Kinetics and mechanism of the comproportionation of hypothiocyanous acid and thiocyanate to give thiocyanogen in acidic aqueous solution

The kinetics of comproportionation of hypothiocyanous acid (HOSCN) and thiocyanate (SCN-) to give thiocyanogen ((SCN)2) in acidic aqueous solutions have been determined by double-mixing stopped-flow UV spectroscopy. Hypothiocyanite (OSCN-) was generated at pH 13 by oxidation of excess SCN- with hypobromite (OBr-), followed by a pH jump to acidic conditions ([H+] = 0.20-0.46 M). The observed pseudo-first-order rate constants exhibit first-order dependencies on [H+] and [SCN-] with overall third-order kinetics. The corresponding kinetics of hydrolysis of (SCN)2 have also been examined. Under conditions of high (and constant) [H+] and [SCN-], the kinetics exhibit second-order behavior with respect to [(SCN)2] and complex inverse dependences on [H+] and [SCN-]. Under conditions of low [H+] and [SCN-], the kinetics exhibit first-order behavior with respect to [(SCN)2] and independence with respect to [H+] and [SCN-]. We attribute this behavior to a shift in the rate-limiting step from disproportionation of HOSCN (second-order dependency on [(SCN)2]) to rate-limiting hydrolysis (first-order dependency on [(SCN)2]). Thus, we have determined the following equilibrium constant by the kinetic method: (SCN)2 + H2O HOSCN + SCN- + H+; Khyd = [HOSCN][SCN-][H+]/[(SCN)2] = khyd/kcomp = 19.8(+/-0.7) s-1/ 5.14(+/-0.07) x 103 M-2 s-1 = 3.9 x 10-3 M2.     

Oxidative decomposition of pentachlorophenol in aqueous solution

OH-radical-induced dechlorination of pentachlorophenol (PCP) has been studies pulse and γ-radiolytically. OH radicals react with PCP by both electron transfer (53%) and addition followed by very rapid HCl-elimination to form phenoxyl radicals. The phenoxyl radicals decay to form products (e.g. chloranil) that unstable in alkaline aqueous solution and release some more Cl⁻, therefore G(Cl⁻) is high. Primary HPLC–MS analysis reveals that some quinones among the final products, whose toxicity remains unclear. Ozone can also oxidize PCP very rapidly, and this oxidation may destroy the benzene ring of PCP.     

Radiation-induced decomposition of cyclotrimethylenetrinitramine in aqueous solutions

Radiation-induced decomposition of cyclotrimethylenetrinitramine, a representative of heterocyclic nitramines, in dilute aqueous solutions saturated with argon, air, and N₂O has been investigated. Dose dependence curves have been obtained, and the initial yields of formation of nitrite ions and hydrogen peroxide have been measured, making up 0.122 and 0.049 in deaerated solutions, 0.050 and 0.065 in aerated solutions, and 0 and 0.074 μmol/J in N₂O-saturated solutions, respectively. The main radiolysis initial steps involve the reaction of dissociative electron capture with nitro-group elimination in the form of nitrite ion and the reaction of hydrogen atom abstraction from the methylene group by hydroxyl radicals and atomic hydrogen.     

Experimental study of decomposition of aqueous nitrosyl thiocyanate

This study has examined the kinetics of the decomposition of nitrosyl thiocyanate (ONSCN) by stopped flow UV-vis spectrophotometry, with the reaction products identified and quantified by infrared spectroscopy, membrane inlet mass spectrometry, ion chromatography, and CN(-) ion selective electrode. The reaction results in the formation of nitric oxide and thiocyanogen, the latter decomposing to sulfate and hydrogen cyanide in aqueous solution. The rate of consumption of ONSCN depends strongly on the concentration of SCN(-) ions and is inhibited by nitric oxide. We have developed a reaction mechanism that comprises three parallel pathways for the decomposition of ONSCN. At high thiocyanate concentrations, two reaction pathways operate including a second order reaction to generate NO and (SCN)(2) and a reversible reaction between ONSCN and SCN(-) producing NO and (SCN)(2)(-), with the rate limiting step corresponding to the consumption of (SCN)(2)(-) by reaction with ONSCN. The third reaction pathway, which becomes significant at low thiocyanate concentrations, involves formation of a previously unreported species, ONOSCN, via a reaction between ONSCN and HOSCN, the latter constituting an intermediate in the hydrolysis of (SCN)(2). ONOSCN contributes to the formation of NO via homolysis of the O-NO bond and subsequent dimerization and hydrolysis of OSCN. Fitting the chemical reactions of the model to the experimental measurements, which covered a wide range of reactant concentrations, afforded estimation of all relevant kinetic parameters and provided an excellent match. The reaction mechanism developed in this contribution may be applied to predict the rates of NO formation from ONSCN during the synthesis of azo dyes, the gassing of explosive emulsions, or nitrosation reactions occurring in the human body.     

