Demetallation of α,β,γ,δ-tetrakis(p-sulfophenyl) porphiniron(III) in mineral acid–alcohol–water media

Demetallation rates of α,β,γ,δ-tetrakis(p-sulfophenyl)porphiniron(III) in hydrochloric acid–ethanol–water, perchloric acid–ethanol–water, and sulfuric acid–alcohol–water media were determined. For a given acidity value H₀ the order of the rates for the three acids was HCl > H₂SO₄ > HClO₄. This is also the order for complex formation between acid anion and iron(III). Consequently ligands as well as protons are involved in the breaking of bonds between the metal and the porphyrin leading to the formation of the activated complex. The log k values for HCl and HClO₄ media were not linearly related to the Hammett acidity function as they were for sulfuric acid–ethanol–water media. The average ΔH? and ΔS?values for the HCl media were 18.4 ± 1.4 kcal/mol and ? 19 ± 3 cal K mol, respectively, in very close agreement with those for H₂SO₄ media despite the difference in H ₀ dependence. For H₂SO₄–alcohol–water media the order of the rates was butanol > propanol > ethanol with little difference between isomeric alcohols.     

Transferts Protoniques de Sels d'Ammonium Substitues—VIII: Sels de Pyridinium 4-Substitues

The deprotonation rate 1/τ of the title compounds, [4 – R – Py H]⁺, where R = NH₂, t-Bu, Me, Cl, Br or CN, is measured using the coalescence of the pyridinic α-protons, in a mixture CF₃COOH/H₂O/HClO₄ of variable acidity Ho, at 38°C. 1/τ is a linear function k/ho of the acidity 1/ho. k is approximately proportional to the water content and independent of the salt concentration, which seems to be evidence for an exchange with an intermediate pyridine hydrate, according to: . After a preliminary ionisation step: k values, like K_A, fit a Hammett relationship (ρ = 5,05), except for R ? NH₂, and are very sensitive to the nature of R (k = 3,44 × 10² for R = NH₂ and k = 3,14 × 10⁸ M^?1 s^?1 for R ? CN), while k_H values (10¹⁰ s^?1) are not.     

The reaction of OH with CO

Hydroxyl radicals were prepared from the photolysis of N₂O at 213.9 nm in the presence of excess H₂. The O(¹D) produced in the primary photolytic act reacts with H₂ to produce OH radicals. If CO is also present, then OH can react either with H₂ or CO: The competition between reactions (1) and (2) was measured by measuring the CO₂ yield at various values of the ratio [CO]/[H₂] at 217–298°K. At 298°K the ratio of the rate coefficients k₁/k₂ increased with pressure from a low-pressure limiting value of 14 to a high-pressure limiting value of 50. The low-pressure limiting value agrees well with the low-pressure values found by others. At lower temperatures our high-pressure values of k₁/k₂ were larger than deduced from the accepted low-pressure Arrhenius expression and could be fitted to the expression The mechanism which seems to fit the results best is with k₁° = k_ak_b/k_-a and k₁^∞ = k_a.     

Kinetic isotope effects in the reactions of bromine atoms with CH4 and CD4

The competitive reactions of Br atoms with CH₄ and CD₄ were studied over the temperature range of 562° to 637°K. Over this temperature interval, the kinetic isotope effect, k_H/k_D, varied from 3.05 to 2.47 for the reactions The rate constant ratio k_H/k_D, expressed in Arrhenius form, was found to equal (1.10 ± 0.05) exp (1030 ± 60/RT). A comparison is presented between the experimental result and the result obtained theoretically from absolute rate theory using the London-Eyring-Polanyi-Sato (LEPS) method of constructing the potential energy surface of the reaction. The agreement between theory and experiment is very poor, and this is believed to arise from the highly unsymmetrical nature of the potential energy surface involved in these reactions. A comparison is also presented between the k_H/k_D values obtained in the Br + CH₄–CD₄ experiments and the available data on the corresponding Cl + CH₄–CD₄ reactions.     

