Química Orgânica 1 · Eliminação e caminhos competitivos
Turn an alcohol into a ring alkene
Condições: Cyclohexanol heated with aqueous sulfuric acid (H₂SO₄), distilling off the alkene; follow dehydration to cyclohexene using the explicit β-H. H₂SO₄ is a stronger acid than H₃O⁺, so in water its first proton sits on hydronium; hydronium and one representative water shuttle are shown, and the HSO₄⁻ counteranion is omitted throughout. Ring drawings specify connectivity, not an impossible trans-cyclohexene geometry.
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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
Begin the acid-mediated pathway
Determine how the neutral alcohol becomes a viable leaving group. Hydroxide is a poor leaving group (its conjugate acid, water, has pKa ≈ 15.7); the protonated alcohol instead releases water (conjugate acid H₃O⁺, pKa ≈ −1.7). Protonated cyclohexanol (pKa roughly −2) is about as acidic as hydronium, so only a small fraction is activated at any moment, and removing the alkene by distillation pulls the equilibrium forward.
Nota: Any hydronium proton can be transferred.
As setas curvas
- The base uses a lone pair to bond to the drawn hydrogen.
- The old bond pair stays on the atom losing the proton.
Continue from the activated substrate
The alcohol oxygen and both attached hydrogens leave together as water.
As setas curvas
- The protonated alcohol leaves as neutral water, not hydroxide.
Complete the alkene-forming step
Both water molecules remain present.
Nota: Remove the separately drawn ring hydrogen next to the carbocation.
As setas curvas
- Water accepts the drawn ring β proton.
- The C–H pair supplies the C=C bond and removes the carbocation charge.
Cyclohexene, water and regenerated acid
The substrate oxygen leaves with its original proton and the acid proton as H₂O. A water molecule accepts the β proton, regenerating hydronium. The net organic change is loss of H and OH as water, with total charge +1 throughout.
O que você vai praticar
Activate a poor leaving group and regenerate aqueous acid through the complete E1 dehydration sequence.
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