Mechanism of the Selective Sulfide Oxidation Promoted by HNO₃/FeBr₃

Abstract

The oxidation of aryl methyl sulfides containing electron withdrawing and electron donating groups (p-NO(2), p-CHO, p-NCS, p-Br, H, p-CH(3), p-OCH(3)) was carried out in homogeneous solution in acetonitrile in the presence of catalytic amounts of HNO(3) and FeBr(3). The FeBr(3) is required for the reaction to proceed for compounds with strongly electron withdrawing groups (p-NO(2) and p-CHO) but is not necessary in the case of all the other compounds, although in the latter cases the yield decreases considerably. The rate of the reaction was measured as a function of substrate, FeBr(3) and HNO(3) concentration. From the experimental data a mechanism is suggested where there are two reaction pathways, one involving the formation of a ternary complex between the substrate, FeBr(3) and NO(3)(-) and one involving a complex formed between the sulfide and the HNO(3). From these complexes HNO(2) is generated, which then combines with HNO(3) to yield N(2)O(4), initiating a catalytic cycle where the sulfide is oxidized and O(2) from the air is stoichiometrically consumed.