Unknown analysis: combining chemical tests and spectra

What you’ll learn

• Build a consistent identification from independent analytical evidence.
• Use combustion, mass and NMR data without over-interpreting one result.

Understand the idea

Core practical 15 extends unknown analysis across the course. Separate an observation from its interpretation and use independent portions where reagents could interfere. In organic analysis, a positive 2,4-dinitrophenylhydrazine test supports an aldehyde or ketone, while a valid Tollens' test can distinguish an aldehyde from the simple ketones considered here. The tests are interpreted as supplied supervised observations; preparation or storage of reactive test reagents is outside this analytical exercise.

Spectra can distinguish structures that share a formula and functional group. A molecular-ion peak gives molecular mass; infrared data suggest functional groups; proton NMR provides chemical environments, integration ratios and splitting. For simple first-order spectra, $n$ equivalent neighbouring hydrogens often produce $n+1$ lines, subject to the stated assumptions. Carbon-13 NMR counts distinct carbon environments rather than necessarily counting every carbon atom separately.

Combustion data can establish elemental ratios: each mole of carbon dioxide represents one mole of carbon atoms and each mole of water represents two moles of hydrogen atoms. If a sample contains only carbon, hydrogen and oxygen, oxygen can be found by mass difference. In inorganic analysis, hydroxide precipitate colour, behaviour in excess reagent and changes on standing can provide complementary evidence. A single colour or a single spectral peak rarely justifies a complete structure.

Key formulae

$$n(\mathrm C)=n(\mathrm{CO_2})$$

$$n(\mathrm H)=2n(\mathrm{H_2O})$$

$$m(\mathrm O)=m_{\mathrm{sample}}-m(\mathrm C)-m(\mathrm H)$$

$$\mathrm{RCHO}+[\mathrm O]\rightarrow\mathrm{RCOOH}$$

Worked example

An unknown has formula $\mathrm{C_4H_8O}$ and molecular-ion peak $m/z=72$. It gives a positive 2,4-DNPH test and a negative valid Tollens' test. Its proton NMR has a 3H triplet at $\delta1.0$, a 3H singlet at $\delta2.1$ and a 2H quartet at $\delta2.4$. The candidates are butanal, butan-2-one and 2-methylpropanal. Identify it using the supplied first-order splitting data.

1. The positive carbonyl test and negative Tollens' result support a ketone rather than either aldehyde candidate.
2. The 3H triplet and 2H quartet match an ethyl group, $\mathrm{-CH_2CH_3}$.
3. The remaining 3H singlet fits a methyl group next to C=O with no adjacent hydrogens across that carbonyl carbon. Together these give $\mathrm{CH_3COCH_2CH_3}$.

Answer: Butan-2-one.