Química Orgânica 1 · Radicais, álcoois e éteres
Anti-Markovnikov addition of HBr to 2-methylpropene with a peroxide
Condições: 2-Methylpropene, HBr, di-tert-butyl peroxide (cat.), heat
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Heat splits the peroxide
The O–O bond of di-tert-butyl peroxide is weak and breaks homolytically on heating, giving two tert-butoxyl radicals.
As setas curvas
- One O–O electron goes to the left oxygen.
- The other O–O electron goes to the right oxygen.
The alkoxyl radical makes a bromine atom
A tert-butoxyl radical removes the hydrogen of HBr. Forming the strong O–H bond while breaking the weak H–Br bond is downhill, and it releases a bromine atom.
As setas curvas
- The oxygen's unpaired electron starts the O–H bond.
- One H–Br electron completes the O–H bond.
- The other H–Br electron stays on bromine, giving Br•.
Where does Br• add?
Br• adds to the CH₂ carbon so that the unpaired electron ends up on the tertiary carbon, the more stable radical.
Br• adds to the CH₂ end of the alkene
The bromine atom pairs with one π electron to form the C–Br bond at the CH₂ carbon. The other π electron stays on the substituted carbon, which becomes a tertiary radical.
As setas curvas
- The bromine's unpaired electron starts the C–Br bond.
- One π electron completes the C–Br bond.
- The other π electron stays on the tertiary carbon.
The tertiary radical takes H from HBr
The carbon radical removes the hydrogen of a second HBr, completing the C–H bond and releasing a new bromine atom that adds to the next alkene.
As setas curvas
- The carbon's unpaired electron starts the C–H bond.
- One H–Br electron completes the C–H bond.
- The other H–Br electron stays on bromine as a new Br•.
1-Bromo-2-methylpropane
Bromine sits on the former CH₂ carbon and hydrogen on the former substituted carbon: anti-Markovnikov addition. The new Br• propagates the chain; the remaining tert-butoxyl radical can start another chain.
O que você vai praticar
Draw the radical chain in which a peroxide initiator makes Br•, Br• adds to the CH₂ end of 2-methylpropene, and the tertiary radical takes H from HBr to give 1-bromo-2-methylpropane.
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