Química Orgânica 1 · Transformações de alcinos
Reduce an internal alkyne to a trans alkene
Condições: But-2-yne with sodium metal in liquid ammonia (dissolving-metal reduction). Two Na atoms, each drawn with its one valence electron, and two NH₃ molecules (the solvent is also the proton source) are explicit. Draw each single-electron move with a fishhook (one-electron) arrow and each proton transfer with ordinary two-electron arrows.
Desenhe este problema você mesmo
Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 5 passos escritos
Transfer the first electron
One of the two π bonds is converted into a carbon lone pair and an unpaired electron at the other carbon: a radical anion. Its lone pair and radical sit as far apart as possible, so the methyl groups end up on opposite sides (trans-bent). Na becomes Na⁺.
As setas curvas
- Sodium gives its single valence electron to an alkyne carbon.
- One electron of a π bond pairs with the incoming electron, making a carbon lone pair.
- The other electron of that π bond remains unpaired at the other carbon.
Protonate the carbanion
Ammonia (pKa about 38) is acidic enough to protonate the very basic carbanion carbon. A neutral vinyl radical remains; it prefers the geometry that keeps the two methyl groups trans.
As setas curvas
- The carbanion lone pair takes a proton from ammonia.
- The N–H pair stays on nitrogen, giving amide ion.
Transfer the second electron
The vinyl radical is reduced to a vinyl anion. The anion keeps the geometry with the two methyl groups trans, and its lone pair sits where the final hydrogen will go, which fixes the E geometry.
As setas curvas
- A second sodium atom gives one electron to the radical carbon, pairing with its unpaired electron.
Protonate the vinyl anion
The second protonation completes the net addition of two hydrogen atoms (two electrons plus two protons) across one π bond.
As setas curvas
- The vinyl anion lone pair takes a proton from a second ammonia.
- The N–H pair stays on nitrogen as a second amide ion.
trans-But-2-ene, two Na⁺ and two NH₂⁻
(E)-But-2-ene forms: the new hydrogens are on opposite sides of the C=C, so the two methyl groups are trans. Each sodium supplied one electron and each ammonia one proton, leaving 2 NaNH₂. Contrast Lindlar hydrogenation, which gives the cis alkene. Sodium cannot reduce the isolated alkene further under these conditions.
O que você vai praticar
Track single-electron transfers from sodium and proton transfers from ammonia through radical-anion and vinyl-radical intermediates to explain trans-alkene selectivity.
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