Química Orgânica 1 · Caminhos de substituição
Convert a tertiary alcohol through its cation
Condições: 1-Methylcyclohexan-1-ol shaken with concentrated HBr at room temperature. HBr is drawn as the proton donor. Follow the SN1 branch to 1-bromo-1-methylcyclohexane; E1 elimination to 1-methylcyclohexene competes and is not requested.
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Protonate the hydroxyl group
As with every alcohol substitution, OH must first become a better leaving group. HBr (pKa approximately −9) protonates the alcohol oxygen almost completely.
As setas curvas
- A lone pair on the alcohol oxygen takes the proton from HBr.
- The H–Br pair stays on bromine as bromide.
Water leaves
The carbon bearing oxygen is tertiary: three carbons surround it, so backside attack is blocked. Instead the C–O bond breaks on its own. The resulting tertiary carbocation is stabilized by its three alkyl neighbors (hyperconjugation), which makes this slow, rate-determining ionization possible.
As setas curvas
- The C–O pair leaves with oxygen as neutral water, leaving the carbon with an empty p orbital.
Bromide captures the cation
The planar carbocation can be approached from either face. The reacting carbon is not a stereocenter in the product (it carries a methyl and two identical ring branches), so no stereochemistry needs to be specified.
As setas curvas
- A bromide lone pair bonds to the empty orbital of the carbocation.
Tertiary alkyl bromide and water
The same ring carbon now carries Br and the methyl group: 1-bromo-1-methylcyclohexane. The alcohol oxygen left as neutral water with its original H and the proton from HBr. Tertiary alcohols react with HBr quickly even at room temperature because this pathway passes through a relatively stable cation; primary alcohols need the SN2 route instead.
O que você vai praticar
Sequence OH protonation, loss of water to a tertiary carbocation, and bromide capture as three separate SN1 events on a ring alcohol.
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