Química Orgánica 2 · Sustitución electrofílica aromática
1-Bromonaphthalene from naphthalene and Br₂
Condiciones: Br2, CCl4
Dibuja este problema tú mismo
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
C1 of naphthalene attacks bromine
Naphthalene is more nucleophilic than benzene (it loses less aromatic stabilization on attack), so Br₂ reacts without a Lewis acid. Attack at C1 is faster than at C2.
Las flechas curvas
- The C1=C2 π pair forms the new C–Br bond at C1; C2 becomes cationic.
- The Br–Br pair moves onto the outer bromine, which leaves as bromide.
Delocalize the charge to C4
Moving the C3=C4 π pair puts the charge on C4. In both this contributor and the first one, the second ring keeps an intact benzene sextet — the key reason C1 attack is favored.
Las flechas curvas
- The C3=C4 π pair shifts toward C2, moving the positive charge to C4.
Why C1?
Attack at C1 gives two arenium contributors that keep a whole benzene ring; attack at C2 gives only one.
Return the charge to C2
Redraw the C2 cation contributor, the one with the charge beside sp³ C1. Its C1–H electrons re-form the C1=C2 π bond when the proton is removed.
Las flechas curvas
- The C2=C3 π pair shifts back toward C4, leaving the positive charge on C2 next to C1.
Bromide removes the C1 proton
Bromide removes the proton from sp³ C1 and the C–H electrons form the C1=C2 π bond, restoring the aromatic naphthalene.
Las flechas curvas
- A bromide lone pair takes the C1 proton, forming HBr.
- The C–H bond electrons form the ring π bond to the neighboring cationic carbon, restoring aromaticity.
1-Bromonaphthalene and HBr
Bromine has replaced the C1 hydrogen to give 1-bromonaphthalene; both rings are aromatic again, and HBr is the by-product.
Qué vas a practicar
Draw bromination of naphthalene at C1, using the arenium contributors to explain why C1 beats C2.
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