Química Orgánica 1 · Resonancia y distribución electrónica
Benzyl cation: four contributors
Condiciones: No reagent: draw the next resonance contributor of the same species.
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 5 pasos escritos
Ring π bond fills the empty orbital
The CH2 carbon has only six electrons and an empty p orbital aligned with the ring π system. The adjacent C1=C2 π pair can swing toward it, giving an exocyclic C=C and moving the positive charge onto the ortho carbon C2.
Las flechas curvas
- The C1=C2 π pair moves to C1–C7, giving the CH2 carbon its octet; C2 is left with six electrons and the + charge.
Pass the charge to the para carbon
The next ring π bond, C3=C4, sits next to the new cation at C2, so it can shift toward C2. The positive charge moves two carbons further, to the para position C4.
Las flechas curvas
- The C3=C4 π pair forms the C2–C3 π bond, completing C2's octet and leaving the + on C4.
Pass the charge to the other ortho carbon
Repeating the same move with the C5=C6 π bond places the charge on C6, the second ortho carbon. Moving it once more would return it to C7, the starting drawing.
Las flechas curvas
- The C5=C6 π pair forms the C4–C5 π bond, so the positive charge now sits on C6.
Where is the charge?
The positive charge alternates: benzylic carbon, ortho, para, ortho. The meta carbons never carry it.
Third ring contributor
With the charge on the second ortho carbon, the benzyl cation has four contributors in total: one with the charge on CH2 and three ring contributors. This delocalization makes benzylic cations about as stable as tertiary ones, which is why benzylic halides ionize readily in SN1 reactions.
Qué vas a practicar
Move ring π bonds toward the empty orbital of the benzyl cation to draw its three ring contributors, placing the positive charge on both ortho carbons and the para carbon.
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