Química Orgânica 1 · Reações radicalares e seletividade
1-Bromo-2-methylcyclopentane by anti-Markovnikov HBr addition
Condições: HBr, 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 bromine radical
A tert-butoxyl radical removes the hydrogen from H–Br. The O–H bond formed is much stronger than the H–Br bond broken, so this step is fast and delivers Br•, the real chain carrier.
As setas curvas
- The unpaired electron on oxygen supplies one electron of the new bond to hydrogen.
- One electron of the H–Br bond completes the new O–H bond.
- The other H–Br electron stays on bromine, giving a bromine radical.
Propagation: Br• adds to the alkene
The bromine radical adds to the ring CH of the double bond, leaving the unpaired electron on the methyl-bearing carbon as a tertiary radical. Addition at the other end would give a less stable secondary radical.
As setas curvas
- The unpaired electron on bromine 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 methyl-bearing ring carbon, which becomes the new radical centre.
Why anti-Markovnikov?
In the radical mechanism bromine adds first, to the carbon that gives the more stable carbon radical; hydrogen arrives last. So Br ends up on the less substituted carbon, the opposite of ionic HBr addition, where H+ adds first.
Propagation: the carbon radical takes H from HBr
The carbon radical abstracts the hydrogen atom from a second HBr, completing the anti-Markovnikov addition and regenerating Br• to add to the next alkene.
As setas curvas
- The unpaired electron on the methyl-bearing ring carbon supplies one electron of the new C–H bond.
- One electron of the weak Br–H bond completes the C–H bond.
- The other Br–H electron stays on Br, which carries the chain on.
1-Bromo-2-methylcyclopentane, tert-butanol and radicals
Bromine and the methyl group are now on adjacent ring carbons: 1-bromo-2-methylcyclopentane, formed as a mixture of cis and trans isomers.
O que você vai praticar
Draw the radical addition of HBr to 1-methylcyclopentene in the presence of peroxide, giving 1-bromo-2-methylcyclopentane.
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