Química Orgânica 1 · Caminhos de substituição
Couple two alcohols into an ether
Condições: Ethanol heated with concentrated H₂SO₄ at about 140 °C. Follow the SN2 branch to diethyl ether; at higher temperature (about 180 °C) elimination to ethylene takes over and is not requested. H₂SO₄ is drawn as H–O–SO₃H, where SO₃H is an unchanged group; the hydrogen sulfate formed takes the last proton back, regenerating the acid catalyst.
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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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Protonate one ethanol
Protonation turns the OH of this ethanol into a water leaving group. Only a small fraction of the ethanol is protonated at any moment; the rest is available as nucleophile.
As setas curvas
- A lone pair on an ethanol oxygen takes a proton from sulfuric acid.
- The acid O–H pair stays on oxygen, giving hydrogen sulfate.
A second ethanol displaces water
The reacting carbon is primary, so there is no carbocation: the neutral alcohol oxygen pushes water out in one backside step. The attacking oxygen keeps its hydrogen during the attack and so becomes positively charged.
As setas curvas
- A lone pair on the neutral ethanol oxygen bonds to the carbon bearing OH₂⁺ from the side opposite oxygen.
- The C–O pair stays on the departing oxygen, which leaves as water.
Remove the proton from the new ether
A protonated ether is a strong acid (pKa approximately −3.5), of similar strength to sulfuric acid, so hydrogen sulfate or another ethanol molecule removes this proton readily. The acid catalyst is returned.
As setas curvas
- A lone pair on the negatively charged hydrogen sulfate oxygen takes the proton from the new ether oxygen.
- The ether O–H pair stays on oxygen, which becomes neutral.
Diethyl ether, water and regenerated acid
The attacking oxygen now joins two ethyl groups: diethyl ether. The other ethanol oxygen left as water, and hydrogen sulfate accepted the ether proton, so H₂SO₄ is regenerated and acts only as a catalyst. This route works for symmetrical ethers from primary alcohols; a mixture of two different alcohols would give three different ethers.
O que você vai praticar
Make a symmetrical ether from a primary alcohol by using one alcohol molecule as the nucleophile and a protonated one as the electrophile.
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