Química Orgânica 2 · Efeitos dos substituintes e orientação
Draw how a nitro group drains the ring
Condições: Nitrobenzene, no reagent. Draw the two equivalent nitro contributors, then pull ring π electrons into the nitro group to place positive charge at an ortho carbon, the para carbon and the other ortho carbon. These are contributors of one molecule, not reaction steps.
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Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 6 passos escritos
Swap the two N–O bonds
Nitrogen already has four bonds and a positive charge, so it cannot accept a fifth bond: one N=O π pair must leave as the other forms. The two N–O bonds are therefore identical in the real molecule.
As setas curvas
- A lone pair on the negatively charged oxygen forms an N=O π bond.
- The existing N=O π pair becomes a lone pair on oxygen, which becomes negative.
Pull a ring π pair into the nitro group
The nitro group is a π acceptor: the ring can push electrons toward it, leaving an electron-poor ortho carbon.
As setas curvas
- The ring π pair next to the nitro group forms a C=N π bond.
- The N=O π pair becomes a lone pair on oxygen so that nitrogen keeps four bonds.
Carry the positive charge to the para carbon
The same π shift used for any ring cation moves the charge two atoms along.
As setas curvas
- The π pair between the meta and para carbons forms a π bond to the cationic ortho carbon. The para carbon becomes positive.
Carry the positive charge to the other ortho carbon
This completes the ring contributors.
As setas curvas
- The π pair between the next meta and ortho carbons forms a π bond to the cationic para carbon. The other ortho carbon becomes positive.
Use the contributors
Across the ring contributors, the ortho and para carbons carry positive charge while the meta carbons never do.
Where the ring is electron-poor
Across the ring contributors, the ortho and para carbons carry positive charge while the meta carbons never do. The δ+ signs at the ortho and para positions in the usual nitrobenzene drawing summarize that pattern; they are partial charges, not extra formal charges. Nitrobenzene is therefore strongly deactivated toward electrophiles and substitutes mainly at the meta positions. The same withdrawal at the ortho and para positions is what makes a nitro group helpful in nucleophilic aromatic substitution at those positions.
O que você vai praticar
Generate the nitro-group and ring contributors of nitrobenzene and use them to explain deactivation and meta direction.
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