Phosphine and Thiophene Cyclopalladated Complexes: Hydrolysis Reactions in Strong Acidic Media

The mechanisms for the hydrolysis of organopalladium complexes [Pd(CNN)R]BF₄ (R=P(OPh)₃, PPh₃, and SC₄H₈) were investigated at 25 °C by using UV/Vis absorbance measurements in 10 % v/v ethanol/water mixtures containing different sulphuric acid concentrations in the 1.3–11.7 M range. In all cases, a biphasic behavior was observed with rate constants k_1obs, which corresponds to the initial step of the hydrolysis reaction, and k_2obs, where k_1obs>k_2obs. The plots of k_1obs and k_2obs versus sulfuric acid concentration suggest a change in the reaction mechanism. The change with respect to the k_1obs value corresponds to 35 %, 2 %, and 99 % of the protonated complexes for R=PPh₃, P(OPh)₃, and SC₄H₈, respectively. Regarding k_2obs, the change occurred in all cases at about 6.5 M H₂SO₄ and matched up with the results reported for the hydrolysis of the 2‐acetylpyridinephenylhydrazone (CNN) ligand. By using the excess acidity method, the mechanisms were elucidated by carefully looking at the variation of k_i,_obs (i=1,2) versus ${c_{{\rm{H}}^ + } }$ . The rate‐determining constants, k_0,A‐1, k_0,A‐2, and k_0,A‐SE2 were evaluated in all cases. The R=P(OPh)₃ complex was most reactive due to its π‐acid character, which favors the rupture of the trans nitrogen–palladium bond in the A‐2 mechanism and also that of the pyridine nitrogen–palladium bond in the A‐1 mechanism. The organometallic bond exerts no effect on the relative basicity of the complexes, which are strongly reliant on the substituent.     

Kinetics and Mechanism of the Nitration of Chlorobenzene with Nitric Acid

The rate of homogeneous nitration of chlorobenzene with 70-90% nitric acid is proportional to the chlorobenzene concentration and activity of nitric acid. The existence of linear correlations between the rate constants k _2ap and the acidity function -H and between logk ^* _2ap values (calculated using NO₂ ⁺ concentration) and the acidity function -(H _R + logaH ₂O) indicates that the nitrating species is nitronium ion generated by reaction of H₃O⁺ with nitric acid monohydrate. Increase in the energy of activation with rise in water concentration is explained by increase of H for equilibrium formation of nitronium ion.     

Chromium(III)-isonicotinate Complexes. Kinetics and Mechanism of Isonicotinate Ligand Liberation in HClO4 Solutions

Chromium(III)-isonicotinate complexes, cis-[Cr(C₂O₄)₂(N-inic)(H₂O)]^- and [Cr(C₂O₄)(H₂O)₃-OH-Cr(C₂O₄)₂(O-inic)]^-(N-inic)(H₂ (N-inic = N-bonded and O-inic = O-bonded isonicotinic acid) were obtained and characterized in solution. Kinetics of acid-catalyzed isonicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO₄ range, at I=1.0 m. The dependencies of the pseudo-first order rate constant on [H⁺] were established: k_obs = k₀+k_HQ_H[H⁺] and k_obs = k_HQ_H[H⁺] for the N-inic and O-inic complex, respectively, where k₀ and k_H are the rate constants of the spontaneous and the acid-catalyzed reaction paths, and Q_H is the protonation constant of the carboxylic group in isonicotinic ligand. The obtained results indicate that N-bonded isonicotinic acid liberation occurs mainly via a spontaneous reaction path and is much slower than O-bonded inic liberation. The mechanisms for these processes are proposed.     

