Geoffrey Stedman

Kinetics, mechanism, and stoicheiometry of the oxidation of hydroxylamine by nitric acid

Hydroxylamine is oxidised by nitric acid to form dinitrogen monoxide and nitrous acid, the proportions varying with reaction conditions. The yield [HNO2]∞/[NH3OH+]0 is a maximum at ca. 4–5 mol dm–3 HNO3, and is also a function of the hydroxylamine concentration. In 5 mol dm–3 HNO3 the limiting yield is ca. 0.85 at very low initial hydroxylamine concentrations, but decreases towards zero at higher values of [NH3OH+]0. Reaction is only observed at sufficiently high nitric acid concentrations; at 25°C the cut-off point is ca. 2.5 mol dm–3 HNO3. The reaction is characterised by an induction period, followed by a rapid autocatalytic process. Addition of nitrite eliminates the induction period, while addition of nitrite scavengers completely prevents any reaction. Nitrous acid is an essential catalyst for the reaction, and the initial rate of reaction obeys the equation d[HNO2]/dt=V0=K[HNO2][NH3OH+]. Isotopic experiments, using 15N-enriched hydroxylamine show that virtually all of the N2O arises from reaction between HNO2 and hydroxylamine. The mechanism suggested involves oxidation of unprotonated hydroxylamine by N2O4 to form the nitroxyl diradical HNO; this is then further oxidised to HNO2, which reacts with hydroxylamine to form N2O.

Electrophilic nitrosation at sulphur and nitrogen in thiourea

Thiourea reacts rapidly with nitrous acid in aqueous solution to form an equilibrium concentration of a coloured S-nitroso-compound. The equilibrium constant has been measured, together with a preliminary value of the rate constant for the nitrosation. The kinetics of the much slower reaction to form nitrogen and thiocyanic acid have been investigated. The mechanism proposed is a rate-determining sulphur-to-nitrogen migration of the nitroso-group in the conjugate base of the S-nitroso-compound.

FORMATION OF AN ADDUCT BETWEEN THIOCYANATE ION AND NITROSYL THIOCYANATE

Nitrosyl thiocyanate reacted with sufficiently high concentrations of thiocyanate ion to form an adduct, [ON(SCN) 2 ] - , which absorbs strongly in the UV region. This species has different properties to that species described in the literature as [NO(SCN) 2 ] - formed in pulse-radiolysis experiments involving (SCN) 2 - and NO. The difference probably arises from different structures, an S-nitroso as compared to an N-nitroso compound.

Kinetics and equilibria in the nitric acid–nitrous acid–sodium thiocyanate system

The nature of the red species formed in acidic solutions containing nitrous acid and thiocyanate ion has been confirmed to be ONSCN. The alternative literature interpretation in terms of ONSCNH+ arises from neglect of activity coefficients and partial protonation of SCN– to HNCS in moderately concentrated mineral acid. The kinetics of the nitrous acid-catalysed oxidation of thiocyanate in moderately concentrated nitric acid is controlled by rate-determining formation of N2O4 at even modest concentrations of thiocyanate. Literature work on this system is discussed.

The reaction of nitrogen dioxide with iron(III) cyanide complexes

The kinetics of reaction of [FeIII2(CN)10]4− with nitric and nitrous acids has been shown to fit the following rate law: rate = k h12[HNO2]12[nitrate]12[FeIII2(CN)2−10] The primary product appears to be a binuclear FeIII, FeII cyanide complexes with an NO+ ligand in the coordination shell of the ferrous ion. The kinetics of reaction and the properties of the product show a close resemblance to those of the reaction [FeIII(CN)6]3− with nitric and nitrous acids follow a similar rate law, though the dependence of rate upon acidity and nitrate concentration has not been determined. Rate = k[HNO2]12[complex] It is concluded that the reaction of iron(III) cyanide complexes with nitric and nitrous acids to form iron(II) cyano nitrosyl products occurs through NO•2 as the active nitrogen species.

Theoretical Estimates of Association Constants for Contact Triple-Ion Formation

Abstract A formula for the calculation of formation constants for ion-triplets in contact, KIT, is derived on the basis of a thermodynamic argument. This approach also yields an expression for the association constants for ion-pairs, ΚIP, within the same approximation, leading to a modification of the familiar Fuoss equation. The formula for K1T is compared with literature data for transition metal cyano complexes.

Kinetic study of the stability of (NH2)2CSSC(NH2)22

The kinetics of the decomposition of formamidine disulfide dihydrochloride, (NH2)2CSSC(NH2)22+ 2Cl–, in aqueous solution at 25 °C are general base catalysed, and rate constants have been measured over the pH range 1.91–9.03. Analysis of the data yields ionisation constants pK1= 5.49, pK2= 7.66 and rate constants for deprotonation of the substrate by OH– and H2O. For the doubly charged cation, reaction occurs through a small concentration of a reactive tautomer, (HN)(NH3+)CSSC+(NH2)2, which is attacked by H2O. The singly charged cation reacts by parallel pathways involving OH– and H2O as attacking bases.

Kinetics and mechanism of the reaction of nitrous acid with 2,4-dinitrophenylhydrazine

Arylhydrazines react with excess nitrous acid to form mixtures of the diazonium ion and aryl azide. By use of the weakly basic 2,4-dinitrophenylhydrazine (DNP), pKa= 1.55, information about the mechanism of the diazotisation reaction has been obtained. Three successive stages can be observed. The initial reaction consists of parallel nitrosations by N2O3 and NO+ of the free base DNP at the terminal nitrogen. This is followed by a stage with the rate independent of [HNO2], almost certainly a tautomerisation, probably to form ArNNNHOH and ArNHNNOH. The former is converted to ArN3 while the latter undergoes a further electrophilic nitrosation by NO+ of a conjugate base species to yield a di-N,N′-nitrosoarylhydrazine, a precursor to the diazonium ion.

Kinetic and product study of the reaction between nitrous acid and hydrazine

The reaction between nitrous acid and hydrazine is a normal N-nitrosation reaction, and does not involve oxidation at NH2NH2 to N2H2 as previously postulated. The N-nitrosohydrazine decomposes by two parallel routes, to HN3 at high acidities and to ammonia and N2O at low acidities. An intermediate in the decomposition has been detected spectrophotometrically. Revised values are provided for the relative reactivity of some nucleophiles to nitrosating agents, and the relation between reactivity and charge in nitrosation reactions is discussed.

Mechanism of the acid catalysed pathway for the nitrosation of hydroxylamine

The profile of rate constant versus[H+] for the acid catalysed pathway of the nitrous acid reaction with the hydroxyammonium ion can be qualitatively interpreted in terms of a reversible O-nitrosation, followed by an O to N migration of the nitroso group in the conjugate base species NH2ONO. There is no need to assume a non-steady-state mechanism involving NO+, as has been proposed previously.