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Laboratório de Química Orgânica · Lab techniques

Tratamentos, rendimentos e solução de problemas

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O que você vai aprender

  • Choose and order quenching reagents for hydrides and organometallics, and push the arrows of each quench reaction.
  • Select the aqueous wash that removes a specific reagent or by-product, and break emulsions and aluminium or magnesium gels.
  • Calculate amounts, limiting reagents, theoretical, percent and overall yields from masses, volumes, densities and concentrations.
  • Diagnose a failed or low-yielding reaction from TLC, IR and observations made during the workup, and choose the fix.
  • Recognise the main hazards of workups (gas evolution, peroxides, pyrophoric reagents, acid dilution, incompatible waste) and the safe practice for each.

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Quench before you work up

A workup starts by destroying whatever reactive reagent is left. Hydrides (NaBH₄, LiAlH₄, NaH) and organometallics (RMgBr, RLi) react with any O–H proton to give hydrogen or an alkane, and the reaction is fast and exothermic. Cool the flask to 0 °C and add the proton source dropwise, starting with the weakest: isopropanol (pKa ≈ 17) before water (15.7), saturated NH₄Cl (NH₄⁺, pKa 9.2) or dilute acid. Ethyl acetate is a useful first quench for LiAlH₄ because it is reduced to ethoxide without giving any H₂. Every mole of LiAlH₄ destroyed by water gives four moles of H₂ (LiAlH₄ + 4 H₂O → LiOH + Al(OH)₃ + 4 H₂), about 24.5 L of gas per mole of H₂ at 25 °C, so the flask must stay open to a bubbler. Grignard and organolithium additions are usually quenched with saturated NH₄Cl: ammonium is acidic enough to protonate the alkoxide and to dissolve magnesium hydroxide salts, but not acidic enough to dehydrate a sensitive tertiary alcohol, which warm strong acid would turn into an alkene.

Washes that do chemistry

Many washes remove an impurity by reacting with it. Sodium thiosulfate reduces leftover I₂ or Br₂ to colourless halide ions. Sodium sulfite reduces excess peracid such as m-CPBA to the carboxylate, which a NaHCO₃ wash then removes. Saturated sodium hydrogensulfite (bisulfite) adds to unhindered aldehydes and methyl ketones, turning them into water-soluble hydroxy sulfonate salts. Aqueous CuSO₄ pulls pyridine into water as a deep-blue copper complex without the strong acid that would harm acid-sensitive groups. Repeated water or 5% LiCl washes remove water-miscible DMF, which is too high-boiling (153 °C) to evaporate. Boron residues from NaBH₄ or hydroboration leave as volatile trimethyl borate, B(OMe)₃ (bp 68 °C), when the crude product is evaporated several times from methanol.

Emulsions and gels

An emulsion is a cloudy middle layer of tiny droplets stabilised by fine solids, soaps or a small density difference. Avoid it by swirling or inverting gently rather than shaking hard; break it by waiting, by adding saturated NaCl (brine), which raises the density and ionic strength of the water layer and pushes organic material out of it, or by filtering through Celite to remove the particles. Aluminium salts after a LiAlH₄ or DIBAL reduction form a gelatinous precipitate that traps product. Two standard fixes: the Fieser workup, where n grams of LiAlH₄ are followed by n mL of water, n mL of 15% NaOH and then 3n mL of water to give a granular, filterable solid; or stirring with saturated aqueous sodium potassium tartrate (Rochelle salt), whose tartrate chelates aluminium and gives two clear layers.

Amounts and yields

Convert every reagent to moles first: mass ÷ molar mass, volume × density ÷ molar mass for a neat liquid, and volume × concentration for a solution. Divide each by its stoichiometric coefficient; the smallest result is the limiting reagent, whatever its mass or volume. An ester needs two equivalents of a Grignard reagent, so the Grignard is limiting unless at least twice as many moles of it are present. Theoretical yield = moles of product the limiting reagent can give (its moles ÷ its coefficient × the product's coefficient) × product molar mass; two PhMgBr per triphenylmethanol means half as many moles of product as of Grignard reagent. Percent yield = actual ÷ theoretical × 100; a value over 100% means the product is still wet or contains solvent or impurity. The yield of a sequence is the product of the step yields: 80%, 70% and 90% give 50.4% overall. Organolithium solutions lose strength on storage, so titrate them and calculate volumes from the measured concentration, not the label.

