Gas chromatography–mass spectrometric identification of iodine species arising from photo-chemical vapor generation

Ultraviolet irradiation of aqueous solutions of iodide/iodate ion containing low molecular weight organic acids generates volatile iodine species that are amenable to detection by atomic spectrometry. In the presence of formic, acetic or propionic acids, photo-chemical generation results in the formation of HI, methyl- and ethyl-iodide respectively, the latter two products being directly identified by gas chromatography–mass spectrometry. Deuterium and ¹³C-labeled reagents were employed to elucidate the provenance of the alkyl group. Use of ¹³CH₃–COOH produced ¹³CH₃–I; deuterated acetic acid (D₃C-COOD) resulted in the formation of CD₃–I. These observations indicate direct transfer of the alkyl group from the carboxylic acid to iodide, consistent with the suggestion that the mechanism of synthesis involves radical induced reactions.     

An NMR relaxation study of hydrogen exchange and its deuterium isotope effects in aqueous carboxylic acid solutions

Exchange-rate measurements of water protons and deuterons in aqueous solutions of acetic, malonic and glutaric acid by the NMR T₂ Carr-Purcell method, employing oxygen-17-enriched water, are reported. In contrast to previous results on acetic-acid solutions, intermolecular proton exchange via solvent between COOH and COO^? groups is found to contribute considerably to the exchange rate for all three acids investigated. The rate constants necessary for the calculation of the proton residence time in the COOH groups have been determined. Observed deuterium isotope effects on the rate constants for hydrogen-ion and acetic-acid-catalyzed exchange are discussed in terms of fractionation-factor theory.     

CNDO/2-calculations on halogenated acetic acids and their dimeric adducts

In dimeric hydrogen bonded aggregates of acetic acid and its halogenated homologues, stabilization energies, geometries and charge distributions depend on the acidity and basicity of the monomers. For stabilization and bond lengths, the effects of basicity predominate; e.g., CH₃COOH.CH₃COOH is more stable than CF₃COOH.CH₃COOH. Calculated charge densities are in good agreement with ¹H NMR chemical-shift measurements. A further correlation is found between the stabilization energy of the adducts investigated, and the pK-values of the corresponding acids in the solvent acetic acid.     

Novel photopolymerizations initiated by alkyl radicals generated from photocatalyzed decarboxylation of carboxylic acids over oxide semiconductor nanoparticles: Extended photo-Kolbe reactions

Polymerizations of vinyl acetate are photocatalyzed by TiO₂ nanoparticles in presence of carboxylic acids including propionic acid, n-butyric acid and pivalic acid. Nuclear magnetic resonance (NMR) analysis using ¹³C-labeled n-butyric acid as the probing molecule demonstrates that the polymerization of vinyl acetate is initiated by alkyl radicals generated from photocatalytic decarboxylation of the carboxylic acid. A universal mechanism is established with extending the photo-Kolbe reaction from acetic acid to the carboxylic acids with longer chains. Kinetics studies find that n-butyric acid has higher initiation rate than acetic acid, indicating more efficient decarboxylation for butyric acid than acetic acid in their aqueous solutions. It is proved that carboxylates participate in the decarboxylation. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectra are obtained with aqueous solutions of the carboxylic acids in contact with a layer of the TiO₂ nanoparticles, and the observations are discussed with respect to the interaction between the TiO₂ and carboxylic acids.     

Kinetic simulation of the enzymatic oxidation of methane

The kinetics of methane oxidation by methane monooxygenase is simulated numerically. Literature data on the distribution of products of the oxidation of deuterated methane CH_{4 ? n} D _n (CH₃D, CH₂D₂, and CHD₃), as well as on the kinetic isotope effect in the competitive oxidation of CH₄ and CD₄ by methane monooxygenase, are analyzed in the framework of a nonradical multistep mechanism. Kinetic schemes whose first step involves two hydrogen atoms of the oxidation substrate are considered. The kinetic models suggested for methane oxidation are in good agreement with experimental data.     

