Química Orgânica 1 · Reações radicalares e seletividade
Stop two chains with one BHT molecule
Condições: BHT (2,6-di-tert-butyl-4-methylphenol) as an antioxidant in a food fat undergoing autoxidation. ROO• (R = a fat-derived alkyl group) is the chain carrier; two are drawn. Follow hydrogen transfer to a peroxyl radical, then the phenoxyl resonance path to the methyl-bearing para carbon, then coupling there with the remaining peroxyl radical. Other phenoxyl reactions are not included.
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Donate the phenolic hydrogen
The phenolic O–H bond (about 340 kJ/mol) is weaker than the ROO–H bond formed (about 365 kJ/mol), so the transfer is downhill and fast. The chain-carrying ROO• becomes a hydroperoxide that cannot propagate.
As setas curvas
- The terminal peroxyl oxygen supplies one electron of the new O–H bond.
- One O–H bonding electron of BHT completes the new O–H bond.
- The other O–H electron stays on the phenol oxygen.
Delocalize the phenoxyl radical to an ortho carbon
Resonance spreads the odd electron from oxygen into the ring. The bulky tert-butyl groups shield the oxygen and both ortho carbons, so this phenoxyl radical does not start new chains.
As setas curvas
- The unpaired electron on the phenoxyl oxygen pairs into a new C=O π bond.
- One electron of the adjacent ring π bond completes C=O.
- The other ring π electron stays on the tert-butyl-bearing ortho carbon.
Continue the delocalization to the para carbon
This contributor places the unpaired electron on the para carbon, the least hindered radical site of the phenoxyl radical.
As setas curvas
- The unpaired electron on the ortho carbon pairs into a new ring π bond.
- One electron of the neighbouring ring π bond completes that bond.
- The other π electron stays on the methyl-bearing para carbon.
Couple the para carbon with a second peroxyl radical
Radical–radical coupling consumes a second chain carrier. Each BHT therefore stops two autoxidation chains.
Nota: Use the para contributor for this coupling; the tert-butyl groups block the oxygen and the ortho carbons.
As setas curvas
- The unpaired electron on the para carbon supplies one electron of the new C–O bond.
- The unpaired electron on the terminal peroxyl oxygen completes the C–O bond.
A peroxy dienone and a hydroperoxide
The para carbon now bonds to the methyl group, a peroxide oxygen and two ring carbons: 4-(alkylperoxy)-2,6-di-tert-butyl-4-methylcyclohexa-2,5-dien-1-one. The original phenol oxygen is a neutral C=O oxygen with two lone pairs. Both products are closed-shell, and two ROO• radicals have been removed from the chain.
O que você vai praticar
Show how a hindered phenol donates its O–H hydrogen to a peroxyl radical, delocalize the phenoxyl radical to the para carbon, and trap a second peroxyl radical there.
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