Q2(a). Sulfonation of naphthalene: kinetic and thermodynamic control

Introduction. Naphthalene reacts with concentrated H₂SO₄ by electrophilic aromatic substitution (SO₃ or HSO₃⁺ is the electrophile). Unlike nitration, sulfonation is reversible. The product therefore depends on temperature: one isomer forms faster, the other is more stable.

Scheme: Low temperature: α (1-) acid, kinetic product. High temperature: β (2-) acid, thermodynamic product.

Kinetic product at low temperature (α-attack)

1. Attack at C-1 (α) gives an arenium ion (σ complex) with two resonance forms that keep the second ring as an intact benzene sextet; attack at C-2 (β) gives only one such form.
2. The α σ complex, and so the transition state leading to it, is lower in energy: lower activation energy, faster reaction.
3. At about 80°C the reverse reaction (desulfonation) is negligible on the time scale used, so the product ratio equals the ratio of rates: naphthalene-1-sulfonic acid is the main product (kinetic control).

Thermodynamic product at high temperature (β-isomer)

1. In the 1-isomer the bulky -SO₃H group is pushed against the peri hydrogen at C-8 (1,8-interaction), which raises its energy. The 2-isomer has no such strain and is the more stable product.
2. At about 160°C protodesulfonation becomes fast: the 1-acid loses SO₃ easily (its higher energy means a smaller barrier for the back reaction), and naphthalene is sulfonated again. Over time the mixture drains into the more stable naphthalene-2-sulfonic acid (thermodynamic control).
3. Proof of reversibility: naphthalene-1-sulfonic acid heated with H₂SO₄ at 160°C is converted into the 2-acid.

Energy level diagram

Figure: Sketch (not to scale). Solid: α path, lower barrier. Dashed: β path, higher barrier but more stable product.

Conclusion. Below about 80°C the faster-formed 1-sulfonic acid is obtained (kinetic control: more stable arenium ion, lower Eₐ). Near 160°C sulfonation is reversible and the more stable 2-sulfonic acid accumulates (thermodynamic control: no peri interaction).

Application. Naphthalene-2-sulfonic acid is fused with NaOH to make 2-naphthol, the coupling component of many azo dyes; the 1- and 2-acids are also the starting points of the naphthylamine sulfonic acids (dye intermediates).