Química Orgânica 1 · Reações radicalares e seletividade
Ethyl hexyl sulfide by thiol–ene addition
Condições: CH₃CH₂SH, ROOR (peroxide), heat or hν
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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: make the thiyl radical
A tert-butoxyl radical removes the hydrogen from the S–H bond of ethanethiol. The S–H bond is weak (about 365 kJ/mol), so this gives the ethylthiyl radical, the chain carrier, quickly.
As setas curvas
- The unpaired electron on oxygen supplies one electron of the new O–H bond.
- One electron of the S–H bond completes the O–H bond.
- The other S–H electron stays on sulfur as the unpaired electron of the thiyl radical.
Propagation: the thiyl radical adds to the alkene
The sulfur radical adds to the terminal CH2 of hex-1-ene, leaving a secondary carbon radical on C2. As with bromine, the addition direction is chosen to give the more stable carbon radical.
As setas curvas
- The unpaired electron on sulfur 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 C2, which becomes the new radical centre.
What carries the chain?
The thiyl radical (RS•) plays the same role as Br• in the peroxide HBr addition: it adds to the alkene, and the weak S–H bond of the next thiol delivers hydrogen and regenerates RS•.
Propagation: the carbon radical takes H from a thiol
The secondary radical abstracts the hydrogen atom from a second ethanethiol. This completes the addition and releases a new thiyl radical for the next alkene.
As setas curvas
- The unpaired electron on C2 supplies one electron of the new C–H bond.
- One electron of the weak S–H bond completes the C–H bond.
- The other S–H electron stays on S, which carries the chain on.
Ethyl hexyl sulfide and a new thiyl radical
Sulfur is attached to the terminal carbon and hydrogen to C2: ethyl hexyl sulfide, the anti-Markovnikov thiol–ene product.
O que você vai praticar
Draw peroxide initiation and the two propagation steps that add ethanethiol across hex-1-ene with sulfur on the terminal carbon, giving ethyl hexyl sulfide (1-(ethylsulfanyl)hexane).
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