Abstract
The reaction of the three tetramethylbenzenedisulfonic acids and some lower homologues in concentrated aqueous sulfuric acid has been studied at 25deg;C. 3,4,5,6‐Tetramethylbenzene‐1,2‐disulfonic acid undergoes both intramolecular disulfonic anhydride formation (the rate of which strongly increases upon increasing the sulfuric acid acidity) and protiodesulfonation (the rate of which decreases slightly with increasing acidity) while the other disulfonic acids only undergo protiodesulfonation. The observed rate order for protiodesulfonation within the tetramethylbenzenedisulfonic acids [1,2‐ > 1,3>> 1,4‐(SO~3~H)~2~] differs strongly from that predicted on the basis of the additivity principle of substituent effects [1,3>>;1,4‐ > 1,2‐(SO~3~H)~2~]. The strongly enhanced reactivity of the 1,2‐disulfonic acid is ascribed to a greater relief of steric strain in the 1,2‐than in the other disulfonic acids, the sulfo group being more bulky than the methyl. Also, within the group of tri‐ and tetra‐methylbenzene‐1,3‐disulfonic acids, the rate of desulfonation increases with an increase in the degree of overcrowding around the sulfo group which is desulfonating. Mechanisms for the two types of reaction of the tetramethylbenzenedisulfonic acids are proposed, based on the variation of the rate constants with the sulfuric acid acidity. It is concluded that the substrate entity undergoing protiodesulfonation is the disulfonate dianion and further that the entity undergoing cyclization to the 1,2‐disulfonic anhydride is the monoprotonated 1,2‐disulfonic acid.