Química Orgánica 2 · Enoles, enolatos y reacciones en alfa
Acid-catalysed α-chlorination of cyclohexanone to 2-chlorocyclohexanone
Condiciones: Cl2, AcOH
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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 6 pasos escritos
Acetic acid protonates the carbonyl
Acid-catalysed halogenation goes through the enol. First the carbonyl oxygen of cyclohexanone takes a proton from acetic acid, the strongest acid present, making the α-hydrogens more acidic.
Las flechas curvas
- A carbonyl oxygen lone pair takes the acidic O–H proton.
- The O–H pair stays on the acetate oxygen.
Acetate removes an α-hydrogen to give the enol
Acetate removes a proton from the α-carbon; the C–H pair forms the C=C bond and the C=O⁺ π pair returns to oxygen, giving the neutral enol. This enolization is the slow step.
Las flechas curvas
- The acetate oxygen takes the α-proton.
- The C–H pair forms the enol C=C bond.
- The C=O⁺ π pair returns to oxygen.
The enol attacks Cl2
The enol is a nucleophilic alkene: the O–H lone pair pushes, the C=C π pair bonds to one Cl atom, and Cl⁻ leaves from the other. The positive charge ends up on oxygen as a protonated carbonyl.
Las flechas curvas
- An oxygen lone pair forms the C=O⁺ π bond.
- The C=C π pair forms the new C–Cl bond.
- The Cl–Cl pair leaves as Cl⁻.
Lose the O–H proton
Acetic acid, a neutral oxygen base, removes the proton from the protonated carbonyl, giving the neutral α-halo ketone. The net by-product is HCl (here drawn as protonated acetic acid and Cl⁻).
Las flechas curvas
- The acetic acid carbonyl oxygen takes the O–H proton.
- The O–H pair stays on the ketone oxygen.
What is the rate-determining step?
Kinetic studies show the rate is independent of [Cl2] and the same for Cl2, Br2 and I2.
2-Chlorocyclohexanone
One Cl has replaced one α-hydrogen. In acid the reaction stops after monohalogenation, because the electron-withdrawing halogen makes the carbonyl oxygen less basic and slows the next enolization.
Qué vas a practicar
Draw acid-catalysed enol formation from cyclohexanone, attack of the enol on Cl2, and loss of the O–H proton to give 2-chlorocyclohexanone.
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