Química Orgânica 1 · Ácidos, bases e ressonância: revisão para a prova
Triphenylmethanol forms the trityl cation in sulfuric acid
Condições: Triphenylmethanol, concentrated H₂SO₄, 25 °C
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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.
Ler todos os 6 passos escritos
Sulfuric acid protonates the hydroxyl group
An oxygen lone pair bonds to a hydrogen of sulfuric acid. The OH becomes OH₂⁺, which can leave as neutral water.
As setas curvas
- A hydroxyl lone pair bonds to the acidic hydrogen.
- The O–H bonding pair stays on oxygen, giving hydrogen sulfate.
Water leaves
The C–O bond breaks with both electrons going to oxygen, releasing water and leaving the central carbon with an empty p orbital.
As setas curvas
- The C–O bonding pair leaves with oxygen as water.
Why does this cation form so easily?
The empty p orbital on the central carbon overlaps with all three benzene rings, so the positive charge is spread over ten carbons.
A ring shares the positive charge
A ring π pair moves toward the cationic carbon, forming an exocyclic C=C bond and leaving the positive charge on a ring carbon ortho to the attachment point.
As setas curvas
- The ring π pair forms a π bond to the cationic central carbon.
Back to the major contributor
The ring-cation contributor is minor: it still has a six-electron carbon, and one ring has lost its aromatic sextet. Moving the exocyclic π pair back into the ring restores three benzene rings with the positive charge on the central carbon.
As setas curvas
- The exocyclic π pair moves back into the ring, filling the ortho carbon's empty orbital.
Triphenylmethyl cation
The major contributor keeps all three benzene rings intact, with the positive charge on the central carbon; minor contributors share it with the ortho and para carbons of each ring, ten carbons in all. Water and hydrogen sulfate are the by-products.
O que você vai praticar
Draw sulfuric acid protonating the OH of triphenylmethanol, loss of water to give the triphenylmethyl cation, one minor contributor that places the positive charge in a ring, and the return to the major contributor.
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