Química Orgânica 2 · Adições à carbonila: revisão para a prova
Two equivalents of methylmagnesium bromide add to ethyl benzoate
Condições: MeMgBr (2.5 equiv), dry Et2O, 0 °C to reflux; then dilute aqueous HCl (H3O+) workup
Desenhe este problema você mesmo
Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 6 passos escritos
The first methyl group adds to the ester carbonyl
The nucleophilic methyl carbon of the Grignard reagent attacks the ester carbonyl carbon, and the C=O π pair moves onto oxygen, giving a tetrahedral alkoxide. Magnesium is left as the bromomagnesium cation BrMg⁺, drawn with a +1 Lewis formal charge on magnesium; its oxidation state stays +2.
As setas curvas
- The C–Mg pair forms the new C–C bond to the carbonyl carbon.
- The C=O π pair moves onto oxygen.
Ethoxide leaves and a ketone forms
The tetrahedral intermediate carries a leaving group. The oxygen lone pair re-forms C=O and pushes out ethoxide, giving acetophenone in the flask while Grignard reagent remains.
As setas curvas
- The oxygen lone pair re-forms the C=O π bond.
- The C–O bond breaks and ethoxide leaves with the pair.
Can the ketone be isolated?
The ketone is formed in the presence of unreacted Grignard reagent and is the more reactive electrophile, so it is consumed as fast as it forms.
A second methyl group adds to the ketone
A second molecule of methylmagnesium bromide attacks the ketone carbonyl carbon, and the C=O π pair moves onto oxygen, giving a tertiary alkoxide. This alkoxide has no leaving group, so it waits for the workup.
As setas curvas
- The C–Mg pair forms the second new C–C bond.
- The C=O π pair moves onto oxygen.
Hydronium protonates the tertiary alkoxide
Dilute aqueous acid is added after the reaction. The alkoxide takes a proton from hydronium, giving the tertiary alcohol and water.
As setas curvas
- An alkoxide lone pair takes the proton from hydronium.
- The O–H pair stays on oxygen, leaving water.
2-Phenylpropan-2-ol
Both new methyl groups sit on the former ester carbonyl carbon; the ethoxy group has left. An ester plus excess Grignard reagent gives a tertiary alcohol with two identical groups from the reagent. The ethoxide is also protonated to ethanol by the aqueous acid.
O que você vai praticar
Draw the addition of methylmagnesium bromide to ethyl benzoate, the loss of ethoxide to give acetophenone, the second addition, and the acidic workup to 2-phenylpropan-2-ol.
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