Relative rates of sulfonation of linear and branched long‐chain alkylbenzenes

Abstract

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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Optimisation of the Linear Alkyl Benzene Sulfonation Process for Surfactant Manufacture

“…Comparing initial LAB composition with composition of the unconverted LAB in the sulfonated product mixture, Cohen et al , observed that the proportion of higher-molecular weight LAB homologues was higher in the recovered LAB after sulfonation than in the initial LAB and that the proportion of 2-phenyl alkanes was higher in the initial LAB than in the recovered LAB. Subsequent work at the University of Wales, Swansea, both with commercial LAB mixtures and with simpler mixtures of model 1-phenyl alkanes of different chain lengths confirmed and quantified the trends:…”