Química Orgânica 2 · Substituição acílica, aminas e sais de diazônio
Why saponification goes to completion
Condições: Ethyl benzoate heated under reflux (a) with aqueous NaOH (1.2 equiv) and (b) with catalytic H2SO4 in aqueous ethanol. Reaction (a) converts all the ester; reaction (b) stops at an equilibrium mixture of ethyl benzoate, benzoic acid and ethanol.
Desenhe este problema você mesmo
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.
A pergunta
Why does the reaction in NaOH go to completion?
Mostrar a resposta e a explicação
Resposta e explicação
Resposta correta The benzoic acid formed is immediately deprotonated to benzoate, which is not electrophilic, so ethanol or ethoxide cannot attack it to re-form the ester.
The final proton transfer (pKa 4.2 versus about 16) is enormously favourable and removes the product from the reversible part of the sequence.
Esta não Hydroxide is a catalyst that is regenerated, and catalysts drive reactions to completion.
Hydroxide is consumed (one equivalent per ester), and no catalyst can change the position of an equilibrium.
Esta não Hydroxide is a stronger nucleophile than water, so the reaction is faster and therefore goes further.
A faster reaction reaches equilibrium sooner but does not change where the equilibrium lies; completeness comes from the irreversible deprotonation.
Esta não Ethanol evaporates from the basic solution but not from the acidic one.
Both mixtures are heated under reflux, which returns ethanol to the flask; the difference is chemical, not physical.
Solução explicada
In base the sequence is addition of hydroxide, loss of ethoxide, then proton transfer from the acid to ethoxide. That last step forms a resonance-stabilised carboxylate that has no reason to react with an alcohol, so the overall reaction is one-way. In acid there is no such sink: the protonated acid can be attacked by ethanol again.
O que você vai praticar
Explain why ester hydrolysis in base is irreversible while acid-catalysed hydrolysis is an equilibrium.
Você acabou de desenhar cada seta deste problema.
Cada um dos 5349 problemas de Orgânica 1 e 2 funciona assim: feedback em cada seta, dicas quando você travar e um modo prova sem dicas. Grátis com uma conta, sem cartão.
Travou na sua própria lição?
Tire uma foto: o primeiro scan é grátis, sem cadastro
Você escaneia toda semana? Scan Standard: por cerca de problemas