Química Orgânica 1 · Rearranjos de carbocátions e expansões de anel
Methanolysis of 1-(1-chloroethyl)-1-methylcyclobutane with ring expansion
Condições: methanol (solvent), 25 °C
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Ionisation to a secondary cation
In polar, protic methanol the C–Cl bond ionises; chloride leaves and a secondary carbocation forms next to the quaternary ring carbon.
As setas curvas
- The C–Cl bonding pair leaves with chlorine as chloride.
Why the ring expands
Both available shifts give a tertiary cation; only the ring-bond shift also relieves cyclobutane strain, so ring expansion wins.
Ring-bond shift expands the ring
One ring C–C bond of the quaternary carbon migrates with its electron pair to the cationic carbon. That carbon is pulled into the ring, which grows from four to five members, and the positive charge moves to the former quaternary carbon, now a tertiary cation bearing a methyl group.
As setas curvas
- The ring C–C bonding pair carries its CH2 carbon over to the cationic carbon.
Methanol captures the tertiary cation
An oxygen lone pair of methanol bonds to the tertiary cationic ring carbon, giving an oxonium ion.
As setas curvas
- A methanol oxygen lone pair forms the new C–O bond.
Methanol removes the O–H proton
A second methanol molecule takes the proton from the oxonium ion, giving the neutral ether.
As setas curvas
- A lone pair of the second methanol forms the new O–H bond.
- The O–H bonding pair stays on the ether oxygen.
1-Methoxy-1,2-dimethylcyclopentane
The product is a cyclopentane: the methoxy group and one methyl sit on the former quaternary carbon, and the old CHCl carbon, still carrying its methyl, is now a ring carbon next to it.
O que você vai praticar
Draw the SN1 methanolysis of 1-(1-chloroethyl)-1-methylcyclobutane, including the ring-bond shift that expands the four-membered ring to a five-membered tertiary cation, giving 1-methoxy-1,2-dimethylcyclopentane.
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