Laboratório de Química Orgânica · Lab techniques
Separações e purificação
Uma lição resolvida gratuita e 40 problemas de prática em que você mesmo desenha cada seta curva.
Travou em separações e purificação? Escaneie o problema.
Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.
Depois, Scan Standard: /mês
O que você vai aprender
- Use pKa values to choose the aqueous acid or base that moves one compound of a mixture into the water layer, and push the arrows of each proton transfer.
- Plan a recrystallization, calculate solvent volume and maximum recovery, and judge purity and identity from melting points.
- Choose between simple, fractional, vacuum and steam distillation, and dry an organic solution correctly.
- Calculate and interpret Rf values, predict elution order on silica and reverse-phase media, and choose an eluent for a column.
Lição gratuita
O texto da lição resolvida. No Arrow Lab, cada etapa vem com as estruturas e as setas desenhadas.
Ler a aula gratuita (5 partes)
Acid–base extraction
Neutral organic compounds prefer the organic layer; ions prefer water. A proton transfer therefore moves a compound between layers. A proton transfer favours the side with the weaker acid, so a base extracts an acid only if the base's conjugate acid is weaker (higher pKa) than the compound. Benzoic acid (pKa ≈ 4.2) is deprotonated by sodium hydrogencarbonate, whose conjugate acid, carbonic acid, has pKa ≈ 6.35; the carbonic acid then loses CO₂, so vent the funnel. Phenols (2-naphthol, pKa ≈ 9.5) need NaOH (water, pKa 15.7). Amines are extracted by dilute HCl as ammonium ions (anilinium pKa ≈ 4.6); amides are not basic enough. Each compound is recovered by reversing the proton transfer: acidify a carboxylate to pH ≈ 2, basify an ammonium ion to pH ≈ 12, several units above its pKa. Several small extractions beat one large one: each portion leaves the fraction V(water) / (V(water) + K × V(organic)) of a neutral solute behind, where K is its partition coefficient. Diethyl ether (density 0.71 g/mL) floats on water; dichloromethane (1.33 g/mL) sinks.
Recrystallization and melting point
A good recrystallization solvent dissolves the compound well when hot and poorly when cold, while impurities either stay dissolved in the cold solvent or never dissolve and are removed by hot gravity filtration. Use the minimum volume of boiling solvent; whatever stays dissolved at the final temperature is lost, so maximum recovery = (mass − cold solubility × volume) / mass. A compound too soluble in one solvent and insoluble in another is recrystallized from the miscible pair: dissolve it in the minimum of the hot good solvent, add the hot poor solvent to the cloud point, then cool. Cool slowly so the growing lattice rejects impurities. If the solution saturates above the solid's melting point, the compound oils out instead of crystallizing. A pure solid melts sharply (range of 1–2 °C); impurities lower and broaden the range. A mixed melting point with an authentic sample confirms identity: no depression means the same compound.
Distillation and drying
Boiling point rises with stronger intermolecular forces: diethyl ether (35 °C) and 1-butanol (118 °C) share the formula C₄H₁₀O, but only the alcohol hydrogen-bonds. Add boiling chips or stir before heating, so the liquid boils smoothly instead of superheating and bumping. Simple distillation separates liquids whose boiling points differ by more than about 70 °C or a volatile liquid from a non-volatile solid; closer pairs need a fractionating column. Vacuum distillation lowers the boiling point of compounds that decompose when heated, and steam distillation co-distils water-immiscible compounds below 100 °C because the vapour pressures of the two liquids add. Azeotropes, such as 95.6% ethanol in water, cannot be separated further by distillation. Before evaporating an organic layer, wash it with brine, then add anhydrous MgSO₄ or Na₂SO₄ until fresh powder flows freely. For amines, basic K₂CO₃ (or neutral Na₂SO₄) is the safe choice: CaCl₂ binds amines and alcohols, and acidic drying agents protonate amines.
TLC and column chromatography
On silica gel, polar compounds bind more strongly and travel less: Rf = distance travelled by the spot ÷ distance travelled by the solvent front. A more polar eluent raises every Rf. Carboxylic acids streak unless a little acetic acid is added to the eluent. To follow a reaction, run starting material, a co-spot of starting material plus reaction mixture, and the reaction mixture side by side; the co-spot proves the two compounds separate. Reverse-phase (C18) plates and columns invert the order: the most polar compound elutes first. For a flash column, choose the eluent that gives the target an Rf of about 0.25–0.35; a compound elutes after roughly 1/Rf column volumes. Aromatic and conjugated compounds show up under 254 nm UV light; saturated compounds need a stain such as KMnO₄.
What to carry forward
Every purification exploits one physical difference: charge (extraction), solubility versus temperature (recrystallization), volatility (distillation) or adsorption to a polar surface (chromatography). Name the difference first, then choose the technique and conditions that magnify it.
Problemas de prática (40)
Praticar é grátis com uma conta.
Desenhe o mecanismo 4
Acid–base extraction
- Benzoic acid is extracted by aqueous sodium hydrogencarbonateDraw the proton transfer that turns ether-soluble benzoic acid into water-soluble benzoate during a bicarbonate wash, and explain from pKa values why it is favourable.
- 4-Chloroaniline is extracted by dilute hydrochloric acidDraw hydronium protonating the amino group of 4-chloroaniline, turning the ether-soluble amine into a water-soluble anilinium ion.
