Química Orgânica 2 · Ferramentas de oxidação e redução
NaBH4 reduction of 4-oxopentanoic acid in aqueous base
Condições: NaOH (1.1 equiv), H2O; NaBH4 (0.5 equiv), 0 °C to 25 °C; then cold aqueous HCl to pH 2
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Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 6 passos escritos
Hydroxide deprotonates the carboxylic acid
The most acidic proton goes first. Hydroxide removes the carboxylic acid O–H proton, giving the carboxylate and water.
As setas curvas
- A hydroxide lone pair forms the new O–H bond to the acidic proton.
- The carboxyl O–H pair stays on oxygen, giving the carboxylate.
Hydride adds to the ketone carbon
The B–H bonding pair of borohydride forms the new C–H bond at the ketone carbon while the C=O π pair moves onto oxygen.
As setas curvas
- The B–H pair carries the hydrogen to the ketone carbon.
- The ketone C=O π pair moves onto oxygen, giving an alkoxide.
Why the carboxylate is not reduced
A carboxylate carbon is flanked by two oxygens sharing a negative charge, so it is a very poor electrophile toward an anionic hydride donor.
Water protonates the alkoxide
In aqueous base the proton source is water: the alkoxide takes a proton from water, giving the secondary alcohol and regenerating hydroxide.
As setas curvas
- An alkoxide lone pair forms the new O–H bond to a water hydrogen.
- The water O–H pair stays on oxygen as hydroxide.
Acidification restores the carboxylic acid
Adding HCl to pH 2 supplies hydronium. One hydronium protonates the carboxylate oxygen, giving the neutral acid, and another neutralises the hydroxide.
As setas curvas
- A carboxylate oxygen lone pair takes a proton from hydronium.
- The hydronium O–H pair stays on oxygen, leaving water.
- Hydroxide takes a proton from a second hydronium.
- That O–H pair stays on oxygen, leaving water.
4-Hydroxypentanoic acid
The ketone has become a secondary alcohol and the acid is back in its neutral form: the product is 4-hydroxypentanoic acid.
O que você vai praticar
Draw deprotonation of 4-oxopentanoic acid by hydroxide, hydride addition to the ketone, protonation of the alkoxide by water, and acidification of the carboxylate, giving 4-hydroxypentanoic acid.
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