Química Orgánica 2 · Sustitución electrofílica aromática
2-bromotoluene from toluene with Br₂ and FeBr₃
Condiciones: Br2, FeBr3
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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
The ring attacks bromine
FeBr₃ (not drawn) polarizes Br₂. The ring carbon ortho to the methyl group attacks bromine; the positive charge lands at once on the methyl-bearing carbon.
Las flechas curvas
- The ring π pair at the carbon bearing the drawn hydrogen forms the new C–Br bond, leaving a positive charge on the neighboring carbon.
- The Br–Br bond pair moves onto the far bromine, which leaves as bromide.
Delocalize the charge to the para carbon
The charge moves to the carbon para to the new C–Br bond. Together with the first contributor (charge on the methyl-bearing carbon) this shows the delocalized arenium ion.
Las flechas curvas
- The neighboring ring π pair shifts toward the cationic carbon, moving the positive charge to the carbon para to the new bond.
Return the charge beside the sp³ carbon
The arenium ion is one hybrid; redraw the contributor with the positive charge back on the carbon next to the sp³ C–H. That contributor shows which π bond the C–H electrons will re-form in the deprotonation.
Las flechas curvas
- The π pair shifts back toward the para carbon, leaving the positive charge on the ortho carbon next to the sp³ C–H.
Ortho vs para
Both ortho and para attack are favored electronically; para usually wins because the ortho positions sit beside the methyl group.
Lose the proton and restore aromaticity
Bromide (released as FeBr₄⁻, regenerating FeBr₃) removes the proton from the sp³ carbon that gained bromine, and the C–H electrons restore the aromatic ring.
Las flechas curvas
- A lone pair on the bromide ion takes the ring proton, forming HBr.
- The C–H bond electrons form the ring π bond to the neighboring cationic carbon, restoring aromaticity.
2-bromotoluene and HBr
Bromine has replaced the hydrogen ortho to the methyl group, giving 2-bromotoluene; 4-bromotoluene forms alongside it and is usually the larger fraction, and almost no meta product forms.
Qué vas a practicar
Draw bromination of toluene at the ortho position, showing the arenium contributor with the charge on the methyl-bearing carbon.
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