Carbon-13 kinetic isotope effects in the decarbonylation of lactic acid of natural isotopic composition in phosphoric acid medium

The¹³C kinetic isotope effect fractionation in the decarbonylation of lactic acid (LA) of natural isotopic composition by concentrated phosphpric acids (PA) and by 85% H₃PO₄ has been studied in the temperature interval of 60–150°C. The values of the¹³C₍₁₎ isotope effects in the decarbonylation of lactic acid in 100% H₃PO₄, in pyrophosphoric acid and in more concentrated phosphoric acids are intermediate between the values calculated assuming that the C₍₁₎–OH bond is broken in the rate-controllin gstep of dehydration and those calculated for rupture of the carbon-carbon bond in the transition state. In the temperature interval of 90–130°C the experimental¹³C fractionation factors determined in concentrated PA approach quite closely the¹³C fractionation corresponding to C₍₂₎–C₍₁₎ bond scission. the¹³C₍₁₎ kinetic isotope effects in the decarbonylation of LA in 85% orthophosphoric acid in the temperature range of 110–150°C coincide with the¹³C isotope effects calculated assuming that the frequency corresponding to the C₍₁₎–OH vibration is lost in the transition state of decarbonylation. A change of the mechanism of decarbonylation of LA in going from concentrated PA medium to 85% H₃PO₄ has been suggested. A possible secondary¹⁸O and a primary¹⁸O kinetic isotope effect in decarbonylation of lactic acid in phosphoric acids media have been discussed, too.     

Carbon-13 kinetic isotope effects in the decarbonylation of liquid formic acid of natural isotopic composition initiated by phosphorus pentoxide

The isotopic composition of the consecutive fractions of carbon monoxide produced in the decarbonylation of liquid formic acid of natural isotopic composition initiated by addition of phosphorus pentoxide has been measured in the temperature interval 19–100°C and the observed gradual decrease of the _PDB values and the increase of thek ₁₂/k ₁₃ ratio of the isotopic specific rate constants (KIE values) for each next fraction of CO have been interpreted in terms of conclusions presented in the first paper from this series¹ concerning the decarbonylation of HCOOH (F.A.) in concentrated and diluted with water phosphoric acid media. The initial fast dehydration of F.A. by phosphoric anhydride, P₂O₅, proceeds at room temperture with about 1% carbon-13 KIE. The (k ₁₂/k ₁₃) values increase with time, as the decarbonylation slows down due to the hydration of phosphorus pentoxide with water generated in dehydration of HCOOH and reach the plateau values characteristic for each reaction temperature. These increasing very slowly with reaction times at intermediate temperatures maximum values of (k ₁₂/k ₁₃) ratios are quite close to values of¹³C KIE observed in the decarbonylation of pure F.A. (k ₁₂/k ₁₃=1.0443 at 81°C). Addition of water to liquid F.A. at 90°C and at 100°C caused the further increase of the¹³C KIE. The detailed discussion of the¹³C KIE in the HCOOH–P₂O₅ system has been given.     

Carbon-13 kinetic isotope effect of the decarbonylation of oxalic acid

Carbon-13 intramolecular kinetic isotope effects in the decarbonylation of oxalic acid dihydrate of natural isotopic composition by SO₃ and by fuming sulphuric acid at room temperature and decarbonylation of oxalic acid dihydrate by 100% H₃PO₄ in the temperature interval 80–150°C have been determined. The obtained isotopic and kinetic results have been compared with the earlier¹³C experimental and theoretical studies in other solvents.     

Carbon isotope effect studies in the mechanism of reactions of alkynes with formic acid

Carbon-13 fractionation observed in the course of carbon monoxide formation in the reaction of phenylacetylene with the large excess of liquid formic acid in the temperature interval 20–100°C has been investigated and compared with the¹³C fractionation in the dehydration of pure liquid formic acid. The anomalous temperature dependence of the¹³C fractionation has been interpreted as caused by the change of the kinetics and of the mechanism of CO formation from the one involving¹³C–H bond rupture rate determining step (operating in the presence of phenylacetylene) to the mechanism according to which HCOOH decarbonylates in liquid state. No large increase of the¹³C fractionation with rising of the reaction temperature from 70 to 134°C has been found in the case of decarbonylation of F.A. in the presence of large excess of phenylacetylene. The¹³C KIE was of 1.020 in the temperature interval 90–133.7°C in this case.     