A new iron(II) thiocyanato thiocyanogen complex

The reaction between Fe³⁺ and SCN^?, in glacial acetic acid medium, gave a dark red solution which differs from the classical FeSCN²⁺, prepared in aqueous medium, with regard to stability and composition. In contrast to [FeSCN]²⁺ which undergoes photochemical decomposition and is bleached by light and reduced by hydroxylammonium chloride, the new complex is stable towards these agents, but it reacts with water. In addition to the charge transfer band at 480 nm exhibited by both solutions, the new complex exhibits a sharp strong band at 253 nm characteristic of the trithiocyanate ion, [SCN^? + NCSSCN]; thiocyanogen being produced by oxidation of SCN^? by Fe³⁺. The reaction with water is attributed to the hydrolysis of thiocyanogen mainly into sulphate. The presence of thiocyanogen in the new complex enhanced polymerixation and subsequent deposition of solid complex. On the basis of chemical, spectral, and thermal analyses the formula K₂[Fe(SCN)₄(NCSSCN)], is assigned to the new compound.     

Ozone decomposition in concentrated aqueous solutions of salts

Effect of high concentrations of electrolytes (Na₂SO₄, NaCl, and NaNO₃) on the rate of ozone decomposition in water was studied. The conversion kinetics of O₃ dissolved in these solutions was analyzed. The rate constants of ozone decomposition were determined.     

Thermal decomposition of formaldehyde diperoxide in aqueous solution

Thermal decomposition of formaldehyde diperoxide (1,2,4,5-tetraoxane) in aqueous solution with an initial concentration of 6.22 × 10^?3 M was studied in the temperatures range from 403 to 439 K. The reaction was found to follow first-order kinetic law, and formaldehyde was the major decomposition product. The activation parameters of the initial step of the reaction (ΔH ^≠ = 15.25 ± 0.5 kcal mol^?1, ΔS ^≠ = ?47.78 ± 0.4 cal mol^?1K^?1, E _a = 16.09 ± 0.5 kcal mol^?1) support a mechanism involving homolytic rupture of one peroxide bond in the 1,2,4,5-tetraoxane molecule with participation of the solvent and formation of a diradical intermediate.     

Photooxidative decomposition of sodium dodecyl sulfate in aqueous solutions

Kinetic features and the mechanism of photooxidative decomposition of sodium dodecyl sulfate in an aqueous solution under the action of pulsed short-wavelength UV light with a continuous spectrum upon introduction of hydrogen peroxide was studied.     

Kinetic model of thiourea dioxide decomposition in aqueous ammonia

The kinetics of the decomposition of thiourea dioxide in aqueous ammonia solution at various temperatures and ammonia concentrations has been investigated. The parameters of the individual steps of the process have been derived from experimental data by solving an inverse kinetic problem. The adequacy of the proposed kinetic model to the experimental data has been verified on the basis of the F-test and calculated correlation coefficients for the rate constants of the individual steps of the process.     

Kinetic features of ammonium persulfate decomposition in aqueous medium

Russian Chemical Bulletin - Decomposition of ammonium persulfate in an aqueous medium (353–368 K) is limited by decomposition of persulfate ions with a rate constant logk 2 =...     

Accurate rate constants for decomposition of aqueous nitrous acid

Decomposition of nitrous acid in aqueous solution has been studied by stopped flow spectrophotometry to resolve discrepancies in literature values for the rate constants of the decomposition reactions. Under the conditions employed, the rate-limiting reaction step comprises the hydrolysis of NO(2). A simplified rate law based on the known elementary reaction mechanism provides an excellent fit to the experimental data. The rate constant, 1.34 × 10(-6) M(-1) s(-1), is thought to be of higher accuracy than those in the literature as it does not depend on the rate of parallel reaction pathways or on the rate of interphase mass transfer of gaseous reaction products. The activation energy for the simplified rate law was established to be 107 kJ mol(-1). Quantum chemistry calculations indicate that the majority of the large activation energy results from the endothermic nature of the equilibrium 2HNO(2) ? NO + NO(2) + H(2)O. The rate constant for the reaction between nitrate ions and nitrous acid, which inhibits HNO(2) decomposition, was also determined.     

