Química Orgânica 1 · Reações radicalares e seletividade
Growing polystyrene: initiation and two propagation steps
Condições: ROOR (peroxide initiator), heat
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 6 passos escritos
Initiation: split the peroxide
Heat or light breaks the weak O–O bond of the peroxide (ROOR, drawn as di-tert-butyl peroxide, assumed) homolytically into two tert-butoxyl radicals. The O–O bond (about 150 kJ/mol) is the weakest bond present.
As setas curvas
- One electron of the O–O bond stays on the first atom.
- The other electron of the O–O bond stays on the second atom, so the bond splits evenly into two radicals.
Initiation: the alkoxyl radical adds to styrene
A tert-butoxyl radical adds to the CH2 end of styrene. The unpaired electron is left on the carbon bearing the phenyl group, where it is a benzylic radical stabilized by the ring.
As setas curvas
- The unpaired electron on oxygen supplies one electron of the new bond to the less hindered alkene carbon.
- One π electron of the C=C completes that bond.
- The other π electron stays on the phenyl-bearing carbon, which becomes the new radical centre.
Head-to-tail growth
Each addition puts the new bond on the CH2 end of styrene so that the radical always ends up benzylic; this gives a regular head-to-tail chain with a phenyl group on every other carbon.
Propagation: add a second styrene
The benzylic radical adds to the CH2 end of another styrene, forming a new C–C bond and moving the radical to the next benzylic carbon. The chain is now two monomer units long.
As setas curvas
- The unpaired electron on the benzylic carbon supplies one electron of the new bond to the less hindered alkene carbon.
- One π electron of the C=C completes that bond.
- The other π electron stays on the new benzylic carbon, which becomes the new radical centre.
Propagation: add a third styrene
The same step repeats: the chain-end benzylic radical adds to a third styrene. Every propagation consumes one monomer and passes the radical to the new chain end.
As setas curvas
- The unpaired electron on the chain-end benzylic carbon supplies one electron of the new bond to the less hindered alkene carbon.
- One π electron of the C=C completes that bond.
- The other π electron stays on the new chain-end carbon, which becomes the new radical centre.
The growing polystyrene radical
Three styrene units are now joined head-to-tail, capped by the tert-butoxy group from the initiator, and the chain still ends in a benzylic radical that will add more monomer until a termination step.
O que você vai praticar
Draw peroxide homolysis, addition of a tert-butoxyl radical to styrene, and two propagation steps in which the benzylic radical adds to further styrene molecules, ending with the growing trimer radical.
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