Química Orgânica 1 · Raciocínio ácido–base
Why bicarbonate fizzes with a carboxylic acid
Condições: Acetic acid with aqueous sodium bicarbonate (NaHCO₃). First draw proton transfer from acetic acid to the negatively charged bicarbonate oxygen. Then, with one explicit water molecule relaying protons, draw the decomposition of carbonic acid into CO₂ and water. Approximate pKa values: acetic acid 4.8, carbonic acid 6.4. Na⁺ is a spectator.
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.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 3 passos escritos
Bicarbonate removes the acetic acid proton
Acetic acid (pKa about 4.8) is a stronger acid than carbonic acid (pKa about 6.4), so bicarbonate is a strong enough base to deprotonate it (K ≈ 10^1.6, about 40).
As setas curvas
- A lone pair on the negative bicarbonate oxygen forms a bond to the carboxyl hydrogen.
- The O–H pair stays on the acetate oxygen, which becomes negatively charged.
Carbonic acid breaks down with a water relay
This is one concerted, cyclic proton shuttle. The solvent water accepts a proton and donates one, so it is regenerated. The departing hydroxyl group becomes a new water molecule. Loss of CO₂ gas from the open container pulls the whole sequence forward.
Nota: One solvent water molecule is now drawn explicitly. It accepts a carbonic acid proton and delivers one of its own protons to the departing hydroxyl group in the same step.
As setas curvas
- A lone pair on the water oxygen forms a bond to a carbonic acid O–H proton.
- The O–H pair becomes a new C=O π bond.
- The other C–OH bond breaks and its pair stays on oxygen, so carbon never exceeds four bonds.
- A lone pair on the departing hydroxyl oxygen forms a bond to a water hydrogen.
- The water O–H pair returns to its oxygen, which ends with two O–H bonds again.
Sodium acetate, carbon dioxide and water
Acetate carries the negative charge on an oxygen with three lone pairs, balanced by Na⁺. Carbon dioxide is linear O=C=O with two lone pairs on each oxygen and no charges. One hydroxyl group left as a new water molecule, and the solvent water now holds the proton it accepted from carbonic acid. The bubbles of CO₂ are the visible sign that a carboxylic acid reacted with bicarbonate.
O que você vai praticar
Follow proton transfer from acetic acid to bicarbonate, then water-assisted breakdown of carbonic acid into carbon dioxide and water.
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