- Recovering 4-chloroaniline by basifying the acid extractDraw hydroxide removing the N–H proton of 4-chloroanilinium so the neutral amine can be back-extracted into ether, and decide how basic the solution must be.
- 2-Naphthol is extracted by aqueous sodium hydroxideDraw hydroxide deprotonating 2-naphthol and the resonance that delocalises the naphthoxide charge into the ring, and explain why bicarbonate cannot do the same job.
Preveja o produto 3
Chromatography
- More ethyl acetate in the eluentPredict how increasing eluent polarity changes Rf values on silica gel.
Acid–base extraction
- Which layer is which?Predict which layer of a two-phase extraction is organic from the solvent densities.
- What precipitates when the extract is acidified?Predict the species that precipitates when a carboxylate extract is acidified, using its pKa.
Classificar 4
Chromatography
- Which spot needs a stain?Classify compounds as visible or invisible under 254 nm UV on a fluorescent TLC plate from their chromophores.
Acid–base extraction
- Which compound survives an acid wash?Recognise that amides, unlike amines, are not extracted by dilute aqueous acid.
- Which compound does bicarbonate extract?Identify which component of a mixture moves into aqueous sodium hydrogencarbonate by comparing its pKa with that of carbonic acid.
Recrystallization and melting point
- Choosing a recrystallization solventClassify candidate solvents by the solubility behaviour needed for recrystallization.
Comparar 3
Chromatography
- Reverse-phase elution orderPredict which compound elutes first from a reverse-phase (C18) column, where polarity order is inverted relative to silica.
- Rank Rf values on silicaRank compounds by Rf on silica gel from their ability to hydrogen-bond with the stationary phase.
Distillation and drying
- Hydrogen bonding and boiling pointRank oxygen compounds by boiling point from the number of hydrogen-bonding O–H groups.
Calcular 6
Chromatography
- How far apart do the bands elute?Estimate the volume of eluent separating two bands on a flash column from their TLC Rf values.
- Calculate an Rf valueCalculate an Rf value from the distances travelled by a spot and the solvent front.
Acid–base extraction
- How much amine is protonated at pH 1?Calculate the ratio of protonated to neutral amine at a given pH from the pKa of its conjugate acid.
- Three small extractions versus one large oneCalculate the mass recovered by repeated extractions from a partition coefficient and compare it with a single extraction.
Recrystallization and melting point
- Best possible recoveryCalculate the maximum percent recovery of a recrystallization from the cold solubility and the solvent volume.
- How much hot water?Calculate the minimum volume of boiling solvent needed to dissolve a sample from its hot solubility.
Encontre o erro 2
Acid–base extraction
- A separation done in the wrong orderDiagnose why washing with hydroxide before bicarbonate fails to separate a carboxylic acid from a phenol.
Recrystallization and melting point
- The product oiled outDiagnose why a low-melting solid separates as an oil during recrystallization and choose a remedy.
Interprete as evidências 4
Chromatography
- Is the reduction finished?Interpret a three-lane TLC plate (starting material, co-spot, reaction) to judge whether a reaction is complete.
Distillation and drying
- Is the solution dry yet?Interpret the appearance of magnesium sulfate to judge when an organic solution is dry.
Recrystallization and melting point
- Reading a melting rangeInterpret a low, broad melting range as evidence of an impure solid.
- Identify an unknown by mixed melting pointInterpret mixed melting points to identify an unknown among compounds with the same melting point.
Planeje uma síntese 6
Chromatography
- Choosing a column eluent from TLCChoose a flash-column eluent from TLC data using the target Rf range and the separation from an impurity.
Distillation and drying
- Drying a solution of an amineChoose a drying agent compatible with a basic amine in an organic solution.
- When is a fractionating column needed?Decide from boiling-point differences which mixture requires fractional rather than simple distillation.
Acid–base extraction
- Separate an acid, a phenol and a neutral compoundOrder the washes and work-up steps that separate a carboxylic acid, a phenol and a neutral compound dissolved in ether.
Recrystallization and melting point
- Removing coloured, insoluble impuritiesPlan how to remove insoluble impurities and decolourising charcoal from a hot recrystallization solution without losing product.
- Recrystallizing from a solvent pairPlan a mixed-solvent recrystallization for a compound too soluble in one solvent and insoluble in another.
Explique por quê 8
Chromatography
- A carboxylic acid streaksExplain why carboxylic acids streak on silica and how an acidic additive to the eluent sharpens the spot.
Distillation and drying
- Boiling chipsExplain why boiling chips or stirring are used when a liquid is heated to boiling.
- Why wash with brine?Explain the purpose of a saturated sodium chloride wash before drying an organic layer.
- The 95% ethanol ceilingExplain why fractional distillation of aqueous ethanol stops at about 95.6% ethanol.
- Steam distillation of limoneneExplain why a water-immiscible, high-boiling oil co-distils with water below 100 °C.
- Why distil under vacuum?Explain how reducing the pressure lets a heat-sensitive, high-boiling liquid be distilled.
Acid–base extraction
- Why vent the funnel?Explain why a separatory funnel must be vented often during a bicarbonate wash of an acidic solution.
Recrystallization and melting point
- Why cool slowly?Explain why a hot recrystallization solution is cooled slowly before the ice bath.
Travou em separações e purificação? Escaneie o problema.
Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.
Depois, Scan Standard: /mês