Organic Chemistry 2 · Electrophilic aromatic substitution
Naphthalene-2-sulfonic acid by sulfonation at 160 °C
Conditions: conc. H2SO4 (contains SO3), 160 °C
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SO₃ attacks C2 of naphthalene
At 160 °C every sulfonation step is reversible, so the 1-sulfonic acid that forms first keeps losing SO₃ again. Attack at C2, the β position, is slower but its product does not revert, so it accumulates. The C1=C2 π pair bonds to sulfur and an S=O π pair moves onto oxygen.
The curved arrows
- The C1=C2 π pair forms the new C2–S bond.
- One S=O π pair moves onto oxygen, which becomes negatively charged.
The charge spreads into the second ring
The cation at C1 can move only by borrowing the C8=C8a π bond, which places the charge on C8 and breaks the second ring’s sextet. C2 attack has just one fully benzenoid contributor, which is why it is the slower pathway.
The curved arrows
- The C8=C8a π pair shifts toward the cationic C1, moving the charge to C8.
Why the 2-isomer at 160 °C?
Under reversible conditions product stability, not rate, decides.
Hydrogen sulfate removes the C2 proton
Hydrogen sulfate removes the proton on the sp³ C2. The C–H pair forms the C1=C2 π bond while the C1=C8a π pair returns to C8, so both rings are aromatic again.
The curved arrows
- The negative oxygen of hydrogen sulfate bonds to the C2 proton.
- The C–H pair forms the C1=C2 π bond.
- The C1=C8a π pair shifts back onto the cationic C8, restoring the second ring’s sextet.
Protonate the sulfonate
The sulfonate oxygen takes the proton back from sulfuric acid, giving naphthalene-2-sulfonic acid and regenerating hydrogen sulfate. The ring proton has simply been relayed to the product.
The curved arrows
- A lone pair on the negative sulfonate oxygen bonds to the proton of sulfuric acid.
- The O–H pair stays on oxygen, regenerating hydrogen sulfate.
Naphthalene-2-sulfonic acid
Naphthalene-2-sulfonic acid, the thermodynamic product, is left once the equilibrium has settled. Its SO₃H group has no peri neighbour, so it is lower in energy than the 1-isomer.
What you'll practice
Draw SO3 attack at C2 of naphthalene, the arenium contributor that moves charge into the second ring, and deprotonation and protonation to naphthalene-2-sulfonic acid, the thermodynamic product.
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