Química Orgánica 1 · Reacciones radicalarias y selectividad
Benzyl bromide from toluene
Condiciones: Br₂, hν
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
Initiation: split Br₂ with light
Light breaks the weak Br–Br bond homolytically: each bromine atom keeps one of the two bonding electrons, giving two neutral bromine radicals and no ions. A second Br₂ molecule waits for the propagation steps.
Las flechas curvas
- One electron of the Br–Br bond stays on the first atom.
- The other electron of the Br–Br bond stays on the second atom, so the bond splits evenly into two radicals.
Propagation: remove a methyl hydrogen
The bromine radical abstracts one of the three methyl hydrogens. The benzyl radical that forms is resonance-stabilized by the ring, so this C–H bond is far weaker than the aromatic C–H bonds.
Las flechas curvas
- The unpaired electron on the bromine radical supplies one electron of the new bond to hydrogen.
- One electron of the methyl C–H bond pairs with it to complete the new bond to hydrogen.
- The other methyl C–H electron stays behind as the unpaired electron of the new radical.
Light versus FeBr3
Conditions decide the site: radical conditions (light, heat, peroxide) brominate the side chain; a Lewis acid such as FeBr3 brominates the ring by electrophilic aromatic substitution.
Propagation: take Br from a second Br₂
The carbon radical attacks Br₂, forming the C–Br bond and releasing a new bromine radical. That radical starts the next cycle, so one initiation event drives many cycles.
Las flechas curvas
- The unpaired electron on methyl C supplies one electron of the new bond to Br.
- One electron of the Br–Br bond completes that new bond.
- The other Br–Br electron stays on the far Br, regenerating the chain-carrying radical.
Benzyl bromide and HBr
The benzyl radical took bromine from Br2, giving benzyl bromide (bromomethyl)benzene and a new bromine radical.
Qué vas a practicar
Draw Br–Br homolysis and the two propagation steps that convert toluene into benzyl bromide with Br2 and light.
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