Thermal decomposition studies of aqueous and nitric solutions of hydroxyurea

Hydroxyurea and its derivatives are important nonsalt forming reductants in partitioning of uranium and plutonium in the nuclear fuel reprocessing operations. There is no experimental data available in open literature describing pressurization due to the thermal decomposition of aqueous and nitric solutions of hydroxyurea at elevated temperatures. Authors studied thermal decomposition of hydroxyurea-nitric acid system and resultant pressurization at various concentrations of nitric acid in an adiabatic calorimeter in closed-vent conditions. During these experiments, pressurization was observed. In this paper, results of these experiments have been discussed.     

Ebulliometric study of the decomposition of urea in aqueous media

Boiling-temperature measurements have been used to study the kinetics of the decomposition of urea. Values for the rate constants in neutral, acid, and basic media have been obtained and compared with literature data. The effect of the reverse reaction on the value of the experimental rate constants is discussed. The profile of the ebulliometric curves is different for the three media and is correlated with the number of various species present in solution. It is concluded that ebulliometric measurements can be useful for kinetic studies.     

Oxidative decomposition of oxalate ion with ozone in aqueous solution

The oxidation of oxalate ions with ozone in aqueous solution has been studied, and the effects of pH, temperature, and reactant concentrations on the reaction rate and efficiency have been estimated. The oxidative decomposition is most effective in alkaline medium (pH ≥ 10) at 50°C. Under these conditions, the consumption of ozone is 0.6±0.1 g per gram of oxalate or 1.1±0.1 mol per mole of oxalate, which corresponds to the stoichiometry (COO^–)₂ + O₃ + H₂O → 2CO₃^2– + O₂ + 2H⁺.     

A new copper(I) thiocyanato thiocyanogen complex

The reaction between Cu²⁺ and SCN^?, in glacial acetic acid at a Cu/SCN ratio, of 1:4, gave a dark red solution that exhibited an absorption band at 496 nm. The colour of the freshly prepared solution was bleached by water with concomitant production of SO₄^2? ion. On standing, the stability of the colour towards water is increased and the position of the absorption band shifts to a higher wavelength. It is inferred that the thiocyanogen produced on oxidation of SCN^? by Cu²⁺ is stable in the non-aqueous medium and is coordinated together with SCN^? in a Cu(I) complex. The bleaching action of water is attributed to the hydrolysis of thiocyanogen, mainly to SO₄^2?. The presence of thiocyanogen enhanced polymerization and subsequent deposition of an orange-brown solid complex. On the basis of chemical, spectral, and thermal analyses the formula K[Cu(SCN)₂(NCS—SCN)], is assigned to the new compound.     

Pseudohalogen chemistry—VII: Addition of thiocyanogen to alkynes

Thiocyanogen does not react with simple alkynes under heterolytic conditions in benzene at 20-25°. Under homolytic conditions, addition occurs readily giving mixtures of E- and Z- dithiocyanatoalkenes with high E:Z ratios; prolonged treatment with excess reagent also leads to mixtures of the dithiocyanatoalkenes but usually with lower E:Z ratios. A radical-chain mechanism, involving preferential anti-addition of thiocyanogen in an initial kinetically-controlled reaction and subsequent thermodynamically-controlled isomerisation of the adducts, is proposed. The influence of substituents on the reaction rates and product ratios is discussed in terms of their steric effects.     

Kinetics and mechanism of the decomposition of N-bromoalcoholamines in aqueous solution

The decomposition reactions of N-bromodiethanolamine, N-bromoethylethanolamine, and N-bromomethylethanolamine in aqueous solution have been studied kinetically under various experimental conditions. The results support a proposed reaction mechanism in which the rate controlling step is assumed to be the formation of an imine which is then hydrolyzed to the final decomposition products.     

The thermal decomposition of aqueous manganese (II) nitrate solution

The thermal decomposition of commercially available aqueous solutions of manganese(II) nitrate was investigated using the conventional thermal analytical techniques of thermogravimetry (TG), differential thermal analysis (DTA), and evolved gas analysis (EGA). Infrared spectra and X-ray diffraction patterns were used to help characterize intermediate species.