Several SO3H-functionalized ionic liquids (Fits) were synthesized and their catalytic performances for catechol (CAT) alkylation with tert-butyl alcohol (TBA) were studied theoretically as well as experimentally. Under optimized reaction conditions, the conversion of CAT was 41.5%, and the selectivity for 4-tert-butyl catechol (4-TBC) could reach 97.1%. This electrophilic reaction exhibits a kinetics-dependent character. The higher frontier electron density at the C-4 site, comparatively lower activation energy barrier (E-a), and higher thermodynamic stability all act in concert to make the reaction proceed preferentially toward 4-TBC. The FILs can lead to a much higher CAT conversion and a similar selectivity to 4-TBC compared to a Beta zeolite catalyst. Because the frontier electron density [f(r)(E)] of CAT was not significantly affected by the solvent, the product selectivity rule would not vary, whereas the addition of a solvent with a higher dielectric constant would slow the reaction by increasing E-a, thus resulting in a lower CAT conversion.