Carbon-building routes and purified products
Introduced| English | Meaning |
|---|---|
| Grignard reagent | An organomagnesium halide containing a carbon-magnesium bond, used in synthesis including formation of carbon-carbon bonds. |
| work-up | Treatment after the main reaction to convert intermediates into the intended product and prepare it for isolation, such as acidic work-up after Grignard addition. |
| Recrystallisation | Purifying a solid by dissolving it in a suitable solvent and allowing crystals to form as conditions change. |
| purity | How much of a sample is the desired substance, checked by melting point or chromatography. |
Decide which carbon fragment to add
- A Grignard reagent 格氏试剂 forms from a halogenoalkane and Mg in dry ether. Water consumes it before carbonyl addition.
- Ethylmagnesium bromide plus ethanal bonds two carbon fragments into a four-carbon skeleton.
The ether solvent for Grignard preparation must be _____.
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Water consumes the reagent to form a hydrocarbon.
Separate addition from work-up
- The carbonyl carbon gains the Grignard carbon group. Acidic work-up 后处理 changes the oxygen-containing intermediate into the alcohol.
- Ethanal with ethylmagnesium bromide gives butan-2-ol. Methanal instead gives propan-1-ol; CO₂ then acid gives propanoic acid.
Ethylmagnesium bromide plus methanal then acid gives which alcohol?
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Methanal contributes one carbon to the two-carbon ethyl group.
Use every analytical observation
- C₃H₆O with C=O, an aldehyde proton and positive Tollens’ test supports propanal. Propanone fails the aldehyde observations.
- Predict NMR environments and compare with all supplied data. One matching observation alone rarely identifies a unique structure.
2.76 g salicylic acid at M 138 is 0.0200 mol, giving theoretical aspirin 3.60 g at M 180. A dry 2.52 g is 70.0% yield.
Choose isolation for the product
- Reflux retains volatile reactants; distillation collects a volatile product. Extraction separates immiscible liquid layers.
- Carbonate washing removes acidic impurities and releases CO₂. Vent the separating funnel; organic solvent is not always the upper layer.
The organic layer is always above the aqueous layer.
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Layer position depends on relative densities; dense organic solvents can lie below water.
Measure yield only after drying
- Recrystallisation 重结晶 uses minimum hot solvent, cooling, filtration and a little cold-solvent wash. Dry the product before weighing.
- Aspirin yield uses limiting salicylic acid in a 1:1 mole ratio. A narrow melting range supports purity 纯度 but does not prove identity alone.
Water added before Grignard addition destroys the reagent rather than improving its alcohol yield.
Track carbon count and functional-group changes through every synthesis step. Choose dry conditions where needed, then separate, purify and dry before calculating yield.
Theoretical aspirin is 3.60 g and dry product 2.52 g. Find percentage yield.
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2.52/3.60 × 100 = 70.0%.
Which two actions improve an honest solid-product yield/purity assessment?
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Dry mass avoids water inflation; melting range supplies additional purity evidence.