Química Orgânica 1 · Adição a alcenos e regioquímica
Hydrate styrene through a benzylic cation
Condições: Styrene (ethenylbenzene) in dilute aqueous H₂SO₄. Hydronium is the proton donor; its counterion stays outside the reacting system. Water is the nucleophile, and another water is the final base. The ring is drawn in one Kekulé form and does not change.
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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 vinyl group
The cation next to the ring is benzylic: its empty p orbital lines up with the ring π system, so the positive charge is shared by the ring carbons ortho and para to the side chain. Protonating the benzylic carbon instead would leave an unstabilized primary cation at the end of the chain.
Nota: Any of hydronium’s three hydrogens can be transferred.
As setas curvas
- The alkene π pair bonds the terminal carbon to H, leaving the empty orbital on the carbon next to the ring.
- The O–H pair returns to oxygen, leaving neutral water.
Water bonds to the benzylic carbon
Although resonance spreads the positive charge into the ring, water bonds at the benzylic carbon: capture at a ring carbon would destroy the ring’s aromaticity. The flat cation is attacked equally from both faces.
As setas curvas
- A lone pair on water bonds to the benzylic carbon.
Deprotonate the oxonium ion
Transferring a proton to water regenerates the hydronium consumed in the protonation.
Nota: Either hydrogen on the positive oxygen can be removed.
As setas curvas
- A lone pair on water accepts a proton from the positive oxygen.
- The O–H pair stays on oxygen, which becomes a neutral hydroxyl oxygen.
1-Phenylethanol and hydronium
OH bonds to the benzylic carbon, and the terminal carbon is now a methyl group. The benzylic carbon is a new stereocenter formed from a flat cation, so the product is racemic. Hydronium is regenerated.
O que você vai praticar
Use benzylic resonance stabilization to place OH next to the ring during acid-catalyzed hydration of a vinylarene.
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