Química Orgânica 1 · Reações radicalares e seletividade
3-Bromo-1-methylcyclohexene: allylic bromination with a regio mixture
Condições: NBS, hν, CCl₄
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Initiation: make a bromine radical
Light splits a trace of Br₂ (kept at low concentration by NBS, which converts HBr back into Br₂) into two bromine radicals. NBS and succinimide are not drawn; a second Br₂ molecule is drawn for the propagation.
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 an allylic hydrogen
1-Methylcyclohexene has three different allylic positions: C3, C6 and the methyl group. Removing a C3 hydrogen gives an allylic radical whose odd electron is shared between C3 (secondary) and C1 (tertiary, bearing the methyl group).
As setas curvas
- The unpaired electron on bromine supplies one electron of the new bond to hydrogen.
- One electron of the allylic C–H bond pairs with it to complete the new bond to hydrogen.
- The other allylic C–H electron stays behind as the unpaired electron of the new radical.
Why a mixture?
An unsymmetrical allylic radical has two reactive ends, and an unsymmetrical alkene has more than one kind of allylic hydrogen; each combination gives a different allylic bromide.
Propagation: trap the allylic radical with Br₂
In the contributor drawn, the odd electron is on C3, and bonding there to bromine from Br₂ gives 3-bromo-1-methylcyclohexene. Bonding at the other end of the same radical (C1) would give 1-bromo-1-methylcyclohex-2-ene instead.
As setas curvas
- The unpaired electron on the allylic carbon 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.
3-Bromo-1-methylcyclohexene, HBr and Br•
Bromine sits at C3, next to the trisubstituted C=C. This is one product of a mixture; it forms as a racemate because the radical carbon is planar.
O que você vai praticar
Draw the NBS chain that removes the allylic hydrogen at C3 of 1-methylcyclohexene and traps the radical at that carbon, giving 3-bromo-1-methylcyclohexene, and explain why other allylic bromides also form.
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