Química Orgánica 2 · Enlaces carbono–carbono con organometálicos
Propiophenone from benzonitrile and ethylmagnesium bromide
Condiciones: EtMgBr, Et2O; then H3O+, heat
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 10 pasos escritos
The ethyl carbon adds to the nitrile carbon
The nucleophilic carbon of the C–Mg bond attacks the electrophilic nitrile carbon. One π pair of C≡N moves onto nitrogen, giving an imine anion paired with MgBr+.
Las flechas curvas
- The C–Mg bond pair forms the new C–C bond to the nitrile carbon.
- One π pair of the triple bond moves onto nitrogen.
Why only one addition?
An ester reacts twice with excess Grignard reagent; the nitrile does not.
Hydronium protonates the imine anion
Aqueous acid is added. The basic imine nitrogen takes a proton from hydronium, giving the neutral imine.
Las flechas curvas
- A nitrogen lone pair takes a proton from hydronium.
- The O–H pair stays on water.
Protonation activates the imine
A second hydronium protonates the imine nitrogen, giving an iminium ion whose carbon is strongly electrophilic.
Las flechas curvas
- The imine nitrogen lone pair takes a proton from hydronium.
- The O–H pair stays on water.
Water adds to the iminium carbon
A water molecule attacks the iminium carbon and the C=N π pair moves onto nitrogen, giving a tetrahedral intermediate with a protonated oxygen.
Las flechas curvas
- A water lone pair forms the new C–O bond.
- The C=N π pair moves onto nitrogen, neutralising it.
Water removes a proton from the oxonium ion
A solvent water molecule deprotonates the new oxonium oxygen, giving a neutral carbinolamine.
Las flechas curvas
- A water lone pair takes a proton from the oxonium oxygen.
- The O–H pair stays on the carbinolamine oxygen.
Hydronium protonates the nitrogen
Protonating the amino group turns it into a neutral ammonia leaving group.
Las flechas curvas
- The nitrogen lone pair takes a proton from hydronium.
- The O–H pair stays on water.
The oxygen lone pair expels ammonia
A lone pair on the hydroxyl oxygen forms the C=O π bond as the C–N bond breaks, releasing ammonia and leaving a protonated ketone.
Las flechas curvas
- An oxygen lone pair forms the C=O π bond.
- The C–N pair leaves with nitrogen as ammonia.
Water deprotonates the protonated ketone
Water removes the proton from the carbonyl oxygen, giving neutral propiophenone and regenerating hydronium.
Las flechas curvas
- A water lone pair takes the proton from the carbonyl oxygen.
- The O–H pair returns to oxygen as a lone pair.
Propiophenone
The nitrile carbon has become the ketone carbon and now carries the new ethyl group: propiophenone (1-phenylpropan-1-one). Ammonia is released and is protonated to ammonium in the acidic solution.
Qué vas a practicar
Draw the single addition of ethylmagnesium bromide to benzonitrile and the acidic hydrolysis of the imine to propiophenone.
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