Química Orgânica 1 · Eliminação e caminhos competitivos
Dehydrate through a benzylic cation
Condições: 1-Phenylethanol heated with a catalytic amount of aqueous sulfuric acid, distilling off the alkene. Follow E1 dehydration to styrene (ethenylbenzene). H₂SO₄ is a stronger acid than H₃O⁺, so in water its proton sits on hydronium; hydronium and the water it becomes are shown, and HSO₄⁻ is omitted. C1 bears the OH and C2 is the methyl carbon. Ph is an unchanged phenyl group; delocalization of the cation into the ring is discussed but not drawn as separate contributors.
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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.
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Protonate the hydroxyl group
As in every acid-catalyzed dehydration, –OH must first become –OH₂⁺ so that it can leave as water.
Nota: Any of hydronium’s three equivalent hydrogens can be transferred.
As setas curvas
- A lone pair on the alcohol oxygen bonds to a hydronium proton.
- The hydronium O–H pair stays on oxygen, which becomes neutral water.
Lose water
The carbon bearing OH is secondary, yet this ionization is comparatively easy. Its empty p orbital lies next to the aromatic ring.
As setas curvas
- The C1–O pair leaves with oxygen as water, leaving a cation at C1, next to the ring.
Why so easy?
Secondary alcohols usually need stronger heating than tertiary ones. Consider what the ring does for the cation at C1.
Remove the C2 proton
C2 is the only carbon next to C1 that carries hydrogens. The new π bond is conjugated with the ring, which adds extra stability to the product.
As setas curvas
- A lone pair on the water oxygen bonds to the separately drawn methyl hydrogen.
- The C2–H pair becomes the C1=C2 π bond, neutralizing C1.
Styrene
C1=C2 is now a vinyl group conjugated with the benzene ring: styrene, C₆H₅–CH=CH₂. The alcohol oxygen left as water, and hydronium is regenerated. Distilling the alkene out as it forms keeps the equilibrium moving toward dehydration.
O que você vai praticar
Connect the easy E1 dehydration of a secondary benzylic alcohol to ring delocalization of its cation, and form a conjugated alkene.
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