Química Orgânica 1 · Reações radicalares e seletividade
Brominate cyclohexene at the allylic position
Condições: Cyclohexene with N-bromosuccinimide (NBS) and light in CCl₄. Initiation has supplied Br•. NBS converts the HBr made in each cycle into Br₂ at low concentration; that Br₂ is drawn, while NBS and succinimide are not. Follow bromine capture after drawing the other allylic resonance contributor.
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Abstract the allylic hydrogen
Allylic C–H bonds are the weakest C–H bonds in cyclohexene because the radical formed is resonance-stabilized: weaker than the other ring CH₂ bonds and far weaker than vinylic C–H bonds. There are four equivalent allylic hydrogens on the two carbons next to the double bond.
As setas curvas
- The bromine radical supplies one electron of the new H–Br bond.
- One allylic C–H electron completes H–Br.
- The other C–H electron stays on the allylic carbon as the unpaired electron.
Why use NBS instead of plenty of Br₂?
The chain needs only a trace of Br₂. NBS reacts with HBr to replenish Br₂ one molecule at a time, so its concentration stays very low throughout.
Show the other allylic contributor
The allylic radical is one delocalized species with its odd electron shared by the two ends of the three-carbon allylic group. Because cyclohexene is symmetric, both contributors have identical connectivity.
As setas curvas
- The unpaired electron on the allylic carbon pairs into a new π bond to the neighboring carbon.
- One electron of the adjacent π bond completes the new π bond.
- The other π electron remains on the far end of the original double bond as the unpaired electron.
Trap the allylic radical
Bonding at the radical carbon in this contributor gives the same constitution as bonding at the radical carbon in the first contributor: in each case Br sits on a carbon next to the remaining C=C.
As setas curvas
- The unpaired electron on the radical carbon supplies one electron of the new C–Br bond.
- One Br–Br bonding electron completes the C–Br bond.
- The other Br–Br electron regenerates the bromine radical chain carrier.
3-Bromocyclohexene, HBr and Br radical
Bromine now bonds to a carbon next to the double bond: 3-bromocyclohexene, the product from either radical terminus. The new stereocenter forms from a planar radical, so the product is racemic. HBr goes on to regenerate Br₂ from NBS, and a new Br• continues the chain.
O que você vai praticar
Explain how NBS keeps Br₂ scarce so allylic substitution outcompetes addition, and show that either terminus of the symmetric cyclohexenyl radical gives the same 3-bromocyclohexene.
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