Preparación, purificación y evidencia analítica combinada
Introducido| English | Significado |
|---|---|
| infrared/ˌɪnfrəˈred/ | infrarrojo |
| distil | destilar |
| reflux/ˈriːflʌks/ | reflujo |
| anhydrous/ænˈhaɪdrəs/ | anhidro |
Can one peak prove which liquid you made?
- Propanal and propanone both have formula C₃H₆O, molecular mass 58 and a carbonyl group. One molecular ion or one C=O absorption cannot distinguish them.
- Combine reaction conditions, purification, mass spectra, infrared 红外 data and chemical tests. Give a supported candidate and explain which competing candidate each observation rejects.
Choose preparation conditions for the product
- Acidified potassium dichromate(VI) changes orange to green when reduced. For a primary alcohol, distil 蒸馏 the aldehyde as it forms; excess oxidant under reflux 回流 gives the carboxylic acid. A secondary alcohol gives a ketone under these conditions.
- For propan-1-ol: CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O; with further oxidation, CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂O. Propan-2-ol gives CH₃COCH₃ + H₂O with [O]. Distillation removes vapour; reflux returns condensed vapour and allows prolonged heating.
¿Qué condiciones favorecen la recolección de propanal a partir de propan-1-ol con dicromato acidificado?
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La destilación elimina el aldehído a medida que se forma, reduciendo la oxidación posterior. El exceso de oxidante bajo reflujo favorece el ácido propanoico. El equipo controlado debe usarse bajo supervisión aprobada.
Purify without confusing bulk water and residual water
- In solvent extraction, use a separating funnel to separate immiscible layers. If the organic layer has density $0.87\,\dfrac{\mathrm g}{\mathrm{cm}^3}$ and the aqueous layer $1.00\,\dfrac{\mathrm g}{\mathrm{cm}^3}$, the organic layer is upper; identify it from the stated densities, not an always-upper rule.
- After bulk separation, a suitable anhydrous 无水的 salt removes residual water. Remove the drying salt, then distil as appropriate. Measure boiling temperature with the thermometer bulb at the vapour take-off. A narrow range near the expected value at the stated pressure supports purity but is not unique structural proof.
A dried liquid boils at 85–86°C at a stated pressure. This is consistent with a relatively pure liquid having that boiling range. It does not prove a single named isomer: compare independent spectral or chemical evidence, and consider whether the apparatus measures vapour temperature correctly.
Dos capas inmiscibles tienen densidades orgánica 0.87 g/cm³ y acuosa 1.00 g/cm³. ¿Qué afirmación sigue?
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El líquido orgánico de menor densidad está arriba en este caso declarado. Un embudo de separación elimina el líquido acuoso en masa; una sal desecante elimina el agua residual después de la separación.
Un rango de ebullición estrecho a la temperatura esperada demuestra de manera única la estructura molecular.
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Un rango estrecho apoya la pureza relativa bajo la presión y las condiciones de medición indicadas. Diferentes estructuras pueden tener temperaturas de ebullición similares; se requiere evidencia independiente de identificación.
Keep mass and fragment evidence distinct
- A singly charged molecular ion at m/z 58 supports molecular mass 58. C₃H₆O gives $3(12)+6(1)+16=58$. The base peak is the most intense peak; it need not be the molecular ion.
- A peak at m/z 29 could be CHO⁺, mass 12 + 1 + 16, or C₂H₅⁺, mass 24 + 5. Check plausible fragmentation and the full pattern. A positive warmed Fehling's/Benedict's test gives a brick-red precipitate and supports propanal over propanone in this two-candidate comparison.
Use C = 12, H = 1 y O = 16. ¿Cuál es el m/z nominal de CHO⁺ con carga simple?
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12 + 1 + 16 = 29 para carga +1. C₂H₅⁺ también tiene masa nominal 29, por lo que este pico único no es una fórmula de fragmento exclusiva.
Combine infrared bands and tests
- Use the supplied wavenumber table: C–H bands differ in alkanes, alkenes and aldehydes; C=C supports an alkene; alcohol O–H is broad while acid O–H is very broad. C=O can occur in aldehydes, ketones and acids; C–X supports a halogenoalkane and N–H an amine. Predict all relevant bonds from a candidate structure before comparing its spectrum.
- For example, supplied broad O–H at $3200–3600\,\dfrac{1}{\mathrm{cm}}$ and C=O at $1680–1750\,\dfrac{1}{\mathrm{cm}}$ can fit HOCH₂CHO; an acid has a much broader O–H region around $2500–3300\,\dfrac{1}{\mathrm{cm}}$. A carbonate test releasing CO₂ supports an acid. For inorganic unknowns use clean aliquots: warming NH₄⁺ with OH⁻ releases NH₃; acidified barium reagent gives white BaSO₄ with sulfate. These observations are not universally unique outside a stated candidate list.
An absorption shows evidence for a bond, not the whole structure. Overlapping bands and ambiguous fragments require more evidence. These are written method and interpretation checks; they do not certify practical performance or replace approved supervised procedures.
Control the preparation, explain each purification step, and combine molecular mass, plausible fragments, infrared bonds and selective tests. State exactly how far the evidence supports the identification.
La lista de candidatos es propanal o propanona. Ambos encajan con C₃H₆O y C=O. La prueba de Fehling caliente produce un precipitado rojo ladrillo. ¿Cuál se apoya?
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El propanal es un aldehído y da esta prueba positiva. La propanona es una cetona y no lo hace bajo estas condiciones. El análisis de fórmula y carbonilo solo no puede distinguirlas.