Química Orgânica 1 · Estereoquímica das reações de alcenos
Open a ring epoxide at its tertiary carbon in acid
Condições: Racemic 1-methylcyclohexene oxide in methanol with a catalytic amount of H₂SO₄. The proton donor is protonated methanol; another methanol is the final base; the HSO₄⁻ counterion is omitted throughout. The drawn enantiomer has the epoxide on the upper (wedge) face. Methanol is the nucleophile: choose the ring carbon it attacks.
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 4 passos escritos
Activate the epoxide oxygen
Neutral methanol is too weak a nucleophile to open an unactivated epoxide. Protonating the ring oxygen turns it into a good neutral leaving group (an alcohol) and weakens both C–O bonds, most of all the bond to the tertiary carbon.
As setas curvas
- An epoxide oxygen lone pair accepts a proton from protonated methanol.
- The O–H bond pair stays on the proton donor oxygen, giving neutral methanol.
Methanol opens the ring at the tertiary carbon
In the protonated epoxide the C–O bond to the tertiary carbon is longer and weaker. That carbon carries more of the positive charge, because a tertiary center stabilizes it best. Methanol therefore attacks the more substituted carbon, the reverse of basic opening. The ring still blocks the upper face, so attack comes from below and inverts the tertiary carbon: its methyl is now on the upper face rather than the lower face.
As setas curvas
- A methanol oxygen lone pair bonds to the tertiary carbon from the lower face, opposite the epoxide oxygen.
- The C–O bond pair at the attacked carbon moves onto the epoxide oxygen, which becomes a neutral hydroxyl oxygen.
Deprotonate the methoxy oxygen
Solvent methanol removes the proton from the oxonium ion. It becomes protonated methanol, so the acid catalyst is regenerated.
As setas curvas
- A solvent methanol lone pair accepts the proton from the positively charged ether oxygen.
- The O–H bond pair stays on the ether oxygen, which becomes neutral.
trans-2-Methoxy-2-methylcyclohexan-1-ol
The OH (wedge, from the original epoxide oxygen) and the OCH₃ (hash, from methanol) are on opposite faces: the two oxygens were delivered anti. The methyl now lies on the same face as the OH. Attack at the tertiary carbon puts OCH₃ on the methyl-bearing carbon; under basic conditions (CH₃O⁻) the less hindered epoxide carbon would be attacked instead. Protonated methanol is regenerated; the racemic epoxide gives the racemic trans product.
O que você vai praticar
Combine acid activation, methanol attack at the more substituted epoxide carbon with inversion, and solvent deprotonation to obtain the trans 2-methoxy alcohol.
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