Aromatic Sulfonation 26: Kinetics and mechanism of the sulfonation of some polyfluorobenzenes in fuming sulfuric acid

Abstract

The mechanism of sulfonation in oleum proposed by Brand et af. -stepwise attachment of so3 and H+ to the substrate, followed by the loss of the proton from the benzene ring -is shown to be inconsistent with the substrate isotope effects reported by these authors. The sulfonation kinetics of p-di-, 1,2,3,4-tetra-, and penta-fluorobenzene in 97-109 % sulfuric acid are discussed in terms of an extrapolation of the mechanism proposed recently for aqueous sulfuric acid. In weak oleum -below 104% sulfuric acid -the rate-limiting step is \ IntroductionThe kinetics of aromatic sulfonation in oleum have been studied by Brand et al. l-3 and Cerfontain 475. The sulfonation rates of trimethylphenylammonium ions 1~2 and of the un-ionized forms of nitrobenzenes 1,

Experimental and molecular modelling studies on aromatic sulfonation

“…The results for toluene effectively eliminate the possibility of a concerted mechanism, C, for the sulfonation as a substantial kinetic isotope effect would be expected because the rate controlling step of this reaction directly involves the cleavage of the C4Ј-H4Ј bond (Scheme 4). It seems likely therefore that the sulfonation reaction proceeds via the stepwise mechanisms A or B (Scheme 4) but as theoretical calculations on a template for the postulated intermediate (12) failed to locate a stationary point on the potential energy surface, with the molecule either dissociating back to toluene (1) and acetylsulfonic acid (9) or breaking to produce toluenesulfonic acid (6) and acetic acid (13), in a similar way to toluenepyrosulfonate intermediate (8), mechanism B appears to be favoured. This is supported by the significant increase in sulfonation rate observed for the trimethylacetylsulfonic acid (10) versus the two other sulfonic acid ( 9) and (11), which can be attributed to steric effects with the former containing the trimethylacetyl group which behaves as a much better leaving group.…”

“…1 The mechanism of the process is complex, reversible in many cases, and depends mainly on (1) the nature of the electrophilic species, which may be derived from sulfuric acid, pyrosulfuric acid, chlorosulfonic acid, acetylsulfonic acid or sulfur trioxide; and (2) the reactivity of the aromatic system under attack. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] At low temperatures, the reverse reaction is very slow, and the forward reaction is essentially irreversible. Several transient intermediate species are thought to be involved but few have been properly characterized.…”

“…Molecular energies of the reactants, intermediates and products(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) involved in the sulfonation of toluene calculated at the ab initio 4-31G/S* level…”