Química Orgânica 1 · Rearranjos de carbocátions e expansões de anel
Ethanolysis of neopentyl bromide with methyl migration
Condições: 1-bromo-2,2-dimethylpropane, ethanol (solvent), reflux
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Ionisation with methyl migration
A free primary cation would be far too unstable, so the C–Br bond breaks only with help from a neighbouring methyl group. That methyl moves with its bonding pair onto the CH2 carbon as bromide leaves, and the positive charge appears on the former quaternary carbon, now a tertiary cation.
As setas curvas
- The C–CH3 bonding pair carries the methyl to the CH2 carbon.
- The C–Br bonding pair leaves with bromine as bromide.
Why the methyl moves as bromide leaves
Neither SN2 (blocked backside) nor a simple SN1 (primary cation) is available; methyl participation lets the molecule ionise straight to a tertiary cation.
Ethanol captures the tertiary cation
An oxygen lone pair of ethanol, the solvent and only nucleophile present, bonds to the tertiary cationic carbon, giving an oxonium ion.
As setas curvas
- An ethanol oxygen lone pair forms the new C–O bond.
Ethanol removes the O–H proton
A second ethanol molecule acts as the base and takes the proton from the oxonium ion, giving the neutral ether and protonated ethanol.
As setas curvas
- An oxygen lone pair of the second ethanol forms the new O–H bond.
- The O–H bonding pair stays on the ether oxygen.
2-Ethoxy-2-methylbutane
The ether oxygen is bonded to the tertiary carbon of a rearranged five-carbon skeleton: 2-ethoxy-2-methylbutane. The ethyl group of the butane chain is the old CH2Br carbon plus the methyl that migrated onto it.
O que você vai praticar
Draw the solvolysis of 1-bromo-2,2-dimethylpropane in ethanol: ionisation with concerted methyl migration to a tertiary cation, capture by ethanol and deprotonation by ethanol, giving 2-ethoxy-2-methylbutane.
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