A kinetic study of the Friedel-Crafts reaction of naphthalene with para-substituted benzenesulphonyl chlorides; the effect of the substituent

The kinetics of the Friedel-Crafts reaction of naphthalene with para-substituted benzenesulphonyl chlorides have been investigated. The rate constants for equations (i) (formation of the sulphonyl cation) and (ii) (sulphone formation) were determined separately. The effect of the para-substituents on k₁ was XC₆H₄SO ₂Cl + AlCl₃ ⇌ ^k1_k-1 XC ₆H₄SO₂⁺·AlCl₄ ^- (i) XC₆H₄SO₂⁺· AlCl^4- + C₁₀H₈ →^k2 XC₆H₄SO₂C₁₀H₇· AlCl₃ + HCl (ii) well expressed by the Yukawa-Tsuno equation, log(k_1X/k_1H) = ρ₁[σ + γ₁(Δσ_R⁺)] where ρ1 = -2.7 and γ₁ = 0.4. On the other hand, the effect of the para-substituents on k₃ (= k₁k₂/k_-1) was expressed by the Brown-Okamoto equation, log(k_3X/k_3H) = ρ₃σ⁺ where ρ₃ = -2.8. Thus, the electron-donating para-substituents gave the greater reaction rates. We concluded that the sulphonyl cation gives large negative ρ values as a result of its high reactivity, and that its low selectivity for α-attack is due to a steric effect. These kinetic results have been rationalised in terms of bifunctional acid-base catalysis in phenyl-sulphonylation.