Química Orgânica 1 · Eliminação e caminhos competitivos
Dehydrate a tertiary alcohol with phosphoric acid
Condições: 2-Methylpropan-2-ol (tert-butyl alcohol) heated with concentrated phosphoric acid (H₃PO₄); the gaseous alkene escapes as it forms. Follow the E1 dehydration to 2-methylpropene. H₃PO₄ is drawn in full, and the dihydrogen phosphate formed in the first step is the base in the last step. The three methyl groups are equivalent; remove the separately drawn methyl hydrogen.
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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.
Ler todos os 4 passos escritos
Protonate the hydroxyl group
Hydroxide is a very poor leaving group, so the C–O bond cannot break yet. Protonation turns –OH into –OH₂⁺, which can leave as neutral water. Only a small fraction of the alcohol is protonated at any moment, but that fraction is all the pathway needs.
As setas curvas
- A lone pair on the alcohol oxygen bonds to the acidic H of phosphoric acid.
- The O–H pair stays on the phosphate oxygen, which becomes dihydrogen phosphate, H₂PO₄⁻.
Lose water
This slow, rate-determining step makes a tertiary carbocation. Three alkyl groups stabilize the positive charge, which is why tertiary alcohols dehydrate under much milder conditions than secondary or primary ones. Only one bond breaks here.
As setas curvas
- The C2–O pair leaves with oxygen as a neutral water molecule, leaving C2 with an empty p orbital.
Remove a β proton
Dihydrogen phosphate is a weak base, but a C–H bond next to a carbocation is very acidic, so a weak base is enough. This proton transfer regenerates H₃PO₄.
As setas curvas
- A lone pair on the negatively charged phosphate oxygen bonds to the separately drawn methyl hydrogen.
- The methyl C–H pair forms the C=C π bond and fills the empty p orbital of the carbocation.
2-Methylpropene, water and phosphoric acid
C1 and C2 now share a π bond: 2-methylpropene, (CH₃)₂C=CH₂. The alcohol oxygen left as water, and the phosphate oxygen that lent its proton got one back, so H₃PO₄ is a catalyst. Dihydrogen phosphate is a poor nucleophile, so it does not trap the cation, and escape of the gaseous alkene pulls the equilibrium toward dehydration.
O que você vai praticar
Carry a tertiary alcohol through protonation, loss of water and β-deprotonation by dihydrogen phosphate, regenerating the H₃PO₄ catalyst.
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