Intriguing salt effects in the formation of hydroxamic acids from aldehydes and nitroso compounds

Addition of inorganic chlorides and perchlorates into the reaction mixture in water leads to a significant increase of the rate constants observed for the formation of N-phenyl hydroxamic acids from nitrosobenzene and formald ehyde or acetaldehyde. The rate enhancements (relative to the reaction rate constant in pure water) vary from the 7.4-fold increase observed on additi on of 2.0 mol dm(-3) Mg(ClO4)(2) into the reaction mixture to the 230.0-fol d increase in the case of 2.0 mol dm(-3) of MgCl2 added. Linear correlation between log hobs for the formation of the N-phenylformohydroxamic acid fro m nitrosobenzene and formaldehyde and the concentration of the anion of the salt added has been observed for MgCl2, CaCl2, LiCl, Mg(ClO4)(2) and NaClO 4, but different correlations were obtained for NaCl, HCl and HClO4. The ob servations are discussed in terms of ion pairing between the unstable catio n reaction intermediates and the anion of the salt. The results obtained su ggest that stabilization of the unstable cationic reaction intermediate by the formation of an ion pair with the salt anion is of key importance for t he formation of hydroxamate from aldehydes and nitrosobenzenes in more conc entrated inert salt water solutions.