The cleavage of di-p-tolyl sulfone with concentrated sulfuric acid has been shown to occur at elevated temperatures (>120°C). The products were confirmed as the corresponding sulfonic acids by gas chromatography-mass spectrometry analysis of the sulfonate ester derivatives. The cleavage reactions of a series of di-p-substituted diaryl sulfones with sulfuric acid have been studied. A method using a double endpoint titration has been used to quantify the amounts of sulfonic acid formed as the reactions proceed. These data have been used to construct second-order rate plots over a range of temperatures and to determine rate constants for the cleavage reactions. The second-order rate plots showed good linearity. Activation energies were determined from Arrhenius plots of the second-order rate constants over a range of temperatures. The presence of electron-donating groups decreases the activation energy of the reaction, whereas the presence of electron-withdrawing groups increases the activation energy of the reaction.
Relative rates of sulfonation of a series of 1-phenylalkanes and isomeric branched-chain alkylbenzenes by gaseous SO 3 have been determined. It has been shown that the composition of the mixture undergoing sulfonation has an important bearing on the actual relative rate constant (k rel ) values obtained. This is taken as evidence for the involvement of pyrosulfonic acids as the actual sulfonating species involved in the sulfonation reaction. Consistent k rel values have been obtained by using an "active solvent" in 10-fold excess as the sulfonating medium. Whereas the branched-chain isomers show very little variation in their rates of sulfonation, the rates of sulfonation of the 1-phenylalkanes increase as the length of the alkyl chain decreases.The commonly accepted mechanism for aromatic sulfonation involves addition of SO 3 to form a zwitterionic Wheland intermediate 1, as illustrated in Scheme 1 for toluene sulfonation. Recent ab initio calculations carried out in these laboratories for toluene sulfonation (1) estimate the enthalpy of the Wheland intermediate to be ca. 20 kcal mol −1 higher than the combined enthalpies of toluene and SO 3 . It follows that for this mechanism the activation energy must be at least 20 kcal mol −1 . However, this activation energy is inconsistent with the fact that in commercial sulfonation of alkylbenzenes the reaction proceeds to ≥90% conversion in a falling film reactor with a residence time of ca. 30 s and an average temperature well below 100°C (2).An alternative reaction pathway involves formation of a 2:1 SO 3 /substrate π-complex (2), which can then undergo rearrangement to a "pyrosulfonic acid Wheland intermediate" 2, stabilized by intramolecular hydrogen bonding (Scheme 2). The activation energy in this case should be much lower than for the pathway via 1 since it avoids the high-energy Wheland intermediate 1. However, the pathway via the 2:1 π-complex is kinetically third-order and, by analogy with other known third-order reactions (3), it is expected to have a very low preexponential factor. Consequently, although this reaction pathway is plausible as an initiation or "priming" process, it is not consistent with the high reaction rate observed in commercial sulfonation.A third possibility (2) is that, following "priming" by a pathway such as that shown in Scheme 2, the reaction proceeds via the pyrosulfonic acid as a continually regenerated sulfonating agent (Scheme 3). The computed enthalpy changes (1) for the steps in this pathway are consistent with a rapid reaction, with k 2 being the most likely rate-determining step.These different reaction pathways have different implications regarding the relative reactivities of alkylbenzenes in sulfonation.In an earlier paper, we reported an investigation of the relative rates of sulfonation of a series of linear alkylbenzenes, involving competition reactions carried out on a multicomponent commercial mixture (4). In the commercial
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