Carbon-13 intermolecular isotope effect in the decarbonylation of liquid extra pure Merck formic acid

Intemolecular¹³C isotope effects in the decarbonylation of extra pure Merck liquid formic acid have been determined in the temperature interval 50–100 °C and compared to¹³C KIE observed in the decomposition of 99.9% liquid formic acid in the temperature range 60–100 °C. A very constants in the Arrhenius and Eyring equations have been calculated and found to be in a good agreement with the corresponding values ofBarham andClark.⁸     

Carbon-14 kinetic isotope effect in the decarbonylation of lactic acid[1-14C]

The carbon-14 kinetic isotope effect for the decarbonylation of lactic acid[1-¹⁴C] in sulfuric acid has been measured in the temperature interval of 20–90°C. The experimental values of (k_12C/k_14C) are compared with the theoretical¹⁴C kinetic isotope effect calculated assuming that one carbon-oxygen stretching vibration is lost in the rate-determining step. The discrepancy between experimentally observed temperature dependence of the¹⁴C kinetic isotope effect and the theoretical one is explained by the possible side reactions which change the apparent degrees of decarbonylation and isotopic composition of CH₃CHOHCOOH[1-¹⁴C] used in experiments aiming at the determination of carbon-14 kinetic isotope effect in the decarbonylation process itself.     

A New Investigation of Aqueous Orthophosphoric Acid Speciation Using Raman Spectroscopy

The Raman spectrum of aqueous phosphoric acid has been investigated at apparentconcentrations of 0.3 to 9.7 mol-dm^–3 at 25°C. A quantitative analysis hasbeen made over this concentration range after the determination of the responsecoefficients of the H₂PO^– ₄ and H₃PO₄ species. In the first step, the spectra wereinterpreted assuming that only two species (H₂PO^– ₄ and H₃PO₄) were present inthe system. The dissociation of phosphoric acid obtained in this case is consistentwith the values Preston and Adams⁽¹⁾ obtained, and which was also found fromRaman spectroscopy. However, a discrepancy exists between the representationsfrom spectra and experimental ones. This discrepancy can be removed if anotherspecies, the anionic dimer H₅P₂O^– ₈ is taken into account. Therefore, in the secondstep, a modified interpretation of the spectra, was used to determine theconcentrations of the H₂PO^– ₄, H₅P₂O^– ₈, and H₃PO₄ species and to deduce the correspondingdegree of dissociation of the acid, as well as the speciation of the solutions as afunction of the apparent concentration of phosphoric acid. As in the results Elmoreand co-workers,² which were deduced from pH measurements, the degree ofdissociation reaches a minimum and then increases significantly for apparentphosphoric acid concentrations greater than 1 mol-dm^–3.     

Kinetics and mechanisms of oxidations by metal ions. Part IX(1) Oxidation of phenylphosphinic acid by chromic acid in perchlorate medium