Study on deuterium exchange in surface CHx species formed from methane over platinum, ruthenium and cobalt under non-oxidative conditions

Dissociative chemisorption of methane over ruthenium, cobalt, platinum and their bimetallic counterparts supported by alumina and NaY was investigated under a wide range of temperatures. The extent of hydrogen loss from methane was monitored by deuterium uptake of the surface CH_x species formed from methane during the course of methane chemisorption. The presence of a high average number of deuterium in the desorbing methane suggested a widespread dissociation of methane. The initial distribution of the deuterated products generally decreased in the sequence CD₄>CHD₃>CH₂D₂. The amount of chemisorbed methane during methane chemisorption increases with temperature and it follows the sequence of reducibility of the supported metals and particle size which, in turn, depends on the support and the alloy formed. CH_x (x=1) species prevailed on alumina supported catalysts, while on NaY supported metals, CH₂ species are dominant when small metal particles are stabilized inside the supercage. Dedicated to Professor Pál Tétényi on the occasion of his 70th birthday     

Multiple Deuteration of Water-Soluble Olefinic Acids with a [Pd(Alizarin Monosulfonate)2] Catalyst

Hydrogenations in aqueous systems with the soluble [Pd(alizarin monosulfonate)₂] catalyst resulted in extensive deuteration of crotonic, trans-2-pentenoic and itaconic acids regardless of whether the deuterium source was D₂ or D₂O. Itaconic acid was deuterated up to 3.6 D/methylsuccinic acid. Detailed ¹H- and ¹³C-NMR studies identified six isotopomers of the deuterated methylsuccinic acid product and revealed an important role of the H/D exchange on the catalytically active Pd-intermediate.     

Catalytic carbonylation of ethylene in the presence of the Pd(acac)2-m-Ph2PC6H4SO3Na(H)-AcOH system

Catalytic systems based on phosphine complexes of palladium have been developed for synthesizing propionic acid (monocarbonylation) and alternating (11) ethylene-carbon monoxide copolymers,i.e., polyketones (polycarbonylation).m-(Diphenylphosphino)benzenesulfonic acid or its sodium salt were used as ligands. Monocarbonylation proceeds at atmospheric pressure in dioxane or acetic acid solvents. Under high pressure, the reaction pathway can change from monocarbonylation, which occurs in the presense of the sodium salt of the ligand, to polycarbonylation when the sodium ion at the sulfo group is completely replaced by a proton. This change in reaction selectivity is observed when the process is performed in acetic acid. When the ligand is present both in the acid and the neutral form, products of di- and oligocarbonylation are formed along with propionic acid and the polyketone. These products were characterized by¹H and¹³C NMR spectra as alternating keto acids C₂H₅(COCH₂CH₂) _n COOH, wheren=1÷3. Kinetic equations were derived for the selective synthesis of propionic acid and polyketones.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 401–404, March, 1994.The authors are grateful to V. A. Zavel'skii who recorded the 1H NMR spectra on a Bruker CXP-200 spectrometer.     

Kinetics,mechanism, and products of the reaction of diphenylcarbonyl oxide with carboxylic acids

The kinetics of the reactions of acetic, benzoic, formic, oxalic, malic, tartaric, trifluoroacetic, and hydrochloric acids with diphenylcarbonyl oxide Ph₂COO was studied. The carbonyl oxide Ph₂COO was generated by flash photolysis of diphenyldiazomethane Ph₂CN₂ in solutions of acetonitrile and benzene at 295 K. The apparent rate constants of the reaction range from 4.6·10⁸ for (COOH)₂ in MeCN to 7.5·10⁹ L mol^–1 s^–1 for acetic acid in a benzene solution. The reaction mechanism was proposed, according to which at the first stage the carbonyl oxide is reversibly solvated by the solvent. Then the solvated carbonyl oxide reacts with the acid molecule by the mechanism of insertion at the O—H bond.     

Dynamics of the enzymatic oxidation of methane

Kinetic isotope effect data for the oxidation of deuterium-substituted methane molecules with methane monooxygenase (MMO) are analyzed in the framework of a multistep nonradical mechanism. New evidence is obtained in favor of the hypothesis of the intermediate formation of a complex containing pentacoordinated carbon. A kinetic scheme whose first step involves two hydrogen molecules of the substrate being oxidized is considered. For coincidence between the calculated and experimental distributions of the oxidation products of partially deuterated methane, the formation of the intermediate complex containing pentacoordinated carbon must be reversible and the rate of the back decomposition of this complex must be substantially higher than the rate of its formation (w _?1 ? w ₁). The experimental distribution of the products of deuterated methane (CH₃D, CH₂D₂, and CHD₃) hydroxylation with MMO, which could not earlier be explained within the widely accepted oxygen rebound mechanism, is quantitatively explained for the first time in terms of the dynamics of a nonradical mechanism using parameters having a simple physical meaning and plausible values.     

