Química Orgânica 1 · Transformações de alcinos
Mercury-catalysed hydration of hex-1-yne
Condições: H₂O, H₂SO₄, HgSO₄ (cat.)
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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.
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Mercury(II) adds to the triple bond
Hg²⁺ is the electrophile. A π pair bonds to mercury at the carbon that leaves the positive charge on the better-stabilized carbon (Markovnikov orientation). Mercury keeps one positive charge.
As setas curvas
- One alkyne π pair forms the C–Hg bond.
Water captures the vinyl cation
A water lone pair bonds to the cationic carbon, so oxygen ends up on the more substituted carbon.
As setas curvas
- A water lone pair bonds to the positive carbon.
Deprotonate the oxonium ion
Solvent water removes a proton from the oxonium ion, giving a neutral mercury-substituted enol.
As setas curvas
- A water lone pair takes the O–H proton.
- The O–H pair stays on oxygen, which becomes neutral.
Protonate the mercury-bearing carbon
Hydronium protonates the enol carbon that carries mercury; the oxygen lone pair forms a C=O π bond as it happens, so no free carbocation forms.
As setas curvas
- The C=C π pair takes a proton from hydronium.
- The O–H pair stays on water.
- An oxygen lone pair forms the C=O π bond.
Mercury(II) leaves
The C–Hg pair becomes a new C=C π bond and the C=O π pair returns to oxygen, releasing Hg²⁺ (the catalyst) and an ordinary enol.
As setas curvas
- The C–Hg bond pair forms the C=C π bond; mercury leaves as Hg²⁺.
- The C=O π pair moves back onto oxygen.
Why the enol does not survive
Keto–enol tautomerism.
Protonate the enol at carbon
Acid-catalysed tautomerization begins: the enol C=C π pair takes a proton from hydronium at carbon while the oxygen lone pair forms C=O.
As setas curvas
- The enol π pair takes a proton from hydronium.
- The O–H pair stays on water.
- An oxygen lone pair forms the C=O π bond.
Deprotonate the carbonyl oxygen
Water removes the O–H proton of the protonated ketone, giving the neutral carbonyl and regenerating hydronium.
As setas curvas
- A water lone pair takes the proton on oxygen.
- The O–H pair stays on oxygen.
The ketone
The keto form is far more stable than the enol (C=O is stronger than C=C), so the product is the ketone. Hg²⁺ and hydronium are regenerated: both are catalysts.
O que você vai praticar
Draw mercury(II)-catalysed Markovnikov hydration of hex-1-yne through the enol and its acid-catalysed tautomerization to hexan-2-one.
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