acid would show four signals; 2-methylpropanoic acid would show areas near 6:1:1.
Possible resolutions:
- The sample is contaminated or mislabelled; the carbonate test may have been performed on a different sample, or the 'broad O–H' may be a misread of a broad impurity (e.g. water).
- The compound is ethyl ethanoate and the carbonate fizzing was due to residual ethanoic acid impurity from incomplete esterification, which would also explain the broad O–H.
- The NMR integration or splitting assignment is wrong.
Most likely conclusion: the compound is ethyl ethanoate, with the acid tests reflecting an acidic impurity. The caveat is that this conclusion depends on the purity of the sample; a pure ester should not react with carbonate. The exam-style judgement is that no single piece of evidence should override a well-resolved NMR pattern, but the conflict must be acknowledged and a purification step (e.g. washing with sodium carbonate to remove acid, then re-running IR) recommended to confirm.