Reactions of the radial cattions of acetic acid and acetic anhydride in CFCl3

The ESR spectra of -irradiated, at –196 °C, solutions of acetic acid and acetic anhydride were studied depending on their concentrations in CFCl₃. The structure of thus produced radical cations is confirmed with the deuterium substituted analogues. It has been shown that the ion-molecular reaction of the radical cation CH₂COOH⁺ in the isolated dimer takes place for the dilute solutions of acetic acid in CFCl₃ resulting in the formation of CH₃COO follwed by its decomposition to CH₃+CO₂ while the radicals CH₂COOH are formed via secondary processes. The reactions of radical cations of acetic oxide have been also studied.     

Die Oxydation von Alkoholen mit Bleitetraacylaten; Einfluss der Acylreste bei der thermischen Reaktion

The competitive thermal oxidation of a 6β-hydroxy steroid (to a 6β, 19-ether) and the thermal oxidative decarboxylation of various carboxylic acids by lead⁴⁺ was investigated. The alcohol oxidation is faster than acetate and benzoate oxidation, about as fast as the oxidative decarboxylation of higher primary carboxylic acids (propionic, butyric acid), and much slower than the oxidative decarboxylation of secondary and tertiary carboxylic acids.     

An Unexplored O2‐Involved Pathway for the Decarboxylation of Saturated Carboxylic Acids by TiO2 Photocatalysis: An Isotopic Probe Study

The aerobic decarboxylation of saturated carboxylic acids (from C₂ to C₅) in water by TiO₂ photocatalysis was systematically investigated in this work. It was found that the split of C¹? C² bond of the acids to release CO₂ proceeds sequentially (that is, a C₅ acid sequentially forms C₄ products, then C₃ and so forth). As a model reaction, the decarboxylation of propionic acid to produce acetic acid was tracked by using isotopic‐labeled H₂¹⁸O. As much as ≈42 % of oxygen atoms of the produced acetic acids were from dioxygen (¹⁶O₂). Through diffuse reflectance FTIR measurements (DRIFTS), we confirmed that an intermediate pyruvic acid was generated prior to the cut‐off of the initial carboxyl group; this intermediate was evidenced by the appearance of an absorption peak at 1772 cm^?1 (attributed to C?O stretch of α‐keto group of pyruvic acid) and the shift of this peak to 1726 cm^?1 when H₂¹⁶O was replaced by H₂¹⁸O. Consequently, pyruvic acid was chosen as another model molecule to observe how its decarboxylation occurs in H₂¹⁶O under an atmosphere of ¹⁸O₂. With the α‐keto oxygen of pyruvic acid preserved in the carboxyl group of acetic acid, ≈24 % new oxygen atoms of the produced acetic acid were from molecular oxygen at near 100 % conversion of pyruvic acid. The other ≈76 % oxygen atoms were provided by H₂O through hole/OH radical oxidation. In the presence of conduction band electrons, O₂ can independently accomplish such C¹? C² bond cleavage of pyruvic acid to generate acetic acid with ≈100 % selectivity, as confirmed by an electrochemical experiment carried out in the dark. More importantly, the ratio of O₂ participation in decarboxylation increased along with the increase of pyruvic acid conversion, indicating the differences between non‐substituted acids and α‐keto acids. This also suggests that the O₂‐dependent decarboxylation competes with hole/OH‐radical‐promoted decarboxylation and depends on TiO₂ surface defects at which Ti_4c sites are available for the simultaneous coordination of substrates and O₂.     

Nuclear magnetic resonance relaxation of methane protons in aqueous solutions

Proton nuclear magnetic resonance longitudinal relaxation rates are reported at 100MHz for CH₄ and CHD₃ in deuterium oxide solutions. The results demonstrate that methane reorients with a correlation time on the order of 0.1 psec, considerably faster than molecule correlation times in aqueous solutions.     

Kinetic Simulation of Methane and Ethane Hydroxylation by Methane Monooxygenase

According to the mechanism of alkane hydroxylation, whose main postulate is the formation of an intermediate complex containing pentacoordinated carbon, the hydroxylation of methane and ethane by methane monooxygenase was kinetically simulated by the numerical method. The published data on the kinetic isotope effects of oxidation of deuterium-substituted methane molecules (CHD₃, CH₂D₂, and CH₃D) and the distribution of products of chiral ethane (R- and S-MeCHDT) oxidation by methane monooxygenase were examined. The kinetic models proposed for the oxidation of isotopically substituted methane and ethane are in good agreement with experimental data.     