Diagnosing a failed reaction

Run a three-lane TLC (starting material, co-spot, reaction mixture). Starting material that stops disappearing means a reagent has been used up or destroyed; a co-spot that looks like one elongated spot means two compounds with nearly the same Rf that need a different eluent. IR answers yes-or-no questions quickly: a broad O–H band near 3300–3400 cm⁻¹ and a C=O band (amide ≈ 1650, ketone ≈ 1715, ester ≈ 1740, acid chloride ≈ 1800 cm⁻¹) tell you which functional groups survived. Common culprits: water in the solvent destroys hydrides and Grignard reagents; an acidic O–H in the substrate consumes one equivalent of a Grignard reagent; old benzaldehyde contains benzoic acid from air oxidation; a basic product disappears into an acid wash and is recovered by basifying and extracting. Biphenyl, from coupling of PhMgBr with bromobenzene, is the usual nonpolar by-product of phenyl Grignard reactions.

Safety in the workup

Never seal a flask that is evolving gas. Add concentrated acid to water, never water to acid, because the heat of dilution boils the first drops of water and spatters acid. Ethers such as THF, diethyl ether and diisopropyl ether form hydroperoxides in air; test old bottles with acidified potassium iodide (a yellow-brown colour of iodine means peroxide) and never distil or evaporate them to dryness, because the peroxide concentrates in the residue. Do not open an old ether bottle with crystals around the cap; report it. Pyrophoric reagents (t-BuLi, neat trialkylaluminiums) are transferred under nitrogen or argon with dry syringes or cannulas, and residues are diluted with a hydrocarbon and quenched slowly with isopropanol in the cold. Keep bleach away from acidic waste (chlorine gas) and from ammonia (chloramines).

What to carry forward

Treat the workup as part of the reaction: destroy reactive reagents gently and in order of increasing acidity, choose each wash for the chemistry it does, keep a mole-based account of every reagent, and read TLC and IR before deciding that a reaction has failed or worked.

Problemas de prática (40)

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Desenhe o mecanismo 8

Aqueous workups and emulsions

  • A bisulfite wash removes benzaldehyde as its water-soluble adductDraw the addition of hydrogensulfite to benzaldehyde through sulfur and the proton transfer that gives the α-hydroxy sulfonate salt, and explain why this removes the aldehyde from the organic layer.
  • Sodium sulfite destroys excess m-CPBA after an alkene oxidationDraw sulfite attacking the terminal oxygen of m-CPBA to give hydrogensulfate and m-chlorobenzoate, then the proton transfer to excess sulfite, and explain why the oxidant must be destroyed before the organic layer is concentrated.
  • Sodium thiosulfate removes iodine from an organic layerDraw the two nucleophilic displacements by which thiosulfate reduces iodine to iodide and becomes tetrathionate, and explain why the brown colour disappears from the organic layer.

Quenching reactive reagents

  • Dilute acid destroys excess sodium borohydrideDraw the proton transfer from hydronium to a borohydride hydrogen that releases H₂ when excess NaBH₄ is quenched with dilute acid, and explain why the quench is done slowly in an open, cooled flask.
  • Ethyl acetate destroys excess LiAlH₄ without releasing hydrogenDraw how excess aluminium hydride reduces ethyl acetate to two ethoxide ions, and explain why ethyl acetate is a gentler first quench than water.

Diagnosing failed reactions

  • A Grignard reagent meets an acidic O–H in the substrateDraw the deprotonation of an alcohol O–H by MeMgBr, the addition of a second equivalent to the ketone and the ammonium chloride workup, and explain why one equivalent of Grignard reagent returned the starting material.
  • Moisture destroys a Grignard reagent: why the alcohol yield was lowDraw the proton transfer from water to 4-methoxyphenylmagnesium bromide that gives anisole, and use it to diagnose and fix a low-yielding Grignard reaction.

Lab safety

  • The potassium iodide test for peroxides in old THFDraw how acidified iodide reduces tetrahydrofuran-2-yl hydroperoxide to oxolan-2-ol while being oxidised to iodine, and explain what a positive test means for the solvent.

Preveja o produto 2

Diagnosing failed reactions

  • The wrong quench for a tertiary alcoholPredict the product formed when a tertiary alcohol from a Grignard reaction is quenched with warm strong acid, and connect it to the spectroscopic evidence.

Lab safety

  • Bleach in the acid wastePredict the toxic gas formed when hypochlorite is mixed with acidic chloride-containing waste.

