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Condensações aldólica, benzoínica e afins no laboratório

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O que você vai aprender

  • Explain why a crossed aldol between an enolisable ketone and a non-enolisable aldehyde gives one main product, and how dehydration drives it.
  • Draw the benzoin, benzilic acid, Cannizzaro and haloform mechanisms run in the teaching lab, naming each intermediate.
  • Use stoichiometry, observations and structure to plan, diagnose and calculate yields for condensation preparations.

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Crossed aldol without self-condensation

A crossed aldol normally gives a mixture, because each carbonyl partner can be both nucleophile and electrophile. The lab version avoids this by pairing an enolisable ketone such as acetone with benzaldehyde, which has no alpha hydrogen. Hydroxide can only make the acetone enolate, drawn here beside benzaldehyde. That enolate meets two electrophiles, but the aldehyde carbon is less hindered and more positive than the ketone carbon, and benzaldehyde is used in excess. The enolate therefore adds to benzaldehyde, and the acetone self-aldol, which is also unfavourable at equilibrium, is a minor pathway.

Dehydration makes it irreversible

The beta-hydroxy ketone from the addition step is not the isolated product. Hydroxide removes an alpha hydrogen from the CH2 between the ketone and the alcohol carbon, and the enolate expels hydroxide from the neighbouring carbon, an E1cB elimination. Hydroxide is normally a poor leaving group, but here the product, benzalacetone (4-phenylbut-3-en-2-one), is an E enone conjugated from the ring through the C=C to the carbonyl. That conjugation makes dehydration downhill, and in aqueous ethanol the product often crystallises out. Benzalacetone still carries an enolisable methyl group, so a second equivalent of benzaldehyde repeats the sequence to give dibenzalacetone.

Umpolung: the benzoin trick

An aldehyde carbon is an electrophile, yet benzoin forms a bond between two aldehyde carbons. Cyanide reverses the polarity of one of them. It adds to benzaldehyde, and once the alkoxide is protonated the former aldehyde hydrogen sits on a carbon bearing both a nitrile and a benzene ring. Hydroxide can remove it, giving the cyanohydrin anion shown: a carbanion that behaves as an acyl anion equivalent. It adds to a second benzaldehyde, water and hydroxide shuttle a proton from the cyanohydrin oxygen to the new alkoxide, and cyanide is expelled to give benzoin. Thiamine does the same job with a thiazolium ylide, avoiding cyanide.

Haloform and Cannizzaro

Two lab reactions turn a carbonyl into a carboxylic acid without a chromium oxidant. In the haloform reaction, bleach chlorinates a methyl ketone three times through its enolates, each faster than the last, giving a trichloromethyl ketone like the one drawn. Hydroxide then adds to the carbonyl and the stabilised trichloromethyl anion leaves, producing chloroform and the carboxylate. In the Cannizzaro reaction an aldehyde with no alpha hydrogen meets concentrated hydroxide: the tetrahedral adduct transfers hydride to a second aldehyde, so one molecule is oxidised to the carboxylate and the other reduced to the alcohol.

What to carry forward

Make only one partner enolisable, let conjugation or precipitation pull the equilibrium, and recognise when a carbon nucleophile comes from umpolung (benzoin) or a carbon leaving group (haloform) rather than an ordinary enolate.

Problemas de prática (15)

Praticar é grátis com uma conta.

Desenhe o mecanismo 10

Benzoin and benzil chemistry

  • Benzilic acid rearrangement of benzilDraw hydroxide addition to benzil, the 1,2-phenyl shift to the neighbouring carbonyl carbon, the proton transfers through hydroxide and water that give the carboxylate, and its protonation on acid workup.
  • Cyanide-catalysed benzoin condensation of benzaldehydeDraw the cyanide-catalysed benzoin condensation: cyanohydrin formation, deprotonation to the acyl anion equivalent, addition to a second benzaldehyde, proton transfer and loss of cyanide.
  • Thiamine (thiazolium ylide)-catalysed benzoin condensationDraw the thiazolium-ylide-catalysed benzoin condensation: ylide formation, addition to benzaldehyde, formation of the Breslow enamine, addition to a second benzaldehyde and release of the catalyst.

Disproportionation and haloform

  • Cannizzaro disproportionation of 4-chlorobenzaldehydeDraw hydroxide addition to 4-chlorobenzaldehyde, the hydride transfer to a second aldehyde and the proton transfer that give 4-chlorobenzyl alcohol and 4-chlorobenzoate.
  • Haloform oxidation of 4-methylacetophenone with bleachDraw the three base-promoted alpha-chlorinations of 4-methylacetophenone, hydroxide attack on the trichloromethyl ketone, loss of the trichloromethyl anion and the proton transfers that give chloroform and 4-methylbenzoic acid.

Aldol-type condensations

  • Claisen–Schmidt condensation of acetophenone with benzaldehyde (chalcone)Draw the base-catalysed aldol addition of the acetophenone enolate to benzaldehyde and the E1cB dehydration that gives chalcone (1,3-diphenylprop-2-en-1-one).
  • Crossed aldol condensation of acetone with benzaldehyde (benzalacetone)Draw the hydroxide-catalysed aldol addition of the acetone enolate to benzaldehyde and the E1cB dehydration that gives benzalacetone (4-phenylbut-3-en-2-one).
  • Second aldol condensation: benzalacetone with benzaldehyde (dibenzalacetone)Draw the enolisation of benzalacetone at its methyl group, the aldol addition to a second benzaldehyde and the E1cB dehydration that gives dibenzalacetone (1,5-diphenylpenta-1,4-dien-3-one).
  • Knoevenagel condensation of benzaldehyde with malononitrileDraw the piperidine-catalysed Knoevenagel condensation: deprotonation of malononitrile, addition to benzaldehyde, proton transfers and E1cB dehydration to 2-benzylidenepropanedinitrile.
  • Perkin condensation of benzaldehyde with acetic anhydride (cinnamic acid)Draw the Perkin reaction: acetate-generated anhydride enolate, aldol addition to benzaldehyde, O-acetylation, E1cB loss of acetate, and water hydrolysis of the mixed anhydride to cinnamic acid.

Preveja o produto 1

  • Chalcone from 4-methoxybenzaldehydePredict which ring ends up on the carbonyl when 4-methoxybenzaldehyde condenses with acetophenone.

Calcular 1

  • Theoretical yield of dibenzalacetoneConvert reagent masses to moles, apply the 2:1 stoichiometry and find the theoretical mass of dibenzalacetone.

Encontre o erro 1

  • A drawn self-aldolDiagnose why the acetone enolate adding to acetone is not the main pathway in the dibenzalacetone preparation.

Trabalhe de trás para frente 1

  • Which partners make this chalcone?Work backwards from a chalcone to the ketone and aldehyde that form it by Claisen–Schmidt condensation.

Explique por quê 1

  • Why the crossed aldol is cleanExplain why benzaldehyde and acetone in NaOH give mainly the crossed condensation product.

Travou em condensações aldólica, benzoínica e afins 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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