Kinetics of the reaction: 2C2H4 → C2H5 + C2H3· heat of formation of the vinyl radical

The initial rates of formation of the major products in the thermal reactions of ethylene at temperatures in the neighborhood of 800 K have been measured in the presence and absence of the additives neopentane and ethane. It has been shown that in the absence of the additive the main initiation process is (1) while in the presence of neopentane and ethane the following additional initiation processes occur: (2) From the ratios of the rates of formation of the major products in the presence and absence of the additive the ratios k_N/k₁ and k_E/k₁ were measured over the temperature range of 750–820 K. Taking values from the literature for k_N and k_E, the following value was obtained for k₁: Previous results using butene-1 as additive were rexamined and shown to be consistent with this measurement. From this measurement the following values were derived: ΔH_f(C₂H₃) = 63.4 ± 2 kcal/mol and D(C₂H₃? H) = 103 kcal/mol.     

New chromium(III)–nicotinate complexes. Kinetics and mechanism of nicotinate ligand liberation in acidic media

Two new chromium(III)–nicotinate complexes, cis-[Cr(C₂O₄)₂(O-nic)(H₂O)]⁻ and cis-[Cr(C₂O₄)₂(N-nic)(H₂O)]⁻, were obtained and characterized in solution (where O-nic=O-bonded and N-nic=N-bonded nicotinic acid). The kinetics of nicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO₄ range, at I=1.0 m. The rate equations were determined and a mechanism is proposed. The rate of Cr–O bond breaking is [H⁺] dependent: k_obs=k_HQ_H[H⁺], where k_H is the acid-catalyzed rate constant and Q_H is the protonation constant of the nonbonded oxygen atom in the O-coordinated ligand. The Cr–N bond breaking proceeds via two paths: spontaneous and acid-catalyzed; k_obs=k₀ + k_HQ_H[H⁺], where k₀ and k_H are the spontaneous and acid catalyzed rate constants and Q_H is the protonation constant of the carboxylic group in the N-bonded nicotinic acid. The results demonstrate by comparison that Cr–N bond breaking is a much slower process than Cr–O bond fission.     

Kinetics and mechanism of oxygen electroreduction on PtCoCr/C catalyst containing 20–40 wt % platinum

It is shown that the mechanism of oxygen electroreduction on the PtCoCr/C systems in 0.5 M H₂SO₄ is similar to that proposed for the Pt/C catalyst. The activity of ternary catalysts is by two and more times higher than that of monoplatinum catalyst. The constant k ₁ is much larger than k ₂ (k ₁ and k ₂ are the rate constants of O₂ reduction to water and H₂O₂, respectively) for all catalysts studied. This indicates that the catalytic systems are selective with respect to O₂ reduction immediately to water in the practically important potential range from 1.0 to 0.6 V. The yield of H₂O₂ increases with a shift of potential in the cathodic direction (<0.7 V) and does not exceed 1%. The sum of rate constants of further conversion of hydrogen peroxide also increases with a shift of potential in the cathodic direction. After a corrosion attack (a treatment in the acid for 24 h), a ratio between the rate constants (k ₁/k ₂) for the PtCoCr/C catalysts increases. This is caused by a considerable increase in k ₁, which is 2.84 × 10⁻² cm/s for the catalyst containing 34 wt % Pt (against 1.5 × 10⁻² cm/s for the untreated catalyst). This can be explained by the reaction proceeding on the particle surface, which was enriched in platinum in the course of corrosion treatment. The properties of platinum clusters on the alloy surface differ from those of monoplatinum as a result of the ligand effect. The amount of oxygen chemisorbed from water on this surface is lower than on Pt/C catalyst. This is the main factor determining an increase in the activity and stability of ternary catalysts.     

