Química Orgânica 1 · Estereoquímica das reações de alcenos
Add bromine across a five-membered ring
Condições: Cyclopentene with Br₂ in CH₂Cl₂ at room temperature, in the dark (no radical initiator). Follow the pathway in which bromine bridges the upper (wedge) face of the ring. Wedges and hashes refer to the plane of the ring. Bromide attack at the other bridge carbon, or bridging from the lower face, gives the enantiomer.
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Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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Form the bromonium bridge
The electron-rich π bond polarizes Br₂ as it approaches. Three pairs move at once: the π pair and a bromine lone pair make the two bridge bonds while the Br–Br pair leaves as bromide. The bridging bromine ends with two bonds, two lone pairs and a +1 charge; both new C–Br bonds point to the same (upper) face of the ring.
As setas curvas
- The alkene π pair forms a C–Br bond.
- A lone pair on the bridging bromine forms the second C–Br bond, so neither carbon is left as a carbocation.
- The Br–Br bond pair leaves as bromide.
Open the bridge from the lower face
The upper face is blocked by the bridge, so bromide can reach a bridge carbon only from below. This backside (SN2-like) attack inverts that carbon. The other C–Br bond is untouched, so the other carbon keeps its configuration. In a five-membered ring this anti opening leaves the two bromines on opposite faces.
As setas curvas
- A bromide lone pair bonds to a bridge carbon from the lower face, opposite the bridge.
- The C–Br bond pair at the attacked carbon returns to the bridging bromine, which becomes neutral again.
trans-1,2-Dibromocyclopentane
The bromines (one wedge, one hash) sit on opposite faces of the ring: trans-1,2-dibromocyclopentane. Unlike the cis isomer, which has an internal mirror plane and is meso, the trans isomer is chiral. Bridging from the lower face or opening at the other bridge carbon gives its mirror image at the same rate, so the product is a racemic mixture of (1R,2R) and (1S,2S). No cis isomer forms, because the bridge never allows attack from the upper face.
O que você vai praticar
Carry bromonium formation and backside bromide opening on cyclopentene through to a chiral trans dibromide and explain why it forms as a racemate rather than a meso compound.
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