Química Orgânica 1 · Reações radicalares e seletividade
Replace C–Br with C–H using tributyltin hydride
Condições: Bromocyclohexane with tributyltin hydride (Bu₃SnH) and catalytic AIBN in benzene at reflux. AIBN has already lost N₂ to give the 2-cyano-2-propyl radical shown. Follow initiation with one Bu₃SnH, then one propagation cycle using the second Bu₃SnH. Bu₃Sn is drawn as one condensed tin group.
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Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 4 passos escritos
Initiate: make the tin radical
The Sn–H bond is weak (about 310 kJ/mol), so even the resonance-stabilized 2-cyano-2-propyl radical can take its hydrogen. The initiator is used in small amounts; each initiation event starts a long chain.
As setas curvas
- The unpaired electron on the initiator carbon supplies one electron of the new C–H bond.
- One Sn–H bonding electron completes the new C–H bond.
- The other Sn–H electron stays on tin, making Bu₃Sn•.
Abstract bromine with the tin radical
Tin has a high affinity for halogens: the Sn–Br bond formed is stronger than the C–Br bond broken. Bromine transfers as an atom (Br•), not as bromide, so no carbocation or ions form.
As setas curvas
- The unpaired electron on tin supplies one electron of the new Sn–Br bond.
- One C–Br bonding electron completes Sn–Br.
- The other C–Br electron stays on the ring carbon as the cyclohexyl radical.
Deliver hydrogen from tin hydride
A strong C–H bond (about 415 kJ/mol) forms at the expense of the weak Sn–H bond, so this step is downhill. The new Bu₃Sn• now abstracts Br from the next alkyl bromide.
As setas curvas
- The unpaired electron on the cyclohexyl radical supplies one electron of the new C–H bond.
- One Sn–H bonding electron completes the C–H bond.
- The other Sn–H electron stays on tin, regenerating the chain carrier.
Cyclohexane, Bu₃SnBr and Bu₃Sn•
The ring carbon bonds to hydrogen in place of bromine, giving cyclohexane. The first tin group carries the bromine as neutral Bu₃SnBr, and the second is the new tin radical that carries the chain. The initiator fragment became isobutyronitrile, (CH₃)₂CHCN. Net reaction: R–Br + Bu₃SnH → R–H + Bu₃SnBr.
O que você vai praticar
Run the Bu₃SnH radical chain: generate Bu₃Sn• from an initiator radical, abstract bromine from carbon, and deliver hydrogen from tin hydride to regenerate the tin radical.
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