Some reactions of the isopropyl radical

The decomposition of ethane sensitized by isopropyl radicals was studied in the temperature range of 496–548°K. The rate of formation of n-butane, isopentane, and 2,3-dimethylbutane were measured. The expression k₁/k₂^½ was found to be where k₁ and k₂ are rate constants of The decomposition of propylene sensitized by isopropyl radicals was studied between 494 and 580°K by determination of the initial rates of formation of the main products. The ratio of k₁₃/k₂^1/2 was evaluated to be where k₁₃ is the rate constant for The isomerization of the isopropyl radical was investigated by studying the decomposition of azoisopropane. The decomposition of the iso-C₃H₇ radical into C₂H₄ and CH₃ was followed by measuring the rate of formation of C₂H₄. On the basis of the experimental data, obtained in the range of 538–666° K, k₁₅/k₂^½ was found: where k₁₅ is the rate constant of      

The reaction of OH with NO2 and the deactivation of O(1D) by CO

NO₂ was photolyzed with 2288 Å radiation at 300° and 423°K in the presence of H₂O, CO, and in some cases excess He. The photolysis produces O(¹D) atoms which react with H₂O to give HO radicals or are deactivated by CO to O(³P) atoms The ratio k₅/k₃ is temperature dependent, being 0.33 at 300°K and 0.60 at 423°K. From these two points, the Arrhenius expression is estimated to be k₅/k₃ = 2.6 exp(?1200/RT) where R is in cal/mole – °K. The OH radical is either removed by NO₂ or reacts with CO The ratio k₂/k^α is 0.019 at 300°K and 0.027 at 423°K, and the ratio k₂/k⁰ is 1.65 × 10^?5M at 300°K and 2.84 × 10^?5M at 423°K, with H₂O as the chaperone gas, where k^α = k₁ in the high-pressure limit and k⁰[M] = k₁ in the low-pressure limit. When combined with the value of k₂ = 4.2 × 10⁸ exp(?1100/RT) M^?1sec^?1, k^α = 6.3 × 10⁹ exp (?340/RT)M^?1sec^?1 and k⁰ = 4.0 × 10¹²M^?2sec^?1, independent of temperature for H₂O as the chaperone gas. He is about 1/8 as efficient as H₂O.     

Experimental study of the D + H2 (v = 1) reaction by CARS spectroscopy

The reactions D + H₂ (v = 0, 1) → HD (v = 0, 1) + H have been studiedin a discharge flow reactor by CARS-spectroscopy. For H₂(v = 0) molecules a rate constant of (4, 0 ± 1, 0) 10^?16 cm³ s^?1 is obtained at 310 K from measured HD (v = 0, 1) product yields. Keeping the degree of vibrational excitation of H₂in the microwave discharge in the range of 1% from the increase of the HD (v = 0, 1) CARS signals a rate of k_{2a, b} = (1, 0 ± 0, 4) 10^?13cm³ s^?1 is derived. The total consumption of H₂ (v = 1) in the presence of D atoms gives a rate k₂ = (1, 9 ± 0, 2) 10^?13 cm³ s^?1 at 310 K. The resultsare discussed in regard to previous measurements and theoretical treatments.     

Experimental versus theoretical evidence for the rate‐limiting steps in uncatalyzed and H+‐ and HO−‐catalyzed hydrolysis of the amide bond

Recent theoretical studies of the alkaline hydrolysis of the amide bond have indicated that the nucleophilic attack of the hydroxide ion at the carbonyl carbon of the amide group is rate limiting. This is shown to be inconsistent with a large amount of experimental observations where the expulsion of the leaving group has been shown to be rate limiting. A kinetic approach has been described, which allows us to diagnose whether the pH‐independent/uncatalyzed hydrolysis of amides involves (a) both the uncatalyzed water reaction (k_w) and H⁺‐ (k_H) and HO^?‐catalyzed (k_OH) water reaction, (b) only the k_w reaction, or (c) only the k + k_OH reaction. The analysis described in this critical review does not favor the recent theoretical claims of the absence of the water reaction in the pH‐independent/uncatalyzed hydrolysis of formamide and urea. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 599–611, 2009     

Cathodic reduction of oxygen on PdCo/C catalyst synthesized based on commercial Pd/C catalyst