Summary The kinetics of chromic acid oxidation of phenylphosphinic acid to phenylphosphonic acid has indicated the formation of an anhydride between HCrO ₄ ^– and phenylphosphinic acid in its active PhP(OH)₂ and inactive PhPH(O)OH forms. The ambiguity about the reactive form of phenylphosphinic acid arises from the fact that protonation of the anhydride leads to the same transition state which disproportionates in the rate-determining step to phosphonium ion and chromium(IV). These, through different reactions in the fast step, yield phenylphosphonic acid and chromium(III) as the final products. That HCrO ₄ ^– is the reactive species of chromium(VI) is confirmed by the fact that k₀ is independent of the inital [Cr^VI] where k₀ is defined by the Equation k₀=k_obs[Cr^VI]/[HCrO ₄ ^– ]; k_obs is the pseudo first-order rate constant with respect to chromium(VI) ([Phenyl-phosphinic acid][Cr^VI]).The plot between k₀ and [H⁺] passes through the origin indicating that the reaction does not occur in the absence of H⁺-ions. Furthermore, the plot between log k₀ and –H₀, the Hammett acidity function, is linear with a slope value of 1.02±0.02 confirming the protonation of the anhydride prior to its rate-limiting disproportionation.The equilibrium constant for the anhydride formation and the composite rate constant kK, K is the protonation constant of anhydride, are reported. The equilibrium constant is almost independent of temperature.Sen Gupta and Chakaladar⁽³⁾ reported values of 8.5, 9.2, 11, 12 and 13, respectively, at 26°, 30.4°, 36°, 39.4° and 46° C. The uncertainty limits were not reported. Nevertheless it is apparent from the data that the values are not greatly influenced by temperature. The statistical mean is 11±2 dm³ mol^–1, in fair agreement with our values.     

Kinetics of the anation reaction of pentaamineaquacobalt(III) by phosphorous acid/hydrogenphosphite

Summary The kinetics of the anation reaction of [Co(NH₃)₅H₂O]³⁺ by H₃PO₃/H₂PO ₃ ^– , to give [CoH₂PO₃(NH₃)₅]²⁺, have been studied at 60, 70 and 80°C, in the acidity range [H⁺](M)=1.5 · 10^–1 –2.0 · 10^–3. Only H₂PO₃ is found to be reactive. The rate data is consistent with an I_d mechanism. The mean value of outer sphere association of [Co(NH₃)H₂O]³⁺ with H₂PO ₃ ^– is 1.5 M^–1. Values of the interchange constants are: 10⁴4k_i(s^–1)= 0.29, 1.47, 5.13, at 60, 70 and 80 °C respectively (H= 1.4 · 10²KJmol^–1, S=8.3 · 10 JK^–1 mol^–1). The first acidity constant of H₃PO₃ at I=1.0 has also been determined: 10²K_a(M)=4.8, 5.2 and 5.5, at 25, 40 and 50 °C respectively.     

Ligand exchange reaction of Zn(II)-acetylacetonate complex with 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid

Ligand exchange reaction of Zn(II)-acetylacetonate complex (Zn-acac₂) with 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (H₂TPPS) has been investigated spectrophotometrically and radiometrically. The exchange reaction was observed by spectral change from H₂TPPS to Zn-TPPS or activity of⁶⁵Zn(acac)₂ extracted into the chloroform phase. The 2nd order rate constants (k ₂) for the exchange reaction at 70 °C and at pH 7.8 were found to be 32.8±2.3 and 31.2±3.2 M^–1·s^–1 from the spectrometric and radiotracer experiments, respectively. For the direct complexation of Zn(II) with H₂TPPS, a similar 2nd order rate constant (k=32.4±4.7 M^–1·s^–1) was obtained as that in the ligand exchange reaction. The activation energies (E) for the exchange and the formation of Zn-TPPS were found to be 69.3±0.2 and 69.4±0.2 kJ·mol^–1, respectively, in the temperature range from 40 to 70 °C.     

Characteristics of oxygen exchange in phosphoric acid

The oxygen exchange between phosphoric acid and water at various temperatures and concentrations has been investigated in the presence and absence of K₂HPO₄. The minimum rate of exchange is observed for a solution concentration of 2–2.5 mole/l. It has been shown that in concentrated solutions of phosphoric acid the formation of H₄PO ₄ ⁺ is possible, and the true rate constant of the acid-catalytic conversion and the basicity constant of H₃PO₄ has been calculated. It has been found that in a solution where the mobility of the oxygen atom towards the anion increases, the value of the limiting current on the anodic dissolution of copper diminishes.     

