Derivatives and combined analytical constraints
Introduced| English | Meaning |
|---|---|
| retention time/rɪˈtenʃn taɪm/ | The time taken for a substance to pass through a chromatography system and reach the detector under specified conditions. |
| co-elution | Two or more substances leaving a chromatography column together, potentially producing overlapping peaks. |
| integrated | Summed over a signal’s range; here, integrated proton-NMR areas give relative proton numbers. |
| provenance/ˈpruːvnəns/ | The recorded origin of data or samples, needed for a defensible conclusion. |
One matching result is not a full identification
- A GC peak with the same retention time 保留时间 as a reference can still come from a different compound or hide co-elution 共洗脱.
- Combine separation evidence with formula, spectra and chemical behaviour.
Retain the acyl carbon in reactions
- Ethanoyl chloride with ethanol gives ethyl ethanoate and HCl. With excess ammonia it gives ethanamide and NH₄Cl.
- Acid ester hydrolysis gives acid and alcohol reversibly. Alkaline hydrolysis gives carboxylate salt and alcohol, driving the reaction toward products.
Hot aqueous NaOH hydrolyses ethyl ethanoate. Which organic products form?
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In alkaline solution the acid product is present as carboxylate salt.
Count environments and assign proton sets
- Carbon-13 NMR counts distinct environments. Symmetry makes propanone have two signals although it has three carbons.
- Ethyl ethanoate gives a 3H acyl-methyl singlet, 2H OCH₂ quartet and 3H terminal-methyl triplet in the simple model.
For C₂H₆O, using C = 12.0000, H = 1.0078 and O = 15.9949 gives 2×12.0000 + 6×1.0078 + 15.9949 = 46.0417.
Three carbon atoms always give three carbon-13 NMR signals.
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Equivalent carbons give the same signal, as the two methyl carbons do in propanone.
An ethyl group commonly gives a 3H triplet and a 2H _____.
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The CH₂ protons are split by three equivalent neighbouring methyl H atoms in the simple n+1 model.
Combine exact and relative information
- Integrated 积分的 proton areas give ratios; splitting gives neighbour evidence; supplied shifts support environments. Exchangeable OH/NH peaks may not follow simple splitting.
- Accurate masses distinguish some formulae with the same nominal mass. Use all given precise atomic masses before rounding.
Interpret the separation limits
- Rf is spot distance divided by solvent-front distance from the same baseline, without units. Compare references under matched conditions.
- GC–MS supplies mass and fragment evidence for separated components. Standards, blanks and sample provenance 来源记录 strengthen a forensic or drug-test interpretation.
“One chromatographic peak proves purity” is false. Multiple components may co-elute.
Make every proposed structure satisfy all observations and state which alternatives the evidence cannot exclude.
A TLC spot travels 3.6 cm and solvent front 6.0 cm from the baseline. Find Rf.
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Rf = 3.6/6.0 = 0.60, with no units.
Which two actions strengthen a retention-time identification?
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Matched references and independent mass evidence reduce ambiguity; provenance and co-elution limits still matter.