The data on the cathodic PdCo/C catalyst prepared by high-temperature synthesis from 20 wt % Pd/C (E-TEK) are shown. According to XRD data, the catalyst represents an alloy with the preferential composition of Pd₂Co. The kinetics and mechanism of oxygen reduction on the PdCo/C catalyst are studied by the methods of rotating disk electrode, rotating ring-disk electrode, and electrochemical impedance. It is shown that oxygen is reduced preferentially to water (k ₁) but in the potential range more negative than 0.6 V, the ratio of constants k ₁/k ₂ decreases, which suggests that the contribution of the reaction that proceeds through the formation of H₂O₂ (k ₂) increases. The activity of PdCo/C catalyst under model conditions in 0.5 M H₂SO₄ was assessed to be 15 mA/mg_cat at a potential of 0.7 V.     

Synthesis and kinetic studies in aqueous solution on chromium(III) complexes with isocinchomeronic acid—potential new biochromium sources

The chromium(III) complexes with a new potential chromium transporting ligand—2,5-pyridinedicarboxylic acid (isocinchomeronic acid, icaH₂):[Cr(icaH)₃]⁰, [Cr(icaH)₂ (H₂O)₂]⁺ and [Cr(icaH)(H₂O)₄]²⁺ (where icaH = N,O-bonded isocinchomeronic acid anion), have been obtained and characterized in solution. The [Cr(icaH)₃]⁰ complex undergoes aquation in acidic media to the diaqua-product. Kinetics of this process was studied spectrophotometrically in the 0.1–1.0 M HClO₄ range, at I = 1.0 M. The first aquation stage, the chelate-ring opening at the Cr–N bond, is a much faster than the second one. The rate laws are of the form: k _obs = k ₁ + k ₋₁/Q ₁[H⁺] and k _obs = k ₂ Q ₂[H⁺]/(1 + Q ₂[H⁺]), where k ₁ and k ₂ are the rate constants for the chelate-ring opening and the ligand liberation, respectively, k ₋₁ is the rate constant of the chelate-ring closure, Q ₁ and Q ₂ are the protonation constants of the pyridine nitrogen and 5-carboxylate group in the one-end bonded intermediate, respectively. The results are discussed in terms of potential pharmaceutical application of the complex.     

Kinetic Study of the Reaction of Leuco Methylene Blue with 2,6-Dimethyl-<Emphasis Type="Italic">p</Emphasis>-benzoquinone in a Reverse Micellar System

The kinetics of the reaction of leuco methylene blue (MBH) with 2,6-dimethyl-p-benzoquinone (DMBQ) were studied in a heptane/bis(2-ethylhexyl)-sulfosuccinate (AOT)/water reverse micellar system. The pseudo-first-order rate constant (k _obsd) obtained in the presence of excess of DMBQ was found to be proportional to the initial concentration of DMBQ for W ₀=3, 5, 10, 15 and 20 (W ₀=[H₂O]/[AOT]). The second-order rate constant (k ₂=k _obsd/[DMBQ]₀) increased with an increase in the W ₀ value, but was almost independent of the concentration of the water pool. A mechanism involving the distribution of DMBQ between the reverse micellar interface and bulk organic solvent was proposed to explain these findings.     

Reactivity and Operational Stability of N‐Tailed TAMLs through Kinetic Studies of the Catalyzed Oxidation of Orange II by H2O2: Synthesis and X‐ray Structure of an N‐Phenyl TAML

