Química Orgânica 1 · Transformações de alcinos
Hydrate a terminal alkyne with mercury(II)
Condições: Hex-1-yne with catalytic HgSO₄ in aqueous H₂SO₄. Hg²⁺ is one opaque ion (sulfate counterions omitted). Hydronium represents the acid; water acts as both nucleophile and base. Use the common open vinyl-cation depiction for mercury addition. Stop at the neutral enol; its tautomerization to hexan-2-one is a separate exercise.
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Add mercury to the terminal carbon
Hg²⁺ is the electrophile. Bonding it to the terminal carbon leaves the positive charge on the more substituted carbon (Markovnikov orientation). Mercury keeps one positive charge.
As setas curvas
- One alkyne π pair forms a bond from the terminal carbon to mercury.
Capture the cation with water
Oxygen, not hydrogen, is added to the more substituted carbon.
As setas curvas
- A water lone pair bonds to the positively charged carbon.
Deprotonate the oxonium ion
The product is an enol that still carries mercury at the terminal carbon. The proton goes to solvent as hydronium.
As setas curvas
- Water takes a proton from the positively charged oxygen.
- The O–H pair stays on the oxonium oxygen, which becomes a neutral OH.
Protonate the carbon bearing mercury
This is the start of an acid-catalyzed tautomerization: the mercury-bearing enol carbon is protonated and the oxygen lone pair stabilizes the other carbon as a protonated carbonyl.
As setas curvas
- The enol π pair takes a proton from hydronium, forming a C–H bond at the mercury-bearing carbon.
- The hydronium O–H pair stays on oxygen, leaving neutral water.
- A hydroxyl oxygen lone pair forms a C=O π bond as the neighboring π pair forms the C–H bond, so no carbocation forms.
Release Hg²⁺
Mercury leaves with no electrons from the C–Hg bond, so it departs as Hg²⁺ and the catalyst is regenerated.
As setas curvas
- The C–Hg pair forms the new C=C π bond, releasing Hg²⁺.
- The C=O π pair returns to oxygen as a lone pair.
Hex-1-en-2-ol, regenerated Hg²⁺ and hydronium
The neutral enol CH₂=C(OH)(CH₂)₃CH₃ carries OH at the more substituted alkene carbon (Markovnikov). A proton replaced mercury at the terminal carbon. Hg²⁺ is free again, and newly formed hydronium replaces the hydronium used, so both catalysts are regenerated. The enol then tautomerizes to hexan-2-one, a methyl ketone.
O que você vai praticar
Carry Hg²⁺-catalyzed Markovnikov hydration of a terminal alkyne from mercury addition through water capture and removal of mercury to the neutral enol.
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