In alkaline media,Fremy’s salt oxidizes alkanols by a hydrogen atom transfer mechanism

In aqueous alkali, Fremy’s salt (potassium nitrosodisulfonate dimer), homolyses nearly exclusively to the monomer radical anion, nitrosodisulfonate (NDS). In this media, NDS almost quantitatively oxidizes benzyl alcohol (PhCH₂OH) to benzaldehyde (PhCHO), itself being reduced to hydroxylamine disulfonate (HNDS). The reaction is very nearly first-order in NDS], alkanol] and in OH⁻]. However, with progressive addition of HNDS, decay kinetics of NDS gradually deviates from first-order. Ultimately, with sufficient excess of HNDS, the reaction becomes second-order in NDS]. The consumption ratio, (ΔPhCH₂OH]/ΔNDS]), is ∼2. PhCD₂OH manifests a large primary kinetic isotope effect (k_H/k_D = 11.6). Substituted benzyl alcohols (RBzCH₂OH) with R-groups withdrawing electron density from the O–H bond accelerated the reaction; those with R-groups donating electron density to the O–H bond retarded the reaction. The conversion of 2-propanol to 2-propanone is much slower compared to that of benzyl alcohol to benzaldehyde. An alpha-H atom transfer mechanism seems logical.