Aromatic sulfonation 28: Degree ofortho‐substitution in the sulfonation of toluene with sulfur trioxide, methanepyrosulfonic acid and fuming sulfuric acid. A mechanistic picture

Abstract

AbstractThe reaction of neat toluene with a number of different sulfonation reagents at 25° has been studied. With sulfur trioxide vapour as reagent, the reaction leads to monosulfonation only. Theo/(o + m + p)ratio varies with the degree of toluene conversion, the limiting values at zero and high conversion being 0.34 ± 0.03 and 0.11 ± 0.01, respectively. The former value is ascribed to the sulfonation of toluene with sulfur trioxide, the latter to sulfonation in the acid phase with toluenepyrosulfonic acid as the sulfonating entity.The heterogeneous reaction of toluene with a deficient amount of a matured mixture of methanesulfonic acid and sulfur trioxide ‐ i.e. with a methanepyrosulfonic acid reagent — leads to monosulfonation only. Theo/(o + m + p)ratio varies from 0.15 ± 0.01 at zero toluene and zero time conversion to 0.11 ± 0.01 at high toluene conversion. The former value is ascribed to sulfonation in the acid phase of dissolved toluene by methanepyrosylfonic acid, the latter to that by both toluene‐ and methane‐pyrosulfonic acids.Heterogeneous sulfonation with a deficient amount of fuming sulfuric acid takes place mainly ( ∼97 %) in the acid phase. Sulfonation in the toluene phase leads to monosulfonic acids only with 41 ± 2 % ofortho‐substitution. Sulfonation in the sulfuric acid phase yields monosulfonic acids and, at the higher sulfuric acid concentrations, also toluene‐2,4‐disulfonic acid. The degree ofortho‐substitution for sulfonation in the acid phase at 25° changes from 48.5 to 40 % on varying the acid concentration from 100.0 to 106.4% H2SO4.