# Sulfonic Resins as Catalysts for the Oxidation of Alcohols with H\({}_{2}\)O\({}_{2}\)/KBr
## 2 Results and Discussion
In fact, homogeneous oxidation has been also described using the much weaker acetic acid as both acid and solvent ([PERSON], 2016), showing that the acid strength is not an important parameter in this case. The kinetic effect of this resin must be related to the accessibility of the sulfonic sites, which seems to be slower in the case of the macroreticular Amberlyst 15; meanwhile, the gel-type resin is efficiently swelled in the biphasic CH\({}_{2}\)Cl\({}_{2}\)/H\({}_{2}\)O reaction medium, leading to the improved diffusion of the reagents. A similar effect had been previously observed in the case of esterification reactions ([PERSON], 2016), where the gel-type resins were more efficient than the macroreticular ones. The amount of catalyst is very important, as shown by the poor result obtained with only 10 mol% (entry 7). In an attempt to use a single solvent reaction medium, the source of bromide was changed from KBr to tetrabutylammonium bromide (TBAB). The oxidation reaction was tested with Dowex 50W\(\times\)8 under exactly the same reaction conditions (20 mol% bromide, 20 mol% sulfonic sites, 1.5 eq 60% H\({}_{2}\)O\({}_{2}\), r.t.), but only CH\({}_{2}\)Cl\({}_{2}\) was used as the solvent. Much worse results were obtained (entry 10), with only 26% conversion after 24 h (total conversion with KBr) and a maximum of 44% after 48 h. The use of the biphasic CH\({}_{2}\)Cl\({}_{2}\)/H\({}_{2}\)O reaction medium with TBAB led to even worse results (only 16% conversion after 48 h, entry 11), and hence the inorganic KBr was kept as the optimal bromide source. The importance of the nature of the bromide confirms the need for this species, as shown in the studies conducted in the homogeneous phase ([PERSON], 2016). In fact, the proposed mechanism suggests that oxidized bromine species (hypobromite and/or molecular bromine/radical) are responsible for the alcohol oxidation; these are generated by a reaction with H\({}_{2}\)O\({}_{2}\) in combination with the sulfonic catalyst. This hypothesis is confirmed by the described oxidation of alcohols with molecular bromine, which can be substituted by the H\({}_{2}\)O\({}_{2}\)/HBr system ([PERSON], 2016). Moreover, recent DFT calculations using the related bromide/PhIO system point to BrO\({}^{-}\) species being responsible for the oxidation of the alcohols ([PERSON], 2016). In Scheme 2, this proposed mechanism has been adapted to the heterogeneous reaction and one crucial detail is that the three components, H\({}_{2}\)O\({}_{2}\), the sulfonic group and bromide, seem to react simultaneously; hence, the diffusion of both H\({}_{2}\)O\({}_{2}\) and bromide from the solution to the sulfonic sites is crucial. This requirement would explain the poor result obtained with TBAB, given that it would be present mainly in the organic phase, whereas the hydrogen peroxide would be in the aqueous phase. The good result with KBr indicates that the diffusion from the aqueous phase to the sulfonic sites is favored (the hydrophilic nature of the resins is demonstrated by the high water content in the commercial samples, see Section 3) and that the presence of both reagents in different phases is clearly detrimental. In contrast with the case of the hydrolysis of epoxides [8], in this case, the swollen capacity of the resin does not seem to have a so crucial role, as both macroreticular Amherlyst 15 and gel-type Dowex 50 W behave in a similar way; furthermore, the fluorinated sulfonic resins are less efficient, despite their swelling ability in the reaction medium (see Supporting Material). The more hydrophobic character of the perfluoroalkyl chains may favor the concentration of the organic phase and hamper the diffusion of the aqueous reagents to the sulfonic sites, at least in the inner part of the particles. The recoverability and reusability of the resins were studied in the case of Amherlyst 15 and Dowex 50W\(\times\)8 (Table 2).