Química Orgânica 2 · Revisão de substituição aromática para a prova
Nitration of chlorobenzene
Condições: Chlorobenzene, conc. HNO3 (1.05 equiv), conc. H2SO4, 30–50 °C
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 7 passos escritos
Sulfuric acid protonates nitric acid
Sulfuric acid, the stronger acid, protonates the hydroxyl oxygen of nitric acid, turning OH into a water leaving group.
As setas curvas
- The hydroxyl oxygen of nitric acid takes the sulfuric acid proton.
- The O–H pair stays on the sulfate oxygen, giving hydrogen sulfate.
Water leaves to give the nitronium ion
The N–O bond to the protonated oxygen breaks and water leaves, while the anionic oxygen forms a second N=O bond. The linear nitronium ion is the electrophile.
As setas curvas
- The N–O pair leaves with oxygen as water.
- A lone pair on the anionic oxygen forms the second N=O bond.
The carbon para to chlorine attacks nitronium
The ring π bond at the carbon para to chlorine forms the C–N bond, and one N=O π pair moves onto oxygen. This slow step gives the arenium ion.
As setas curvas
- The ring π pair forms the C–N bond at the carbon para to chlorine.
- One N=O π pair moves onto oxygen, so nitrogen keeps an octet.
Chlorine shares the positive charge
Because attack was para, the positive charge can reach the carbon bearing chlorine, and a chlorine lone pair forms a C=Cl⁺ π bond. In this contributor every atom has an octet. Meta attack never places charge on that carbon.
As setas curvas
- The ring π bond shifts toward the cationic carbon, moving the charge to the carbon bearing chlorine.
- A chlorine lone pair forms a π bond to the ring.
Deactivating, yet ortho/para
Chlorine withdraws electron density inductively, which slows every attack, but its lone pair stabilises only the ortho and para arenium ions. Rate is set by induction, position by resonance.
Hydrogen sulfate restores aromaticity
Hydrogen sulfate removes the proton from the sp³ ring carbon. The C–H electrons, the ring π bond and the C=Cl⁺ π bond shift together, restoring the aromatic ring and a neutral chlorine.
As setas curvas
- A hydrogen sulfate oxygen lone pair takes the ring proton.
- The C–H pair forms a ring π bond.
- The neighbouring π bond shifts along the ring.
- The C=Cl⁺ π pair returns to chlorine as a lone pair.
1-Chloro-4-nitrobenzene
1-Chloro-4-nitrobenzene is the main product, sulfuric acid is regenerated, and the nitric acid oxygen has left as water.
O que você vai praticar
Draw nitronium formation in mixed acid, para attack by chlorobenzene with the chlorine lone pair stabilising the arenium ion, and deprotonation by hydrogen sulfate to give 1-chloro-4-nitrobenzene.
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