Complexes of cadmium with phosphoric acid

We have investigated the behaviour of¹⁰⁹Cd, in two-phase systems: (HDEHP–C₆ H₆/ H₃ PO₄–HClO₄–LiOH, =0.2), as a function of the independent equilibrium parameters which define the system: pH, equilibrium concentration of H₃ PO₄ and total concentration of HDEHP in the organic phase. The data have been interpreted in terms of the existence of phosphoric complexes characterized by their order 1 with regard to H₃ PO₄ and their charge z. The l and z. values are: 0<1<2, z=–2, 0, 1, 2 for the following ranges: 0.7<pH<2.7 and CH₃ PO₄<4M. Stability constants of the predominant complexes have been obtained. Finally, a formulation of these complexes has been proposed on the basis of partial charge of the atoms. Some complexes, could be formulated as hydroxy-phosphoric species, resulting from competition between hydroxy and dihydrogenophosphate anions. In concentrated phosphoric acid (CH₃ PO₄=4M), complexation of cadmium is not more than 25%.     

Kinetics of oxidation of pyruvic acid by [ethylenebis(biguanide)]silver(III) in aqueous acidic media

The oxidation of pyruvic acid by the title silver(III) complex in aqueous acidic (pH, 1.1–4.5) media is described. The reaction products are MeCO₂H and CO₂, together with a colourless solution of the Ag⁺ ion. The free ligand, ethylenebis(biguanide) is released in near-quantitative yield upon completion of the reduction. The parent complex, [Ag(H₂L)]³⁺ and one of its conjugate bases, [Ag(HL)]²⁺, participate in the reaction with both pyruvic acid (HPy) and the pyruvate anion (Py^–) as the reactive reducing species. Ag⁺ was found to be catalytically inactive. At 25.0°C, I=1.0moldm^–3, rate constants for the reactions [Ag(H₂L)]³⁺+HPy (k ₁), [Ag(H₂L)]³⁺+Py^– (k ₂), [Ag(HL)]²⁺+HPy (k ₃) and [Ag(HL)]²⁺+Py^– (k ₄) arek ₁=(94±6)×10^–5dm³mol^–1s^–1, (k ₂ K _a+k ₃ K _a1)= (1.3±0.1)×10^–5s^–1 and k ₄=(58±4)×10^–5dm³mol^–1s^–1, respectively, where K _a1is the first acid dissociation constant of the [Ag(H₂L)]³⁺ and K _a is for pyruvic acid. A comparison between the k ₁ and k ₄ values is indicative of the judgement that k ₂k ₃. A one-electron inner-sphere redox mechanism seems more justified than an outer-sphere electron-transfer between the redox partners.     

A method of calibration of the formic acid monomer concentration in the gas phase

Pyrolysis of t-butyl formate, (CH₃)₃C-O-CHO, has been carried out in a carrier gas stream of Ar or N₂ in a temperature range of 200–400°C. Between 200 and 300°C, the pyrolysis yielded an equimolar mixture of HCOOH and (CH₃)₂C=CH₂. The results have been used as a calibration method for determining the concentration of the gas-phase HCOOH monomer without interference from the formation of the formic acid dimer. Using this technique, the gas-phase infrared absorption cross-section of HCOOH at 1105 cm^–1 (peak to valley) for the resolution of 0.5 cm^–1 has been determined to be 6.76×10^–19 cm² molecule^–1.     

Kinetic investigation of sulfur(IV) oxidation by peroxo compounds R-OOH in aqueous solution

