AbstractAn exploratory study has been made on the SO3 sulfonation of a number of alkylarenes in [D2]dichloromethane as an aprotic solvent, mostly in the presence of [D8]1,4‐dioxane as a reactivity moderator, in order to obtain information on sulfonic acid and sulfonic anhydride formation. 1,2‐Dimethylbenzene (1) with 1.0 mol‐equiv. of SO3 yields predominantly the 4‐sulfonic acid (1‐4‐S), whereas with 4.0 mol‐equiv. of SO3, 18 mol‐% of the corresponding (intermolecular) sulfonic anhydride (1‐4‐SO2)2O is additionally formed. 1,4‐Dimethylbenzene (2) with 0.8 mol‐equiv. of SO3 yields predominantly 2‐2‐S, whereas with 4.0 mol‐equiv. of SO3 the sulfonic anhydride (2‐2‐SO2)2O and its 6‐S and 6,6′‐S2 derivatives are formed, the eventual main product being (6‐S‐2‐2‐SO2)2O. Reaction of 1‐ethyl‐4‐methylbenzene (3) with 2.0 mol‐equiv. of SO3‐[D8]dioxane at −78 → −40 °C leads to the predominant formation of the 2‐ and 3‐S derivatives in a molar ratio of 26:74, with small amounts of the three corresponding intermolecular anhydrides. Upon performing the sulfonation of 3 at 0 → 22 °C, the relative amounts of the anhydrides are larger, and in addition the 3,5‐S2 derivatives of the intermolecular anhydrides are formed. The absence of the isomeric 2,6‐S2 derivative is ascribed to the buttressing effect of the 2‐sulfo on the 1‐ethyl group, hampering the sulfonation at C(6). Reaction of diphenylmethane (4) with 4.2 mol‐equiv. of SO3 at −20 → 25 °C yields initially 4‐4′‐S and subsequently 4‐4′,4″‐S2. Sulfonation of 1,2‐diphenylethane (5) with 4.0 mol‐equiv. of SO3 at −78 → 22 °C for 4.5 h leads mainly to the cyclic dimeric disulfonic anhydride [5‐4′,4″(‐SO2)2O]2, which upon hydrolysis affords 9% of 5‐2′,4″‐ and 91% of 5‐4′,4″‐S2. However, sulfonation of 5 at 0 → 22 °C for ca. 0.8 h was found to lead to the formation of the intermolecular sulfonic anhydride (4′‐S‐5‐4″‐SO2)2O. On sulfonation of 1,2‐di‐p‐tolylethane (6) with 1.0 mol‐equiv. of SO3, only ca. 20% of the substrate was sulfonated to give, after hydrolysis, 3% of the 2′‐ and 12% of the 3′‐sulfonate, whereas with 4.0 mol‐equiv. of SO3 the initial monosulfonic acids are almost completely converted to yield the 2′,3″,5″‐S3, 3′,3″,5″‐S3, and 3′,3″,5′,5″‐S4.