Química Orgânica 1 · Adição a alcenos e regioquímica
Hydrate an alkene without a free carbocation
Condições: Hex-1-ene with Hg(OAc)₂ in water/THF, followed by NaBH₄. Draw the oxymercuration stage only: ionization, mercurinium formation, water attack, and deprotonation. The NaBH₄ demercuration (replacement of HgOAc by H, a radical process) is summarized, not arrow-pushed. OAc = acetate; Ac is drawn as one group. Water is the nucleophile and a second water is the base.
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Ionize mercuric acetate
Mercury(II) acetate partly dissociates in the aqueous or alcoholic solvent. The resulting ⁺HgOAc is the electrophile that the alkene attacks.
As setas curvas
- The Hg–O pair leaves with the acetate oxygen, giving acetate and the ⁺HgOAc cation.
Form the mercurinium ion
The alkene attacks mercury to give a three-membered mercurinium ion instead of an open carbocation, much as Br₂ gives a bromonium ion. Without a free carbocation there is no carbocation rearrangement. The bond between mercury and the more substituted carbon is longer and weaker because that carbon can better carry positive charge.
As setas curvas
- The alkene π pair supplies one of the two carbon–mercury bonds.
- Mercury’s drawn electron pair supplies the other bond; exchanging the two carbon assignments gives the same bridge.
Water opens the bridge
The solvent oxygen bonds to the more substituted carbon, which carries more of the positive charge in the bridged ion. Attack from the side opposite mercury makes the addition anti; that relationship is lost later in demercuration.
Nota: Use a solvent oxygen as the nucleophile.
As setas curvas
- A water oxygen lone pair bonds to the more substituted carbon from the face opposite mercury.
- The breaking C–Hg bond pair returns to mercury, which stays bonded to the terminal carbon.
Deprotonate the oxonium ion
The solvent removes the proton, leaving a neutral organomercury alcohol.
As setas curvas
- A lone pair on a second water molecule accepts a proton from the positive oxygen.
- The O–H pair stays on the positive oxygen, which becomes neutral.
Hydroxy organomercurial, then hexan-2-ol
Oxygen bonds to the more substituted alkene carbon and HgOAc to the terminal carbon. In the separate NaBH₄ step (not drawn), H replaces HgOAc, giving hexan-2-ol: net Markovnikov addition of H and OH with no rearrangement. The alcohol-bearing carbon becomes a stereocenter, and the product is racemic.
O que você vai praticar
Follow oxymercuration through a bridged mercurinium ion and choose the carbon that water opens.
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