Química Orgânica 1 · Relações estereoquímicas
Add HBr next to an existing stereocenter
Condições: (R)-4-Methylhex-1-ene with HBr in a nonpolar solvent, no peroxides. Follow the ionic Markovnikov pathway; the drawing shows bromide bonding to the upper face (toward the viewer) of the C2 cation. No hydride shift occurs: transferring H from the neighboring CH₂ carbon would give another secondary cation. No product ratio is claimed.
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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Protonate the alkene
Adding H to terminal C1 gives the more stable secondary cation at C2. The C4 stereocenter is not involved and stays R.
As setas curvas
- The π pair forms the new C1–H bond.
- The H–Br bond pair leaves with bromide.
Compare the two faces of the cation
C2 is planar, and bromide can bond to either face. Because the molecule already contains the (R)-C4 center, the two faces are not mirror images of each other: top-face attack gives (2R,4R) and bottom-face attack gives (2S,4R). The two products share the 4R center and differ only at C2, so they are diastereomers. Diastereomeric transition states have different energies, so the two form at different rates; with the stereocenter two carbons away the preference is small, and no ratio is claimed.
Capture the cation
Bromide is the only nucleophile present. Follow the upper-face branch shown.
As setas curvas
- A bromide lone pair forms the C2–Br bond on the upper face.
(2R,4R)-2-Bromo-4-methylhexane
At C2: Br > CH₂ (toward C4) > CH₃ > H. With the wedged Br toward the viewer, the implicit H points away; Br → C3 → C1 runs clockwise, so C2 is R. C4 is still R: its bonds were never touched, and its CH₂CHBrCH₃ branch still outranks ethyl. Bottom-face capture gives the (2S,4R) diastereomer. Because both products contain the 4R center, the crude product is optically active. Bromide gave up one lone pair to make the bond and is neutral with three lone pairs.
O que você vai praticar
Classify the two products formed when a new stereocenter is created in a molecule that already contains one.
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