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Laboratório de Química Orgânica · Classic laboratory preparations

Ésteres, amidas e éteres no laboratório

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O que você vai aprender

  • Draw the Fischer esterifications and anhydride acetylations of the teaching lab with the real acid or base of each medium.
  • Explain how excess reagent, water removal and an irreversible deprotonation decide the yield of esterification and saponification.
  • Use acid–base reasoning to design the washes and extractions of a workup.
  • Plan Williamson ether syntheses from phenols, alcohols and halohydrins.

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Fischer esterification in a flask

Isoamyl acetate, the banana-smelling ester drawn here, is made by heating 3-methylbutan-1-ol with excess acetic acid and a few drops of concentrated sulfuric acid. The acid catalyst protonates the carboxyl carbonyl so that the weakly nucleophilic alcohol can add; a series of proton transfers then lets water leave from what was the acid’s OH group. Every step is reversible and the equilibrium constant is only about 4, so a 1:1 mixture stops near two-thirds conversion. The lab drives the reaction by using the cheaper partner, acetic acid, in excess or by removing water. The alcohol oxygen ends up in the ester; the water oxygen comes from the acid.

Anhydrides and acid chlorides need no acid

Acetic anhydride and acetyl chloride carry a good leaving group, acetate or chloride, on the acyl carbon, so their acyl substitutions run essentially one way and need no equilibrium tricks. Aspirin is made by warming salicylic acid with acetic anhydride: the phenol oxygen adds to a carbonyl carbon, the tetrahedral intermediate expels acetate, and a proton transfer gives the phenyl ester and acetic acid. A few drops of phosphoric or sulfuric acid only speed up attack by the weakly nucleophilic phenol. Amines are nucleophilic enough that aniline and acetic anhydride give acetanilide in water at room temperature, with sodium acetate buffering the acid that forms.

Workup is chemistry too

After an esterification the crude ester still contains leftover carboxylic acid and the acid catalyst. Shaking the organic layer with 5% sodium bicarbonate converts acetic acid (pKa 4.8) into acetate and carbonic acid, which loses carbon dioxide, so the separatory funnel must be vented. Acetate is an ion and moves into the aqueous layer; the neutral ester, with no acidic proton, stays in the organic layer. The same logic runs in reverse: acidifying an aqueous carboxylate or phenoxide solution regenerates the neutral compound, which precipitates or can be extracted. Brine and a drying agent such as MgSO4 then remove dissolved water before distillation.

Williamson in practice

A teaching-lab ether is made by turning an alcohol or phenol into its conjugate base and letting it displace a halide from a primary alkyl halide by SN2. Phenols (pKa about 10) are acidic enough that hydroxide does the deprotonation, so 2-naphthol with KOH in ethanol and then iodoethane gives 2-ethoxynaphthalene (nerolin), drawn here. Ordinary alcohols need sodium hydride instead. Choose the disconnection that puts the halide on the primary carbon, because a secondary or tertiary halide treated with an alkoxide mostly eliminates. An alkoxide and a halide on neighbouring ring carbons close to an epoxide when they can line up anti to each other.

What to carry forward

Acid catalysis plus excess reagent drives the reversible Fischer esterification; anhydrides and acid chlorides acylate essentially irreversibly; base hydrolysis ends in an irreversible carboxylate; acid–base washes move ionised species into water; and a Williamson ether needs an alkoxide or phenoxide plus a primary halide.

Problemas de prática (15)

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Desenhe o mecanismo 10

Esterification and acetylation

  • Acetanilide from aniline and acetic anhydrideDraw the addition–elimination of aniline to acetic anhydride, with acetate as the base, giving acetanilide (N-phenylacetamide) and acetic acid.
  • Aspirin: acetylation of salicylic acid with acetic anhydrideDraw the phosphoric acid–catalysed acetylation of the phenol oxygen of salicylic acid by acetic anhydride, giving acetylsalicylic acid (aspirin) and acetic acid.
  • Benzocaine from 4-aminobenzoic acid and ethanolDraw the Fischer esterification of 4-aminobenzoic acid in ethanol with its amino group kept protonated, then the bicarbonate step that frees the amine, giving benzocaine (ethyl 4-aminobenzoate).
  • Fischer esterification: isoamyl acetate from acetic acid and 3-methylbutan-1-olDraw the acid-catalysed Fischer esterification of acetic acid with 3-methylbutan-1-ol, using sulfuric acid and hydrogensulfate for every proton transfer, to give 3-methylbutyl acetate (isoamyl acetate) and water.
  • Fischer esterification: methyl benzoate from benzoic acid and methanolDraw the acid-catalysed esterification of benzoic acid with methanol, choosing the oxygen that must be protonated to move forward, to give methyl benzoate and water.

Ether synthesis

  • Epoxide closure: cyclopentene oxide from trans-2-chlorocyclopentanolDraw the deprotonation of trans-2-chlorocyclopentan-1-ol by hydroxide and the intramolecular backside attack of the alkoxide that closes the three-membered ring of cyclopentene oxide.
  • Williamson synthesis of nerolin: 2-naphthol and iodoethaneDraw the deprotonation of 2-naphthol by hydroxide and the SN2 attack of the naphthoxide oxygen on iodoethane that gives 2-ethoxynaphthalene (nerolin).
  • Williamson synthesis of propoxybenzene from phenol and 1-bromopropaneDraw hydroxide removing the phenol proton and the phenoxide displacing bromide from 1-bromopropane to give propoxybenzene.

Hydrolysis and saponification

  • Saponification of methyl salicylate to salicylic acidDraw the hydroxide-promoted hydrolysis of methyl salicylate, including deprotonation of the phenol and of the new carboxylic acid, then the acidic workup that returns salicylic acid.
  • Soap making: saponification of one ester of glyceryl tripalmitateDraw hydroxide cleaving one outer ester of glyceryl tripalmitate, ending with the proton transfer that gives palmitate (hexadecanoate) and 1,2-dipalmitoylglycerol.

Calcular 1

  • Percent yield of an aspirin preparationCalculate the theoretical and percent yield of aspirin from the mass of salicylic acid used.

Encontre o erro 1

  • An old aspirin sample that smells of vinegarDiagnose the hydrolysis of an aryl ester from its odour and a ferric chloride test.

Interprete as evidências 1

  • Reading the IR spectrum of a distilled esterInterpret an IR spectrum to decide whether a distilled ester is free of the starting acid and alcohol.

Planeje uma síntese 1

  • Pushing a Fischer esterification to completionPlan reaction conditions that raise the equilibrium yield of a Fischer esterification.

Explique por quê 1

  • Why the bicarbonate wash worksExplain how a sodium bicarbonate wash separates leftover acids from a neutral ester.

Travou em ésteres, amidas e éteres no laboratório? 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.

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