Química Orgânica 1 · Eliminação e caminhos competitivos
Explain the E alkene from an E1 dehydration
Condições: Butan-2-ol heated with aqueous sulfuric acid. H₂SO₄ is a stronger acid than H₃O⁺, so in water its proton sits on hydronium; hydronium and the water it becomes are shown, and the HSO₄⁻ counteranion is omitted. Follow the branch to (E)-but-2-ene, the major product; (Z)-but-2-ene and but-1-ene are minor, and no ratio is claimed. Carbons are numbered as in the name (C1–C4); remove the separately drawn hydrogen on the neighboring CH₂ carbon.
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Protonate the hydroxyl group
Protonation turns the poor –OH leaving group into –OH₂⁺.
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
A secondary cation forms more slowly than a tertiary one, so this dehydration needs stronger heating than the tertiary-alcohol cases. No neighboring shift would give a more stable cation here: moving a C3 hydrogen to C2 would only make another secondary cation.
As setas curvas
- The C2–O pair leaves with oxygen as water, leaving a secondary carbocation at C2.
Why E?
Before the proton is removed, the C2–C3 single bond can still rotate. The E2 requirement for an anti-periplanar leaving group no longer applies, because the leaving group is already gone.
Remove the C3 proton
The water formed in the first step is the base, and it becomes hydronium again: the acid is a catalyst.
As setas curvas
- A lone pair on the water oxygen bonds to the separately drawn hydrogen on the CH₂ carbon next to the carbocation.
- The C3–H pair becomes the C2=C3 π bond, neutralizing C2.
(E)-But-2-ene
The new C=C bond has its methyl groups on opposite sides: (E)-but-2-ene. The alcohol oxygen left as water, and hydronium is regenerated. Removing a hydrogen from the methyl group next to the carbocation instead would give the less substituted but-1-ene, a minor product.
O que você vai praticar
Show why E1 dehydration of butan-2-ol favors (E)-but-2-ene: the planar cation can rotate before deprotonation, so the lower-energy geometry wins.
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