Química Orgânica 1 · Reações de alcenos e alcinos: revisão para a prova
Oxymercuration of 3-methylbut-1-ene
Condições: 3-Methylbut-1-ene, Hg(OAc)2, H2O/THF; then NaBH4, NaOH
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Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
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Mercury(II) acetate loses acetate
One Hg–O bond breaks, giving acetate and the electrophilic acetoxymercury cation, AcOHg⁺. Mercury is Hg(II) here and throughout the mechanism; the + drawn on mercury is the Lewis formal charge of this cation, not a change in oxidation state.
As setas curvas
- The Hg–O pair leaves with oxygen as acetate.
The alkene bridges to mercury
The π pair bonds to mercury at the CH2 carbon while mercury bonds back to the other alkene carbon, giving a three-membered mercurinium ion. This bridge is the conventional drawing of the alkene π electrons shared with an Hg(II) cation: mercury stays Hg(II), and the + on mercury is the Lewis formal charge of the ion.
As setas curvas
- The π pair forms the C–Hg bond at the CH2 carbon.
- Mercury bonds to the second alkene carbon, closing the bridge.
Why no rearrangement?
The bridged ion shares the positive charge between mercury and the two carbons.
Water opens the mercurinium ion
Water attacks the more substituted carbon of the bridge, which carries more of the positive charge, from the face opposite mercury. The C–Hg pair returns to mercury, which is left with no formal charge and is still Hg(II), bonded to carbon and to acetate.
As setas curvas
- A water lone pair forms the C–O bond at the secondary carbon.
- The C–Hg bond to that carbon breaks; the pair returns to mercury.
Water removes the proton
A second water takes a proton from the oxonium ion, giving the neutral hydroxyalkylmercury compound.
As setas curvas
- A water lone pair takes the proton.
- The O–H pair stays on the alcohol oxygen.
Hydroxyalkylmercury acetate
OH is on the CH carbon and HgOAc on the CH2 carbon. NaBH4 then replaces HgOAc with H, giving 3-methylbutan-2-ol: Markovnikov hydration with the original skeleton. The new stereocentre forms as a racemate.
O que você vai praticar
Draw ionisation of mercury(II) acetate, formation of the bridged mercurinium ion, opening by water at the more substituted carbon, and deprotonation, and explain why oxymercuration gives 3-methylbutan-2-ol without the rearrangement seen in acid-catalysed hydration.
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