Química Orgânica 1 · Misturas estereoquímicas e suas medidas
Make a racemate from an achiral alkene
Condições: But-1-ene in dilute aqueous H₂SO₄. The proton donor is hydronium; its HSO₄⁻ counterion is omitted throughout. A water molecule captures the cation and a second water removes a proton from the resulting oxonium ion. The drawing follows capture on the upper face (toward the viewer). No hydride shift occurs: transferring H from the neighboring CH₂ carbon would give another secondary cation.
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Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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Protonate the alkene
Protonation at C1 gives the more stable secondary cation at C2. No stereocenter exists yet: C2 is sp² and planar.
As setas curvas
- The π pair forms the new C1–H bond.
- The O–H bond pair returns to oxygen, leaving neutral water.
Predict the stereochemical outcome
The butan-2-yl cation is achiral and planar at C2. Its two faces are mirror images, so water attack on either face passes through enantiomeric transition states of exactly equal energy. The two rates are identical: (S)- and (R)-butan-2-ol form 50:50, a racemic mixture with 0% ee and no optical rotation.
Capture the cation
Follow upper-face capture; bottom-face capture proceeds identically and gives the mirror image.
As setas curvas
- A water lone pair forms the C2–O bond on the upper face.
Deprotonate the oxonium ion
The oxonium oxygen has three bonds and one lone pair (+1). The second water removes a proton and becomes hydronium, so acid is regenerated.
As setas curvas
- A lone pair on the second water accepts a proton from the oxonium oxygen.
- The O–H bond pair stays on oxygen, restoring its second lone pair.
(R)-Butan-2-ol, formed with its enantiomer
The drawn upper-face branch gives (R)-butan-2-ol: OH > CH₂CH₃ > CH₃ > H, and with the implicit H pointing away, OH → ethyl → methyl is clockwise. The lower-face branch gives (S)-butan-2-ol at the same rate, so the isolated alcohol is racemic (50:50, 0% ee, [α] = 0). Oxygen is neutral with two bonds and two lone pairs; hydronium is regenerated.
O que você vai praticar
Explain why capture of an achiral planar carbocation gives a racemic alcohol.
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