Química Orgânica 2 · Substituição eletrofílica aromática
Benzenesulfonic acid from benzene in hot sulfuric acid
Condições: conc. H2SO4 (contains SO3), Δ
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O mecanismo, passo a passo
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The ring attacks sulfur trioxide
Hot concentrated sulfuric acid contains SO₃ (from 2 H₂SO₄ ⇌ SO₃ + H₃O⁺ + HSO₄⁻), whose sulfur is strongly electrophilic. A ring π pair bonds to sulfur and one S=O π pair moves onto oxygen.
As setas curvas
- The ring π pair at the carbon bearing the drawn hydrogen forms the new C–S bond; the neighboring carbon becomes cationic.
- One S=O π pair moves onto oxygen, which becomes negatively charged.
Delocalize the charge to the para carbon
The arenium charge is delocalized over the two ortho carbons and the para carbon; the attached SO₃⁻ group is anionic, so the intermediate is a zwitterion.
As setas curvas
- The neighboring ring π pair shifts toward the cationic carbon, moving the positive charge to the carbon para to the new bond.
Return the charge beside the sp³ carbon
The arenium ion is one hybrid; redraw the contributor with the positive charge back on the carbon next to the sp³ C–H. That contributor shows which π bond the C–H electrons will re-form in the deprotonation.
As setas curvas
- The π pair shifts back toward the para carbon, leaving the positive charge on the ortho carbon next to the sp³ C–H.
Hydrogen sulfate removes the ring proton
Hydrogen sulfate, present in the acid, removes the proton from the sp³ carbon bearing sulfur, and the ring becomes aromatic again, giving the benzenesulfonate ion.
As setas curvas
- The negative oxygen of hydrogen sulfate takes the ring proton, becoming sulfuric acid.
- The C–H bond electrons form the ring π bond to the neighboring cationic carbon, restoring aromaticity.
Protonate the sulfonate
The sulfonate oxygen takes a proton back from sulfuric acid, giving benzenesulfonic acid and regenerating hydrogen sulfate, so the proton is simply relayed from the ring to the product.
As setas curvas
- A lone pair on the negative sulfonate oxygen takes the proton from sulfuric acid.
- The O–H electrons stay on the sulfuric acid oxygen, regenerating hydrogen sulfate.
Reversibility
Sulfonation is reversible: heating the sulfonic acid in dilute aqueous acid removes SO₃ again.
Benzenesulfonic acid
The SO₃H group has replaced a ring hydrogen, giving benzenesulfonic acid; hydrogen sulfate is regenerated.
O que você vai praticar
Draw sulfonation of benzene by SO₃: ring attack, arenium contributors, deprotonation by hydrogen sulfate and protonation of the sulfonate to benzenesulfonic acid.
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