Temperature dependences of J(C,H) and J(C,D) in 13CH4 and some of its deuterated isotopomers

The nuclear spin—spin coupling constants J(C,H) and J(C,D) have been measured over the temperature range 200–370 K for the methane isotopomers ¹³CH₄, ¹³CH₃D, ¹³CHD₃ and ¹³CD₄. The coupling constants increase with increasing temperature for any one isotopomer and decrease with increasing secondary deuterium substitution at any one temperature. The results are entirely attributable to intramolecular effects and the data have been fitted by a weighted least-squares regression analysis to a spin—spin coupling surface thereby yielding a value for ¹J_e(C,H), the coupling constant at equilibrium geometry, and values for the bond length derivatives of the coupling. We find that ¹J_e(C,H) = 120.78 (±0.05) Hz which is about 4.5 Hz smaller than the observed value in ¹³CH₄ gas at room temperature. Results are also reported for J(H,D) in ¹³CH₃D and ¹³CHD₃ for which no temperature dependence was detected.     

Carbon-13 kinetic isotope effects in the decarbonylation of formic acid of natural isotopic composition in phosphoric acid media

The¹³C kinetic isotope effect (K.I.E.) in the decarbonylation of formic acid of natural isotopic composition in 85% orthophosphoric acid, in 100% H₃PO₄, and in pyrophosphoric acid has been measured in different temperature intervals ranging from 19 to 133 °C. In 85% H₃PO₄ the carbon-13 K.I.E. is determined by the fractionation of carbon isotopes expected for C–O bond rupture (k ₁₂/k ₁₃=1.0531 at 70°C). In 100% H₃PO₄ the¹³C K.I.E. indicates that C–H bond rupture is the major component of the reaction coordinate motion (thek ₁₂/k ₁₃ lay in the range of 1.026–1.017 over the range 30–70 °C). In pyrophosphoric acid the fractionation factor for¹³C equals 1.010 at 19 °C. Activation parameters for the decarbonylation of H¹²COOH in phosphoric acid media have been determined also and suggestions concerning the intimate mechanisms of decarbonylation of formic acid in dilute and concentrated phosphoric acids are made.     

Homogeneous Rhodium–Copper–Halide Catalytic Systems for the Oxidation and Oxidative Carbonylation of Methane

Homogeneous catalytic systems for the oxidation and oxidative carbonylation of methane in aqueous trifluoroacetic acid solutions were developed. The system included rhodium compounds and copper compounds as cocatalysts. Methanol, methyl trifluoroacetate, formic acid, and acetic acid were the reaction products. The process was accompanied by the intense oxidation of CO to CO₂. Regularities of the process were studied, optimum process parameters were determined, and conditions at which methyl trifluoroacetate was formed with 90% selectivity were found. Kinetic isotope effects with respect to the solvent were determined. For the formation of organic products, k _H/k _D was 1, whereas it was 1.6 for the oxidation of CO. Studies were also performed with the use of compounds containing the ¹⁸O isotope. Conceivable mechanistic models of this process are discussed.     

Palladium-catalyzed 1,4-acetoxy-trifluoroacetoxylation of 1,3-dienes

Palladium-catalyzed oxidation of conjugated dienes in acetic acid containing CF₃COOH/LiOOCCF₃ results in the formation of 1-acetoxy-4-trifluoroacetoxy-2-alkenes.     

Kinetics and mechanism of the ruthenium(III)-catalysed oxidation of aliphatic acids by peroxodiphosphate in acetate buffers

Summary The kinetics of oxidation of aliphatic acids (AAs), such as propionic acid, butyric acid, isobutyric acid and valeric acids, by peroxodiphosphate (PDP) using ruthenium(III) as catalyst in aqueous H₂SO₄ at constant ionic strength and different acidities were studied. The ruthenium(III)-catalysed oxidation is first order in [PDP] and fractional order in [AA]. The order with respect to [Ru^III] is fractional. An analysis of the rate dependence upon [H^–] suggests that H₃P₂O ₈ ^– is the active oxidizing species in the oxidation. A mechanism consistent with the rate law is proposed.