Ionic liquids (LI) are proposed as an alternative to conventional organic solvents. Among them, there is a subgroup called “task specific ionic liquid” (TSIL), in which a functional group is covalently attached to the cation or to the anion. This is allows to “direct” towards a specific action its chemical properties in a reactive system. In recent years, the 1- (4-sulfonic acid) -3-methyl butylimidazolium [bsmim] with bisulfate counterion [HSO4] is one of the most studied TSIL. This work aims to contribute to a better comprehension of the relationship between the structure of this TSIL and their catalytic behavior. The objective is to identify which part of the structure of the TSIL is responsible for the catalytic action observed in specific reactive systems and to understand how the addition of a functional group such as -SO3H influences on the structure and therefore its behavior. The 1-butyl-3-methylimidazolium [bmim] was also synthesized with the same counterion. Both LIs were tested in two reactive systems: esterification reactions and synthesis of benzoxanthones. The yields obtained were compared with respect to the action of sulfuric acid. Furthermore, cyclic voltammetry was used to identify the species present in the ILs. The results indicate that TSIL show an improved catalytic activity in both types of reactions in comparison with bmim HSO4. The relationship between the voltammetric results and the final performance allowed to interpret the behavior of these LIs. The IL's synthesis and purification method influence on the species present which condition the results on the reactive system.