Summary Normal and rapid-scan stopped-flow spectrophotometry in the range of 260–300 nm was used to study the kinetics of sulfur(IV) oxidation by peroxo compounds R-OOH (such as hydrogen peroxide, R=H; peroxonitrous acid, R=NO; peroxoacetic acid, R=Ac; peroxomonosulfuric acid, R=SO ₃ ^– ) in the pH range 2–6 in buffered aqueous solution at an ionic strength of 0.5 M (NaClO₄) or 1.0 M (R=NO; Na₂SO₄). The kinetics follow a three-term rate law, rate=(k_H[H]+k_HX[HX]+k_p)[HSO ₃ ^– ][ROOH] ([H] = proton activity; HX = buffer acid = chloroacetic acid, formic acid, acetic acid, H₂PO ₄ ^– ). Ionic strength effects (I=0.05–0.5 M) and anion effects (Cl^–, ClO ₄ ^– , SO ₄ ^2– ) were not observed. In addition to proton-catalysis (k_H[H]) and general acid catalysis (k_HX[HX]), the rate constant k_p characterizes, most probably, a water induced reaction channel with k_p=k_HOH[H₂O]. It is found that k_Hf(R) with k_H(mean)=2.1·10⁷ M^–2 s^–1 at 298 K. The rate constant k_HX ranges from 0.85·10⁶ M^–2 s^–1 (HX=ClCH₂–COOH; R=NO; 293 K) to 0.47·10⁴ M^–2 s^–1 (HX=H₂PO ₄ ^– ; R=H; 298 K) and the rate constant k_p covers the range 0.2·M^–1 s^–1 (R=H) to 4.0·10⁴ M^–1 s^–1 (R=NO). LFE relationships can be established for both k_HX, correlating with the pK_a of HX, and k_p, correlating with the pK_a of the peroxo compounds R-OOH. These relationships imply interesting aspects concerning the mechanism of sulfur(IV) oxidation and the possible role of peroxonitrous acid in atmospheric chemistry. A UV-spectrum of the unstable peroxo acid ON-OOH is presented.     

Kinetics and mechanism of the oxidation of a ternary complex involving nitrilotriacetatocobaltate(II) and malonic acid by N-bromosuccinimide

The kinetics of oxidation of [Co^IINM(H₂O)]^3– (N = nitrilotriacetate, M = malonate) by N-bromosuccinimide (NBS) in aqueous solution have been found to obey the equation: d[Co^III]/dt = k ₁ K ₂[NBS][Co^II]_T/{1 + K₂[NBS] + (H⁺/K₁)} where k ₁ is the rate constant for the electron transfer process, K ₁ the equilibrium constant for dissociation of [Co^IINM(H₂O)]^3– to [Co^IINM(OH)]^4– + H⁺, and K ₂ the pre-equilibrium formation constant. Values of k ₁ = 1.07 × 10^–3 s^–1, K ₁ = 4.74 × 10^–8 mol dm^–3 and K ₂ = 472 dm³ mol^–1 have been obtained at 30 °C and I = 0.2 mol dm^–3. The thermodynamic activation parameters have been calculated. The experimental rate law is consistent with a mechanism in which the deprotonated [Co^IINM(OH)]^4– is considered to be the most reactive species compared to its conjugate acid. It is assumed that electron transfer takes place via an inner-sphere mechanism.     

Studies on the kinetics and mechanism of dissociation of copper(II) and nickel(II) complexes of the macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene in perchloric acid media

Summary Acid catalysed dissociation of the copper(II) and nickel(II) complexes (ML²⁺ of the quadridentate macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene (L) has been studied spectrophotometrically. Both complexes dissociate quite slowly with the observed pseudo-first order rate constants (k_obs) showing acid dependence; for the nickel(II) complex (k_obs)=k_O+k_H[H⁺], the k_o path is however absent with the copper(II) complex. At 60°C (I=0.1M) the k_H values areca 10^–4 M^–1 s^–1 for both complexes; k _H ^Cu /k _H ^Ni =ca. 3.9, comparable to some other square-planar complexes of these metal ions. The rate difference is primarily due to H values [copper(II) complex, 29.4±0.5 kJ mol^–1; nickel(II) complex, 35.6±1.5 kJ mol^–1] with highly negative S values [for copper(II), –215.5 ±6.1 JK^–1 mol^–1 and for nickel(II), –208.1 ±5.6 JK^–1 mol^–1] which are much higher than the entropy of solvation of Ni²⁺ (ca. –160 JK^–1 mol^–1) and Cu²⁺ (ca. –99 JK^–1 mol^–1) ions; significant solvation of the released metal ions and the ligand is indicated.     