The catalytic activity of the N‐tailed (“biuret”) TAML (tetraamido macrocyclic ligand) activators [Fe{4‐XC₆H₃‐1,2‐( N COCMe₂ N CO)₂NR}Cl]^2? ( 3 ; N atoms in boldface are coordinated to the central iron atom; the same nomenclature is used in for compounds 1 and 2 below), [X, R=H, Me ( a ); NO₂, Me ( b ); H, Ph ( c )] in the oxidative bleaching of Orange II dye by H₂O₂ in aqueous solution is mechanistically compared with the previously investigated activator [Fe{4‐XC₆H₃‐1,2‐( N COCMe₂ N CO)₂CMe₂}OH₂]^? ( 1 ) and the more aggressive analogue [Fe(Me₂C{CON(1,2‐C₆H₃‐4‐X) N CO}₂)OH₂]^? ( 2 ). Catalysis by 3 of the reaction between H₂O₂ and Orange II (S) occurs according to the rate law found generally for TAML activators (v=k_Ik_II[Fe^III][S][H₂O₂]/(k_I[H₂O₂]+k_II[S]) and the rate constants k_I and k_II at pH 7 both decrease within the series 3 b > 3 a > 3 c . The pH dependency of k_I and k_II was investigated for 3 a . As with all TAML activators studied to‐date, bell‐shaped profiles were found for both rate constants. For k_I, the maximal activity was found at pH 10.7 marking it as having similar reactivity to 1 a . For k_II, the broad bell pH profile exhibits a maximum at pH about 10.5. The condition k_I?k_II holds across the entire pH range studied. Activator 3 b exhibits pronounced activity in neutral to slightly basic aqueous solutions making it worthy of consideration on a technical performance basis for water treatment. The rate constants k_i for suicidal inactivation of the active forms of complexes 3 a – c were calculated using the general formula ln([S₀]/[S_∞])=(k_II/k_i)[Fe^III]; here [Fe^III], [S₀], and [S_∞] are the total catalyst concentration and substrate concentration at time zero and infinity, respectively. The synthesis and X‐ray characterization of 3 c are also described.     

Primary photochemical processes in the photolysis of alkyl nitrites at 366 NM and 23°c in the presence of 15NO

The alkyl nitrites, C₂H₅ONO, n-C₃H₇ONO, n-C₄H₉ONO, and i-C₄H₉ONO were photolyzed at 23°C in the presence of ¹⁵NO at 366-nm incident radiation. The quantum yields of the corresponding isotopically-enriched alkyl nitrites were measured by mass spectrometry. The results indicated that only part of the absorption leads to photodecomposition. The remainder forms an electronically excited state which isotopically exchanges with ¹⁵NO. The indicated reactions of the electronically excited state RONO*, are where k₃/k₂ = 0.50 ± 0.10, 0.62 ± 0.20, 0.42 ± 0.06, and 0.24 ± 0.03 torr, and that k_2a/k₂ = 1.0, 1.0, 0.64 ± 0.04, and 0.56 ± 0.03, respectively, for C₂H₅ONO, n-C₃H₇ONO, n-C₄H₉ONO, and i-C₄H₉ONO.     

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

The total rate constant k₁ has been determined at P = 1 Torr nominal pressure (He) and at T = 298 K for the vinyl‐methyl cross‐radical reaction: (1) CH₃ + C₂H₃ → Products. The measurements were performed in a discharge flow system coupled with collision‐free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C₂H₄ and CH₄, respectively. The kinetic studies were performed by monitoring the decay of C₂H₃ with methyl in excess, 6 < [CH₃]₀/ [C₂H₃]₀ < 21. The overall rate coefficient was determined to be k₁(298 K) = (1.02 ± 0.53) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C₂H₃ + H and C₂H₃ + C₂H₃. The present result for k₁ at T = 298 K is compared to two previous studies at high pressure (100–300 Torr He) and to a very recent study at low pressure (0.9–3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH₃ + C₂H₅ and with a value for k₁ estimated by the geometric mean rule employing values for k(CH₃ + CH₃) and k(C₂H₃ + C₂H₃). Qualitative product studies at T = 298 K and 200 K indicated formation of C₃H₆, C₂H₂, and C₃H₅ as products of the combination‐stabilization, disproportionation, and combination‐decomposition channels, respectively, of the CH₃ + C₂H₃ reaction. We also observed the secondary C₄H₈ product of the subsequent reaction of C₃H₅ with excess CH₃; this observation provides convincing evidence for the combination‐decomposition channel yielding C₃H₅ + H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C‐H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 304–316, 2000