Química Orgânica 2 · Adições à carbonila: revisão para a prova
Dimethyl ketal of cyclohexanone
Condições: CH3OH, HCl (cat.)
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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
The carbonyl oxygen is protonated
HCl in methanol gives protonated methanol, the strongest acid present. It protonates the carbonyl oxygen, making the carbonyl carbon much more electrophilic toward the weak nucleophile methanol.
As setas curvas
- The carbonyl oxygen lone pair takes the acidic proton.
- The O–H pair stays on methanol.
Methanol adds to the carbonyl carbon
A methanol oxygen lone pair attacks the activated carbonyl carbon, and the C=O⁺ π pair moves onto oxygen.
As setas curvas
- The methanol oxygen lone pair forms the new C–O bond.
- The C=O π pair moves onto the protonated oxygen, which becomes neutral OH.
Methanol removes the extra proton
Another methanol takes the proton from the oxonium ion, giving the neutral hemiketal.
As setas curvas
- A methanol oxygen lone pair takes the O–H proton.
- The O–H pair stays on the methoxy oxygen.
The hydroxyl group is protonated
Protonated methanol now protonates the hemiketal OH, turning it into water, a good leaving group.
As setas curvas
- The hydroxyl oxygen lone pair takes the acidic proton.
- The O–H pair stays on methanol.
Water leaves, pushed by the methoxy oxygen
The methoxy oxygen lone pair forms a C=O⁺ π bond as water leaves, giving a resonance-stabilised oxocarbenium ion.
As setas curvas
- A methoxy lone pair forms the C=O⁺ π bond.
- The C–O pair leaves with water.
Why the reaction can be pushed to the ketal
Every step is reversible; the equilibrium is driven by excess methanol and removal of water.
A second methanol traps the oxocarbenium ion
A second methanol oxygen attacks the oxocarbenium carbon and the C=O⁺ π pair returns to the methoxy oxygen.
As setas curvas
- The methanol lone pair forms the second C–O bond.
- The C=O⁺ π pair returns to the methoxy oxygen.
Methanol removes the last proton
Methanol takes the proton from the new oxonium ion, giving the ketal and regenerating the acid catalyst.
As setas curvas
- A methanol lone pair takes the O–H proton.
- The O–H pair stays on the methoxy oxygen.
1,1-Dimethoxycyclohexane
1,1-Dimethoxycyclohexane forms with water; protonated methanol (the catalyst) is regenerated.
O que você vai praticar
Draw the acid-catalysed formation of 1,1-dimethoxycyclohexane from cyclohexanone through the hemiketal and the oxocarbenium ion.
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