Química Orgânica 1 · Adição a alcenos e regioquímica
Oxymercuration–demercuration of 1-methylcyclohexene
Condições: Hg(OAc)2, H2O/THF; then NaBH4, NaOH
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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 7 passos escritos
Ionize mercuric acetate
Mercury(II) acetate loses one acetate in the polar solvent, giving the electrophile AcOHg⁺.
As setas curvas
- The Hg–O pair leaves with acetate.
Form the mercurinium ion
The alkene attacks mercury and mercury bonds back to the second carbon, giving a bridged three-membered mercurinium ion instead of an open carbocation, so no rearrangement can occur.
As setas curvas
- The π pair forms the bond from the less substituted carbon to mercury.
- A mercury electron pair bonds to the more substituted carbon, closing the bridge.
Which carbon does water attack?
The bridge is unsymmetrical.
Water opens the bridge
The water oxygen bonds to the more substituted carbon, which carries more of the positive charge in the unsymmetrical bridge. It attacks from the face opposite mercury.
As setas curvas
- A water oxygen lone pair bonds to the more substituted carbon.
- The C–Hg bond to that carbon breaks; mercury keeps the pair and stays on the other carbon.
Deprotonate the oxonium ion
A second water molecule removes the O–H proton, giving the neutral organomercury compound.
As setas curvas
- A solvent lone pair takes the proton.
- The O–H pair stays on oxygen.
Demercuration with NaBH4
Borohydride replaces mercury by hydrogen and mercury is reduced to the metal, releasing acetate. (This step really runs through radicals; it is summarized here as one hydride transfer.)
As setas curvas
- The B–H pair delivers hydride to the carbon that held mercury.
- The C–Hg pair goes to mercury.
- The Hg–O pair leaves with acetate, and mercury ends as Hg(0).
1-methylcyclohexan-1-ol
OH sits on the tertiary ring carbon and H on its neighbour: the Markovnikov alcohol, the same one aqueous acid gives, but without a free carbocation.
O que você vai praticar
Draw oxymercuration of 1-methylcyclohexene through the mercurinium ion, water attack at the methyl-bearing carbon, deprotonation, and demercuration to 1-methylcyclohexan-1-ol.
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