Química Orgânica 1 · Caminhos de substituição
Solvolysis with a methyl shift
Condições: 3-Chloro-2,2-dimethylpentane warmed in methanol. Follow the rearranged SN1 branch to 2-methoxy-2,3-dimethylpentane; elimination and unrearranged substitution compete and no ratio is claimed. A second methanol molecule enters only as the final proton acceptor. Connectivity only: the stereocenter formed at the carbon that originally bore chlorine is not assessed.
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Ionize the secondary chloride
Methanol is a weak nucleophile and a polar protic solvent, so the reaction starts by heterolysis rather than backside attack.
As setas curvas
- The C–Cl pair leaves with chlorine, giving a secondary carbocation.
Move a methyl group
The quaternary carbon next to the cation has no hydrogen to offer, but it carries three methyl groups. A methyl shift turns the secondary cation into a tertiary one.
Nota: The three methyl groups on the quaternary carbon are equivalent; any one can undergo the methyl shift.
As setas curvas
- The bond electrons between the quaternary carbon and a methyl group form a new bond between that methyl group and the cationic carbon. The formerly quaternary carbon is left with an empty orbital.
Solvent captures the tertiary cation
Solvent is present in huge excess, so methanol rather than chloride is the likely nucleophile.
As setas curvas
- A methanol oxygen lone pair bonds to the empty orbital on the tertiary carbocation.
Hand the proton to solvent
A protonated ether (pKa approximately −3.5) and protonated methanol (pKa approximately −2.5) are acids of similar strength; the large excess of solvent methanol carries the proton away.
As setas curvas
- A second methanol uses a lone pair to take the O–H proton.
- The O–H pair stays on the ether oxygen, which becomes neutral.
Rearranged ether
A methyl group now bonds to the carbon that originally bore chlorine. The methoxy group bonds to its neighbor, which retains two methyl groups: 2-methoxy-2,3-dimethylpentane. Protonated methanol and chloride together make up the HCl formed. A C–C σ bond supplied the electrons for the methyl shift; no methyl anion or radical was ever free.
O que você vai praticar
Carry a secondary cation next to a quaternary carbon through a 1,2-methyl shift, methanol capture and proton transfer to solvent.
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