Química Orgánica 2 · Efectos de los sustituyentes y orientación
Ortho bromination of anisole through an oxonium contributor
Condiciones: Br2, 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.
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The ring attacks bromine ortho to methoxy
Anisole is so electron-rich that Br2 in acetic acid reacts without a Lewis acid. The ipso–ortho π pair forms the C–Br bond at the ortho carbon, putting the positive charge directly on the carbon bearing OCH3.
Las flechas curvas
- The ring π pair between the ipso and ortho carbons forms the new C–Br bond at the ortho carbon.
- The Br–Br bond pair leaves with the far bromine atom as Br⁻ (the Lewis acid in the reagents only helps polarize this bond).
Oxygen donates into the cationic carbon
Because the cation sits next to the methoxy oxygen, an oxygen lone pair can form a C=O+ π bond. This oxonium contributor is the most important one: all atoms have full octets. Meta attack can never produce it.
Las flechas curvas
- A methoxy oxygen lone pair forms a π bond to the cationic ipso carbon, giving an oxonium contributor in which every atom has an octet.
The key σ-complex contributor
The oxonium contributor has every atom with an octet.
Bromide removes the ring proton
Bromide removes the hydrogen on the sp3 carbon. The C–H pair forms a ring π bond and the C=O+ π pair returns to oxygen, restoring the aromatic ring.
Las flechas curvas
- A bromide lone pair takes the hydrogen on the sp3 ring carbon.
- The C–H bond pair becomes a ring π bond, starting to restore the aromatic sextet.
- The π pair to the positive heteroatom moves back onto it as a lone pair, neutralizing it.
2-bromoanisole
Ortho attack gives 2-bromoanisole and HBr. In practice para attack (4-bromoanisole) is the major product for steric reasons, but both are stabilized by the same oxygen donation.
Qué vas a practicar
Draw ortho bromination of anisole, including the oxonium contributor of the σ complex.
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