Preparation and Characterization of Highly Proton-Conductive Composites Composed of Phosphoric Acid-Doped Silica Gel and Styrene-Ethylene-Butylene-Styrene Elastomer

Highly proton-conductive elastic composites have been successfully prepared from H₃PO₄-doped silica gel and a styrene-ethylene-butylene-styrene (SEBS) block elastic copolymer. Ionic conductivities of the composites depended on the concentration of H₃PO₄ and the heat treatment temperature of the H₃PO₄-doped silica gel. It was found that H₃PO₄ added is present mainly as free orthophosphoric acid in the silica gel. The composite composed of H₃PO₄-doped silica gel with a molar ratio of H₃PO₄/SiO₂ = 0.5 heat-treated at temperatures below 200°C and SEBS elastomer in 5 mass% showed a high conductivity of 10^–5 S cm^–1 at 25°C in an dry N₂ atmosphere. The water adsorption during a storage in 25% relative humidity at room temperature for 1 day enhanced the ionic conductivities of composites by about one order of magnitude. Lower conductivities obtained in the composite with the H₃PO₄-doped silica gel heat-treated at 250°C for 1 h were due to the formation of crystalline Si₃(PO₄)₄. The temperature dependence of conductivity of the composites was the Vogel-Tamman-Fulcher type, indicating that proton was transferred through a liquidlike phase formed in micropores of the H₃PO₄-doped silica gels. The temperature dependence of the modulus of the composite was similar to that of the SEBS elastomer. The thermoplastically deforming temperature of the composite was around 100°C, which was higher by 30°C than that of the SEBS elastomer.     

Synthesis,thermodynamic properties and kinetics of the acid-catalyzed dissociation of nickel(II) diamido polyamino complexes

The ligands 1,10-N,N-bis(2-hydroxymethylbenzoyl)-1,4,7,10-tetraazadecane (L₁) and 1,11-N,N-bis(2-hydroxymethylbenzoyl)-1,4,8,11-tetraazaundecane (L₂) have been synthesized. The stability constants of Ni^II complexes of ligands L₁ and L₂ have been studied at 25 °C using pH titrations. The kinetics of general acid (HCl, 0.04–2.34 mol dm^–3) or buffer (DEPP or DESPEN, 0.05 mol dm^–3, pH 4.83–5.72)-catalyzed dissociation of these Ni^II complexes have been investigated at 25 °C using a stopped-flow spectrophotometer. The ionic strength of solution was controlled at I = 2.34 mol dm^–3 (KCl + HCl) and I = 0.1 mol dm^–3 (KNO₃, buffer), respectively. The kinetic dissociation of Ni^II complexes catalyzed by HCl obeys the equilibrium k _obs = k _1d + k _2H[H⁺], whereas in buffer solution the observed rate constant k _obs = k _d + k _1H[H⁺]. At pH < 1.5, both the proton-assisted and direct protonation pathways contribute to the rates, whereas solvation is the dominant pathway at pH > 6. In the 4.8–5.7 pH range, the complexes dissociate mainly through a proton-assisted pathway.     

Carbon isotope effect studies in the mechanism of reactions of alkynes with formic acid

The¹³C fractionation has been studied in the reaction of phenylacetylene with the excess of liquid Merck formic acid at 30 and 40 °C to see the contribution of the¹³C fractionation in the formolysis of transient -formoxystyrene to the experimentally observed global¹³C fractionation. The¹³C fractionation has been investigated also in the hydration of 1 ml of PhCCH with 1 ml of formic acid in the temperature interval of 80–100°C. The¹³C KIE equal to 1.0168 at 91.75 °C and 1.0167 at 100°C indicate that the self-decomposition of formic acid in such experimental conditions is already largely suppressed. The isotope effect is discussed within the framework of the sequence of reaction steps leading to acetophenone and carbon monoxide production listed in part I.