Química Orgánica 2 · Efectos de los sustituyentes y orientación
Bromination of thiophene at C2
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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Thiophene attacks bromine at C2
Thiophene is more reactive than benzene because sulfur donates into the ring, so Br2 in acetic acid suffices. The C2=C3 π pair forms the C–Br bond at C2 and leaves a cation at C3.
Las flechas curvas
- The C2=C3 π pair forms the new C2–Br bond.
- The Br–Br bond pair leaves with the far bromine atom as Br⁻ (the Lewis acid in the reagents only helps polarize this bond).
Delocalize the charge onto sulfur
Attack at C2 gives three contributors: the cation at C3, at C5, and a sulfonium contributor with C=S+ in which all atoms have octets. Attack at C3 gives only two, so C2 attack is faster.
Las flechas curvas
- The remaining C=C π pair shifts toward the cation, putting the charge on the carbon next to sulfur.
- A sulfur lone pair forms a C=S+ π bond, so every atom has an octet.
Why C2 and not C3?
Count the σ-complex contributors.
Bromide removes the C2 proton
Bromide removes the hydrogen on sp3 C2; the electrons flow back through the ring to sulfur and restore the aromatic thiophene.
Las flechas curvas
- A bromide lone pair takes the hydrogen on C2.
- The C–H bond pair becomes a ring π bond, starting to restore the aromatic sextet.
- A ring π pair shifts one bond along to make room for the new π bond.
- The π pair to the positive heteroatom moves back onto it as a lone pair, neutralizing it.
2-bromothiophene
Substitution at C2 gives 2-bromothiophene and HBr. The same C2 (α) preference holds for pyrrole and furan.
Qué vas a practicar
Draw bromination of thiophene at C2, including the sulfonium contributor of the σ complex.
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