HA-L + FeCl4 -→ [A-L] -+ HCl + FeCl3. (6) For the HBr/FeBr3 pair, ΔG°/ΔH° = -2.9/-3.2 kcal•mol -1 , which is consistent with our experimental results suggesting this pair to be more reactive. However, this value is about half as much as one would expect based on the relative pKa values of HCl and HBr in DCE, (ΔpKa[DCE], HBr-HCl = 4.5). The other two combinations are predicted to be less reactive than HCl/FeCl3, where the CF3COOH/FeCl3 and HF/BF3 combinations give ΔG°/ΔH° = 8.4/16.8 and 7.5/7.1 kcal•mol -1 , respectively. The former case is consistent with experimental results from Table (entry 2) showing no product formation with the CF3COOH/FeCl3 pair. The use of HBF4 (2.5 mol%) as the catalyst resulted in only trace product formation (<5% by 1 H NMR analysis). These results suggest that the pairing of a Lewis acid with a Brønsted acid generally increases the Brønsted acidity significantly in organic media and that careful choice of the pairing could provide a level of control over the overall reactivity of the pair.