Química Orgânica 2 · Rotas de síntese: reagentes, grupos protetores e planejamento em várias etapas
tert-Butyl benzoate from benzoic acid and isobutylene
Condições: Benzoic acid, 2-methylpropene (isobutylene, excess), conc. H₂SO₄ (0.05 equiv), CH₂Cl₂, sealed flask, 25 °C
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Protonation gives the tert-butyl cation
The alkene π pair bonds to the acidic proton of H₂SO₄ at the CH₂ end, so the positive charge lands on the carbon with three methyl groups: the tertiary tert-butyl cation.
As setas curvas
- The π pair forms the new C–H bond at the CH₂ carbon.
- The O–H pair stays on oxygen, giving hydrogensulfate.
Why not an SN2 route?
A tert-butyl group cannot be attached by SN2 because the tertiary carbon is too hindered; it must be delivered as a cation.
The carbonyl oxygen captures the cation
A lone pair on the carbonyl oxygen of benzoic acid bonds to the tert-butyl cation. The carbonyl oxygen is the more nucleophilic oxygen of a carboxylic acid because the cation it forms is resonance-stabilised: the positive charge is shared with the OH oxygen. That cation is simply tert-butyl benzoate protonated on its C=O oxygen, so removing the O–H proton in the next step gives the ester directly.
As setas curvas
- The carbonyl oxygen lone pair forms the O–C bond to the cation.
Hydrogensulfate removes the O–H proton
Hydrogensulfate takes the proton from the OH group. That O–H pair becomes the new C=O π bond and the old C=O π pair moves onto the oxygen bonded to tert-butyl, giving the neutral ester and regenerating H₂SO₄.
As setas curvas
- The hydrogensulfate oxygen lone pair takes the proton.
- The O–H pair forms the new C=O π bond.
- The C=O⁺ π pair moves onto the tert-butyl-bearing oxygen.
tert-Butyl benzoate
The carboxylic acid is protected as its tert-butyl ester and the acid catalyst is regenerated. The bulky tert-butyl group shields the carbonyl from nucleophiles and bases; trifluoroacetic acid later removes it as isobutylene, returning the acid.
O que você vai praticar
Draw the acid-catalysed addition of benzoic acid to 2-methylpropene through the tert-butyl cation, the way a carboxylic acid is protected as its tert-butyl ester.
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