Classificar 1

  • Which solvent needs a peroxide test?Classify common laboratory solvents as peroxide formers from their structure.

Comparar 1

  • Gentlest to harshest proton sourceRank common quenching proton sources by acid strength from their pKa values to plan a stepwise quench.

Calcular 9

Yield and limiting reagent

  • Use the titrated concentrationCalculate the volume of an organolithium solution to dispense from its titrated, not labelled, concentration.
  • Theoretical yield when two equivalents are neededFind the limiting reagent and theoretical yield of a reaction that consumes two equivalents of one reagent.
  • Which reagent limits the Williamson ether synthesis?Identify the limiting reagent of a three-component reaction by converting masses and a liquid volume into moles.
  • Millimoles of a liquid reagentConvert the volume of a neat liquid reagent into millimoles using its density and molar mass.
  • Overall yield of a three-step sequenceCalculate the overall yield of a linear multistep synthesis from the individual step yields.
  • Percent yield of cyclohexeneCalculate a percent yield from the isolated mass and the theoretical yield rather than from the starting mass.
  • Theoretical yield of cyclohexeneCalculate the theoretical yield in grams for a one-to-one reaction in which the product loses a small molecule.

Quenching reactive reagents

  • The last addition of a Fieser workupApply the Fieser n : n : 3n recipe to calculate the volume of each addition in a LiAlH₄ workup.
  • How much hydrogen does a LiAlH₄ quench release?Calculate the volume of hydrogen released when lithium aluminium hydride is destroyed with water, using the balanced equation and the molar gas volume.

Encontre o erro 5

Diagnosing failed reactions

  • Where did the amine go?Diagnose the loss of a basic product into an acidic aqueous wash and identify the species present in the water layer.
  • A LiAlH₄ reduction that returned starting materialCombine an IR spectrum with an observation made during the reaction to diagnose a hydride reduction that never started.
  • An old bottle of benzaldehydeDiagnose a low Grignard yield caused by an acidic impurity that formed in a reagent on storage, and choose how to purify the reagent.
  • A reduction that stopped half-wayDiagnose a reaction that has stalled from TLC plates taken at two times, and choose the next action.

Yield and limiting reagent

  • A 112% yieldDiagnose why a crude percent yield exceeds 100% and choose the fix.

Interprete as evidências 4

Diagnosing failed reactions

  • The non-polar by-product of a phenyl GrignardIdentify the coupling by-product of a phenyl Grignard reaction from its properties and explain how it is removed.
  • Which carbonyl survived?Rank carbonyl compound classes by C=O stretching wavenumber so that a crude IR band can be assigned to the right functional group.
  • What does the crude IR show?Use the presence of both O–H and C=O stretches in a crude IR spectrum to recognise an incomplete alcohol oxidation.
  • A co-spot shaped like a snowmanInterpret an elongated co-spot to decide whether a reaction spot is starting material or a new compound with a similar Rf.

Planeje uma síntese 5

Aqueous workups and emulsions

  • Removing pyridine from an acid-sensitive productChoose a non-acidic wash that removes pyridine from an organic layer containing an acid-labile group.
  • Getting rid of DMFPlan a workup that removes a high-boiling, water-miscible solvent from an organic product.
  • A layer that will not separateChoose the first, simplest remedy for an emulsion in a separatory funnel.
  • Breaking an aluminium gelChoose a chelating workup that dissolves aluminium salts after a hydride reduction without exposing an acid-sensitive product to strong acid.

Lab safety

  • Cleaning up after tert-butyllithiumChoose the safe way to destroy residues of a pyrophoric organolithium reagent.

Explique por quê 5

Lab safety

  • Acid into water, not water into acidExplain why concentrated sulfuric acid is diluted by adding the acid to water.
  • Why not distil ether to dryness?Explain why peroxide-forming solvents must never be distilled or evaporated to dryness.

Quenching reactive reagents

  • Why saturated NH₄Cl for a Grignard workup?Explain the two advantages of saturated ammonium chloride over plain water or strong acid for quenching a Grignard addition that gives a tertiary alcohol.
  • Why quench at 0 °C, one drop at a time?Explain why the quench of a hydride reduction in ether is done in an ice bath with dropwise addition.

Aqueous workups and emulsions

  • Why evaporate from methanol?Identify the volatile boron compound that lets boron residues be removed by repeated evaporation from methanol.

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