Química Orgânica 1 · Caminhos de substituição
Lose a configuration through a planar cation
Condições: (S)-(1-Bromoethyl)benzene in water, with a little acetone to dissolve it. Follow the SN1 substitution branch to 1-phenylethanol; elimination to styrene is minor and not requested. In the first drawing Br and the phenyl group lie in the page, the methyl wedge points toward you and the hydrogen points away. A second water molecule enters only as the final proton acceptor.
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 5 passos escritos
Ionize the benzylic bromide
Water is a weak nucleophile but a good ionizing solvent, and the cation that forms is stabilized by the phenyl ring, so ionization comes first.
As setas curvas
- The C–Br pair leaves with bromine, giving a benzylic carbocation and bromide.
Predict the stereochemical outcome
The carbocation is sp² hybridized: the positively charged carbon and the three atoms bonded to it lie in one plane, drawn here in the plane of the page, and the phenyl ring lies in that plane too so its π system can share the charge. The empty p orbital has one lobe in front of the page and one behind.
Nota: The planar cation is redrawn flat in the page; its two faces are in front of and behind the page.
Capture the cation on the front face
Draw capture on one face; the drawing that follows shows the new C–O bond as a wedge. Capture from the back face happens just as often.
As setas curvas
- A water lone pair bonds to the cationic carbon from the face in front of the page.
Deprotonate the oxonium ion
The alkyloxonium ion (pKa approximately −2) and hydronium (pKa approximately −1.7) are similar in strength, and water is present in large excess.
Nota: Transfer an O–H proton from the oxonium ion to water. The two O–H protons on the oxonium ion are equivalent.
As setas curvas
- A second water takes one O–H proton from the oxonium ion.
- The O–H pair stays on the alcohol oxygen, which becomes neutral.
Alcohol from front-face capture
Front-face capture gives the (S)-alcohol drawn here: OH on a wedge, H on a hash, phenyl and methyl in the page. Capture from the back face gives its mirror image, (R)-1-phenylethanol, at the same rate. The sample is therefore racemic, or close to it, and optically inactive. Hydronium and bromide make up the HBr formed.
O que você vai praticar
Explain why an SN1 reaction of one enantiomer gives both enantiomers of the product by following the planar carbocation.
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