Química Orgánica 1 · Adición a alquenos y regioquímica
Alkoxymercuration–demercuration of propene
Condiciones: Hg(OAc)2, CH3OH; then NaBH4, NaOH
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 7 pasos escritos
Ionize mercuric acetate
Mercury(II) acetate loses one acetate in the polar solvent, giving the electrophile AcOHg⁺.
Las flechas 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.
Las flechas 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.
Why no rearrangement worry?
Compare with acid-catalysed addition of methanol.
Methanol opens the bridge
The methanol 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.
Las flechas curvas
- A methanol 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 methanol molecule removes the O–H proton, giving the neutral organomercury compound.
Las flechas 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.)
Las flechas 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).
2-methoxypropane
The methoxy group is on C2 and H on C1: 2-methoxypropane, the Markovnikov ether, formed without a free carbocation.
Qué vas a practicar
Draw alkoxymercuration of propene: mercurinium ion, methanol attack at C2, deprotonation, and demercuration to 2-methoxypropane.
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