Química Orgânica 1 · Rearranjos de carbocátions e expansões de anel
HBr converts 1-cyclobutylethanol into 1-bromo-1-methylcyclopentane
Condições: HBr (48% aq., excess), 25 °C
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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 7 passos escritos
HBr protonates the alcohol
An oxygen lone pair takes the proton of HBr, making an alkyloxonium ion with water as a good leaving group.
As setas curvas
- An oxygen lone pair forms the new O–H bond.
- The H–Br pair leaves with bromine as bromide.
Water leaves
The C–O bond breaks and water departs, giving a secondary cation next to the cyclobutane ring.
As setas curvas
- The C–O bonding pair leaves with oxygen as water.
Ring expansion
A C–C bond of the cyclobutane ring migrates to the cationic carbon, which is pulled into the ring. The four-membered ring becomes a five-membered ring, and the positive charge moves to the former ring CH carbon: a secondary cyclopentyl cation next to the CH carrying the methyl.
As setas curvas
- The ring C–C bonding pair carries its CH2 carbon to the cationic carbon.
Secondary to secondary?
Ring expansion keeps the cation secondary but removes most of the cyclobutane strain, and it puts the charge next to a CH bearing a methyl, from which a hydride shift gives a tertiary cation.
1,2-Hydride shift to the tertiary cation
The hydrogen on the neighbouring ring carbon, which also carries the methyl group, moves with its bonding pair to the cationic carbon. The charge moves to the methyl-bearing carbon, giving the tertiary 1-methylcyclopentyl cation.
As setas curvas
- The C–H bonding pair carries the hydrogen to the cationic carbon.
Bromide captures the tertiary cation
A bromide lone pair bonds to the tertiary cationic ring carbon.
As setas curvas
- A bromide lone pair forms the new C–Br bond.
1-Bromo-1-methylcyclopentane
Two shifts turned a strained secondary cation into a tertiary cyclopentyl cation, so the product is 1-bromo-1-methylcyclopentane: the former carbinol carbon now carries both the methyl and the bromine inside a five-membered ring.
O que você vai praticar
Draw the reaction of 1-cyclobutylethanol with HBr: protonation, loss of water, ring expansion to a cyclopentyl cation, a hydride shift to the tertiary cation and bromide capture giving 1-bromo-1-methylcyclopentane.
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