# Sulfonic Resins as Catalysts for the Oxidation of Alcohols with H\({}_{2}\)O\({}_{2}\)/KBr
## 2 Results and Discussion
In the second reaction with Amherlyst 15 (entry 2), the maximum conversion was only 63%, and the important attrition of the resin, due to its poor mechanical strength, made it impossible to recover the resin after the second run. Dowex 50W\(\times\)8 showed the same loss of conversion in the second run (entry 4), but its better mechanical properties allowed the recovery of the resin after further runs. The drop in conversion was ascribed to the exchange of sulfonic acid with KBr, leading to -SO\({}_{3}\)K and HBr (Scheme 3), an equilibrium already proposed in the mechanism of the homogeneous phase [24]. Despite being disfavored due to the pK\({}_{\text{a}}\) difference [24], the filtration and separation of the sulfonic resin produce a cumulative effect, and as a consequence, the number of sulfonic sites in the successive runs is lower, with the already known effect on conversion (Table 1, entry 6 vs. entry 7). This was confirmed by the complete reactivation (entry 5) obtained by treatment with HCl at 80 \({}^{\circ}\)C for 3 h, a method able to reverse the equilibrium from -SO\({}_{3}\)K to the acidic -SO\({}_{3}\)H form (Scheme 3). Thus, the possibility of using a lower KBr/sulfonic molar ratio was tested with two experiments (Table 3). The reaction rate and final conversion depend on the amount of KBr, whereas the increase in the sulfonic amount has no effect, but it should be positive for the recycling of the resin. A deeper recycling study was carried out under these conditions (Figure 1). As can be seen, the excess of resin allows full recycling in the second run and the decrease in performance is observed in the third run. As shown above, the reactivation of the resin with HCl allows the recovery of the performance in the fourth and a fifth runs, with another drop in the conversion in the sixth run. The second reactivation is less efficient than the first one, and the conversion declines faster, with a nearly inactive resin in the ninth run. This result seems to indicate a permanent deactivation mechanism, different from the cation exchange proposed to form potassium sulfonate. The analysis of the resin after nine runs shows a loss of 25% of the initial sulfur, due to a side reaction. The more important loss in activity might be due to the loss of the most accessible sulfonic groups, whereas the remaining ones are less available due to diffusion problems. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Entry** & **Sulfonic Solid** & **Run** & **Conversion (\%) 2** \\ \hline
1 & Amberlyst 15 & 1 & \textgreater{}99 \\
2 & & 2 & 63 \\
3 & Dowex 50W\(\times\)8 & 1 & \textgreater{}99 \\
4 & & 2 & 62 \\
5 & & 3 3  & \textgreater{}99 \\ \hline \hline \end{tabular} \({}^{1}\) Reaction conditions: 0.5 mmol of methyl 9(10)-hydroxystearate (**1a + 1b**), 0.1 mmol of KBr, 0.75 mmol of 60% H\({}_{2}\)O\({}_{2}\), 0.1 mmol of sulfonic groups, 1 mL of CH\({}_{2}\)Cl\({}_{2}\)/H\({}_{2}\)O (9:1 \(v/v\)), r.t., 24 h. 2 determined by gas chromatography 3 After the treatment of the resin with HCl at 80 \({}^{\circ}\)C for 3 h.  
\end{table} 
Table 2: Recoverability and reusability of the resins Amberlyst 15 and Dowex 50W\(\times\)8 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Entry** & **mol\% Dowex 50W\(\times\)8** & **mol\% KBr** & **t (h)** & **Conversion (\%) 2** \\ \hline
1 & 20 & 20 & 6 & 75 \\  & & & 24 & \textgreater{}99 \\
2 & 20 & 10 & 24 & 76 \\  & & & 48 & 86 \\
3 & 40 & 20 & 6 & 76 \\  & & & 24 & \textgreater{}99 \\ \hline \hline \end{tabular} \({}^{1}\) Reaction conditions: 0.5 mmol of methyl 9(10)-hydroxystearate (**1a + 1b**), Dowex 50W\(\times\)8, KBr, 0.75 mmol of 60% H\({}_{2}\)O\({}_{2}\), 1 mL of CH\({}_{2}\)Cl\({}_{2}\)/H\({}_{2}\)O (9:1 \(v/v\)), rt. 2 determined by gas chromatography.  
\end{table} 
Table 3: Variation in the KBr/sulfonic ratio in the oxidation of methyl 9(10)-hydroxystearate (**1a + 1b**) 1.