Química Orgânica 2 · Adições à carbonila: revisão para a prova
Ethylene ketal of butan-2-one
Condições: Butan-2-one, ethane-1,2-diol (1.5 equiv), TsOH·H2O (0.02 equiv), toluene, reflux with Dean–Stark removal of water
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 9 passos escritos
TsOH protonates the carbonyl oxygen
A carbonyl oxygen lone pair takes the acidic proton of TsOH. The protonated ketone is far more electrophilic than the neutral one, which a weak nucleophile such as an alcohol needs.
As setas curvas
- A carbonyl oxygen lone pair takes the proton from TsOH.
- The O–H pair stays on the sulfonate oxygen, giving tosylate.
A diol oxygen adds to the carbonyl carbon
One OH oxygen of ethane-1,2-diol attacks the activated carbonyl carbon. The C=O π pair moves onto the protonated oxygen, which becomes a neutral OH.
As setas curvas
- A diol oxygen lone pair forms the new C–O bond.
- The C=O π pair moves onto the protonated oxygen.
Tosylate removes the proton from the added oxygen
The newly bonded oxygen carries a positive charge. Tosylate removes its proton, giving the neutral hemiketal and regenerating TsOH.
As setas curvas
- A tosylate oxygen lone pair takes the O–H proton.
- The O–H pair stays on the ether-type oxygen, which becomes neutral.
TsOH protonates the hemiketal OH
To replace the OH, it must become a good leaving group. TsOH protonates the hydroxyl oxygen, turning it into a water molecule still attached to carbon.
As setas curvas
- A hydroxyl oxygen lone pair takes the proton from TsOH.
- The O–H pair stays on the sulfonate oxygen.
Why remove water?
Every step of ketal formation is reversible, and water is a product; removing it is what drives the equilibrium to the ketal.
Water leaves, giving an oxocarbenium ion
A lone pair of the ring-to-be oxygen forms a C=O⁺ π bond while the C–O bond to the protonated oxygen breaks, releasing water. The oxocarbenium ion is the electrophile for ring closure.
As setas curvas
- The ether oxygen lone pair forms the C=O⁺ π bond.
- The C–O bond breaks and water leaves with the pair.
The second OH closes the five-membered ring
The OH at the other end of the diol chain attacks the oxocarbenium carbon. The C=O⁺ π pair returns to oxygen, and a five-membered ring containing two oxygens forms.
As setas curvas
- The hydroxyl oxygen lone pair forms the ring C–O bond.
- The C=O⁺ π pair returns to the ring oxygen.
Tosylate removes the last proton
Tosylate takes the proton from the positively charged ring oxygen, giving the neutral ketal and regenerating TsOH.
As setas curvas
- A tosylate oxygen lone pair takes the proton.
- The O–H pair stays on the ring oxygen, which becomes neutral.
2-Ethyl-2-methyl-1,3-dioxolane
The former carbonyl carbon is now bonded to two oxygens of a five-membered ring. The carbonyl oxygen left as water, and TsOH is regenerated, so it is a catalyst.
O que você vai praticar
Draw the acid-catalysed formation of 2-ethyl-2-methyl-1,3-dioxolane from butan-2-one and ethane-1,2-diol, using TsOH and tosylate for every proton transfer, and explain why water is removed.
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