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

Abstract

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 XC₆H₄SO₂Cl + AlCl₃ [graphic omitted] XC₆H₄SO₂⁺·AlCl₄^–(i), XC₆H₄SO₂⁺·AlCl₄^–+C₁₀H₈ [graphic omitted] XC₆H₄SO₂C₁₀H₁₀H₇·AlCl₃+ HCl (ii) well expressed by the Yukawa–Tsuno equation, log(k_1^X/k_1^H)=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_3^X/k_3^H)=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 phenylsulphonylation.