Química Orgânica 1 · Reações radicalares e seletividade
Chloromethane from methane: initiation, propagation, termination
Condições: Cl₂, hν
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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 6 passos escritos
Initiation: split Cl₂ with light
Light breaks the weak Cl–Cl bond homolytically: each chlorine atom keeps one of the two bonding electrons, giving two neutral chlorine radicals and no ions. A second Cl₂ molecule waits for the propagation steps.
As setas curvas
- One electron of the Cl–Cl bond stays on the first atom.
- The other electron of the Cl–Cl bond stays on the second atom, so the bond splits evenly into two radicals.
Propagation: a chlorine radical removes a hydrogen
A chlorine radical takes one hydrogen atom from methane, making H–Cl and a methyl radical. The H–Cl bond formed (about 432 kJ/mol) is only slightly weaker than the CH3–H bond broken (about 439 kJ/mol), so this step is nearly thermoneutral and fast.
As setas curvas
- The unpaired electron on the chlorine radical supplies one electron of the new bond to hydrogen.
- One electron of the carbon–H bond pairs with it to complete the new bond to hydrogen.
- The other carbon–H electron stays behind as the unpaired electron of the new radical.
Classify the steps
Initiation makes radicals from a non-radical, propagation keeps the number of radicals constant, and termination destroys radicals.
Propagation: take Cl from a second Cl₂
The carbon radical attacks Cl₂, forming the C–Cl bond and releasing a new chlorine radical. That radical starts the next cycle, so one initiation event drives many cycles.
As setas curvas
- The unpaired electron on carbon supplies one electron of the new bond to Cl.
- One electron of the Cl–Cl bond completes that new bond.
- The other Cl–Cl electron stays on the far Cl, regenerating the chain-carrying radical.
Termination: two chlorine radicals combine
When two chain carriers meet, each supplies one electron to a new Cl–Cl bond. Two radicals are destroyed and the chain stops; termination is rare because radical concentrations are low.
As setas curvas
- The unpaired electron on one chlorine radical supplies one electron of the new Cl–Cl bond.
- The unpaired electron on the other chlorine radical completes the bond.
Chloromethane and HCl
Adding the two propagation steps gives the net reaction CH4 + Cl2 → CH3Cl + HCl; the radicals cancel. The termination step re-forms Cl2 and removes two chain carriers.
O que você vai praticar
Draw every stage of the radical chlorination of methane: Cl–Cl homolysis, hydrogen abstraction, chlorine transfer, and a termination step, all with fishhook arrows.
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