Química Orgânica 1 · Reações radicalares e seletividade
(1-Bromoethyl)benzene by benzylic bromination
Condições: Br₂, hν
Desenhe este problema você mesmo
Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 5 passos 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.
As setas 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 benzylic hydrogen
The bromine radical removes a hydrogen from the CH2 attached to the ring. The benzylic C–H is unusually weak (about 357 kJ/mol) because the radical formed is delocalized into the aromatic π system, onto the ortho and para carbons.
As setas curvas
- The unpaired electron on the bromine radical supplies one electron of the new bond to hydrogen.
- One electron of the benzylic C–H bond pairs with it to complete the new bond to hydrogen.
- The other benzylic C–H electron stays behind as the unpaired electron of the new radical.
Why the benzylic position?
Benzylic and allylic radicals are resonance-stabilized, so their C–H bonds are the weakest in the molecule and react first.
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.
As setas curvas
- The unpaired electron on benzylic 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.
(1-Bromoethyl)benzene and HBr
Bromine sits on the benzylic carbon. The planar benzylic radical is attacked from either face, so (1-bromoethyl)benzene forms as a racemate.
O que você vai praticar
Draw the radical chain that brominates ethylbenzene at the benzylic CH2, giving (1-bromoethyl)benzene (Br2 and light on ethylbenzene).
Você acabou de desenhar cada seta deste problema.
Cada um dos 5349 problemas de Orgânica 1 e 2 funciona assim: feedback em cada seta, dicas quando você travar e um modo prova sem dicas. Grátis com uma conta, sem cartão.
Travou na sua própria lição?
Tire uma foto: o primeiro scan é grátis, sem cadastro
Você escaneia toda semana? Scan Standard: por cerca de problemas