Nitrosation of N-methylhydroxylamine by nitroprusside. A kinetic and mechanistic study.
The kinetics of the reaction between aqueous solutions of Na2[Fe(CN)5NO].2H2O (sodium pentacyanonitrosylferrate(II), nitroprusside, SNP) and MeN(H)OH (N-methylhydroxylamine, MeHA) has been studied by means of UV-vis spectroscopy, using complementary solution techniques: FTIR/ATR, EPR, mass spectrometry and isotopic labeling (15NO), in the pH range 7.1-9.3, I=1 M (NaCl). The main products were N-methyl-N-nitrosohydroxylamine (MeN(NO)OH) and [Fe(CN)5H2O]3-, characterized as the [Fe(CN)5(pyCONH2)]3- complex (pyCONH2=isonicotinamide). No reaction occurred with Me2NOH (N,N-dimethylhydroxylamine, Me2HA) as nucleophile. The rate law was: R=kexp [Fe(CN)5NO2-]x[MeN(H)OH]x[OH-], with kexp=1.6+/-0.2x10(5) M(-2) s(-1), at 25.0 degrees C, and DeltaH#=34+/-3 kJ mol(-1), DeltaS#=-32+/-11 J K(-1) mol(-1), at pH 8.0. The proposed mechanism involves the formation of a precursor associative complex between SNP and MeHA, followed by an OH--assisted reversible formation of a deprotonated adduct, [Fe(CN)5(N(O)NMeOH)]3-, and rapid dissociation of MeN(NO)OH. In excess SNP, the precursor complex reacts through a competitive one-electron-transfer path, forming the [Fe(CN)5NO]3- ion with slow production of small quantities of N2O. The stoichiometry and mechanism of the main adduct-formation path are similar to those previously reported for hydroxylamine (HA) and related nucleophiles. The nitrosated product, MeN(NO)OH, decomposes thermally at physiological temperatures, slowly yielding NO.
Gutiérrez
María Marta
MM
Department of Chemistry, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, Funes y Roca, Mar del Plata, B7602AYL, Argentina.
Alluisetti
Graciela Beatriz
GB
Olabe
José Antonio
JA
Amorebieta
Valentín Tomás
VT
D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D006863:Hydrogen-Ion Concentration; D006898:Hydroxylamines / Q000737:chemistry*; D007700:Kinetics; D013058:Mass Spectrometry / Q000379:methods; D008956:Models, Chemical; D009599:Nitroprusside / Q000737:chemistry*; D015538:Nitrosation; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D013997:Time Factors
D006898:Hydroxylamines; D009599:Nitroprusside; C018338:N-methylhydroxylamine