Química Orgânica 1 · Estereocentros e configuração
Assign R/S through activation and displacement
Condições: (S)-Butan-2-ol with p-toluenesulfonyl chloride (TsCl) and pyridine, then the isolated tosylate with NaCN in DMSO. Solid wedge points toward the viewer, hash away; oxygen and the ethyl group lie in the drawing plane. Sulfonylation is drawn as the standard net substitution at sulfur; p-Tol is the unchanged para-tolyl group. Pyridinium chloride from the first stage stays in the drawing as a spectator. Follow only the SN2 branch of the cyanide stage; competing E2 is not requested.
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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 oxygen
Oxygen bonds to sulfur; nothing happens at the stereocenter. The alcohol C–O bond is untouched, so the four groups at the stereocenter keep their spatial arrangement. Priorities OH > CH₂CH₃ > CH₃ > H; with H pointing away, OH → ethyl → methyl runs counterclockwise: S.
As setas curvas
- An alcohol oxygen lone pair forms the new O–S bond.
- The S–Cl bond pair leaves with chloride.
Neutralize the oxonium ion
Oxygen now carries three bonds and one lone pair (formal charge +1). Pyridine removes the proton so the tosylate is neutral. C2 still has not been touched.
As setas curvas
- The pyridine nitrogen lone pair accepts the O–H proton.
- The O–H bond pair returns to oxygen, which becomes neutral again.
Displace the tosylate
The (S)-tosylate keeps the alcohol’s arrangement: OTs still ranks first, ethyl second, methyl third, so it is still S. Cyanide is added at this separate second stage. Backside attack is the only event that changes the arrangement at C2.
As setas curvas
- The cyanide carbon lone pair attacks C2 from the side opposite the C–O bond.
- The C2–O bond pair leaves with the sulfonate oxygen as the new C–C bond forms.
(R)-2-Methylbutanenitrile
The other three groups at the stereocenter flip like an umbrella turning inside out: C–CN forms on the face opposite the old C–O bond. New priorities: CN (C with N,N,N) > CH₂CH₃ (C with C,H,H) > CH₃ > H. The priority order is the same as before, now in the mirror-image arrangement: with H pointing away, CN → ethyl → methyl runs clockwise, so the product is R. Retention (S) and then inversion give the net S → R result. The cyanide carbon is neutral with four bonds; the p-toluenesulfonate oxygen has three lone pairs and a −1 charge, balanced by Na⁺.
O que você vai praticar
Assign the descriptor at each stage of an alcohol → tosylate → nitrile sequence, separating a retained C–O bond from backside C–O cleavage.
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