Química Orgânica 1 · Caminhos de substituição
Close an epoxide from the back side
Condições: (1R,2R)-2-Chlorocyclohexan-1-ol, one enantiomer of the trans isomer, with aqueous NaOH. OH is on a solid wedge (toward you) and Cl on a hashed wedge (away). Follow deprotonation by hydroxide, then intramolecular SN2 to cyclohexene oxide (7-oxabicyclo[4.1.0]heptane). In the reacting chair conformer OH and Cl are both axial (trans-diaxial, anti-periplanar); the flat ring drawing shows only which face each group is on. Na⁺ is a spectator.
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Hydroxide removes the O–H proton
An alcohol (pKa approximately 16) and water (pKa approximately 15.7) are similar acids, so only a small fraction of the alcohol is deprotonated at any moment. That is enough: each alkoxide that forms can close the ring, and more is produced as it is consumed.
As setas curvas
- A lone pair on hydroxide takes the alcohol O–H proton.
- The alcohol O–H pair stays on oxygen, giving an alkoxide.
Why the trans isomer?
The new C–O bond must form at the back of the C–Cl bond. The alkoxide oxygen is on the front face and Cl on the back face of the ring, so oxygen is already positioned on the side opposite chlorine. In the chair, this corresponds to both groups being axial and anti-periplanar.
The alkoxide closes the ring
This is an intramolecular Williamson ether synthesis. The nucleophile and the electrophile are on neighboring carbons, so a three-membered ring forms despite its strain. The carbon bearing chlorine is inverted: chlorine left from the back face and oxygen arrived on the front face.
As setas curvas
- A lone pair on the alkoxide oxygen bonds to the carbon bearing chlorine from the front face, opposite chlorine.
- The C–Cl pair leaves with chlorine as chloride.
Cyclohexene oxide
Oxygen now bridges two neighboring carbons on the front face of the ring, so both C–O bonds are wedges: the epoxide ring is necessarily cis-fused to the six-membered ring. Chloride left with the negative charge, and the alcohol proton is now part of water. The epoxide has a mirror plane through oxygen and the middle of the epoxide C–C bond, so it is achiral (meso): the other trans enantiomer gives the same product. Treating cyclohexene with Cl₂ and water gives the trans chlorohydrin, so this sequence converts cyclohexene into its epoxide in two stages.
O que você vai praticar
Explain why a trans cyclic halohydrin closes to an epoxide: the alkoxide must reach the C–Cl carbon from the face opposite chlorine, as in any SN2.
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