Aqueous Sulfuric Acid Medium Research Articles

Acid-catalysed aryl hydroxylation of phenylazopyridines: reaction intermediates, kinetics and mechanism 1

A kinetic and product analysis study of the reactions of the three isomeric phenylazopyridines (PAPys) in aqueous sulfuric acid media (30–97 wt% H2SO4) is reported. The final products obtained from the reaction of 4-(phenylazo)pyridine (4-PAPy) are the hydroxylated product 4-(4-hydroxyphenylazo)pyridine, the reduction products 4-aminophenol and 4-aminopyridine, and a small amount of a dimerized product. 3-(Phenylazo)pyridine is unreactive, but 2-(phenylazo)pyridine gives the equivalent 2-(4-hydroxyphenylazo)pyridine, 4-aminophenol and 2-aminopyridine products. This product pattern, an oxidized azo-compound and two reduced amines, is similar to that found in the disproportionation of di-p-substituted hydrazinobenzenes observed in benzidine rearrangement studies. Consequently it has been proposed that the corresponding [N′-(4-hydroxyphenylhydrazino)]pyridines were formed as reaction intermediates in the present system; this is confirmed by showing that [N′-4-(4-hydroxyphenylhydrazino)pyridine synthesized independently gave the same products as 4-PAPy under the same conditions. The kinetic study shows that the 4-isomer reacted faster than the 2-isomer at all the acid concentrations investigated (the 3-isomer being inert). Rate maxima are observed, at ≈72 wt% H2SO4 for 4-PAPy and ≈86 wt% H2SO4 for 2-PAPy. To facilitate the kinetic analysis, values of pKBH22+ for the protonation of the substrates and the possible hydroxy products at the azo-group were determined, using the excess acidity method; the first protonation occurs on the pyridine nitrogen in the pH region. An excess acidity analysis of the observed pseudo-first-order rate constants as a function of acidity indicate an A2 mechanism, with the diprotonated substrate and either one HSO4– ion or one H2O molecule in the activated complex. The proposed mechanism thus involves nucleophilic attack of HSO4– or H2O at an aryl carbon of the diprotonated substrate in the slow step, resulting in an intermediate hydrazo species which gives the observed products in a subsequent fast step (cf. benzidine rearrangement).

Aqueous polymerization of methyl methacrylate initiated by ceric ion reducing agent systems in sulfuric acid medium

Polymerization of methyl methacrylate (MMA) was carried out in aqueous sulfuric acid medium at 30°C using ammonium ceric sulfate (ACS)/methyl ethyl ketone (MEK) and ammonium ceric sulfate/acetone as redox initiator systems. A short induction period was observed with both the initiator systems, as well as the attainment of limiting conversion for polymerization reactions. The rate of ceric ion consumption, R Ce , was first order with respect to Ce(IV) concentration in the concentration range (0.5-5.5) X 10 -3 m, and 0.5 order with respect to reducing agent concentration in the concentration ranges (0.0480-0.2967M) and (0.05-0.3912M) for Ce(IV)-MEK and Ce(IV)-acetone initiator systems, respectively. A fall in R Ce was observed at higher reducing agent concentrations. The plots of R Ce versus reducing agent concentrations raised to the half power yielded straight lines passing through the origin, indicating the absence of complex formation between reducing agents and Ce(IV). The addition of sodium sulfate to maintain constant sulfate ion concentration in the reaction medium could bring down the R Ce values in the present reaction systems. The rate of polymerization of MMA, R p , increased with increase in Ce(IV), reducing agent, and monomer concentrations for the Ce(IV)-MEK initiator. The rate of polymerization of MMA is independent of Ce(IV) concentration and increased with an increase in reducing agent and monomer concentrations for the Ce(IV)-acetone initiator. At higher concentrations of reducing agent (0.4-0.5M), a steep fall in R p values was observed with both the initiator systems. The orders with respect to Ce(IV), MEK, and MMA using the Ce(IV)-MEK initiator were found to be 0.23, 0.2, and 1.29, respectively. The orders with respect to Ce(IV), acetone, and MMA using the Ce(IV)-acetone initiator were found to be zero, 0.42, and 1.64, respectively. Maintaining constant [SO 4 2- ] in the reaction medium could bring down R p values for the Ce(IV)-MEK initiator system. On the other hand, a rise in R p values with an increase in [Na 2 SO 4 ] could be observed when constant [SO 4 2- ] was maintained in the reaction medium for the Ce(IV)-acetone initiator system. A kinetic scheme involving direct attack of Ce(IV) on reducing agent, production of radicals, initiation, propagation, and termination of the polymeric radicals by bimolecular interaction is proposed. An oxidative termination of primary radicals by Ce(IV) is also included.

Photometric and fluorimetric study of the acid-base behavior of 2,2?-diquinolyl and 2,2?,2?-terpyridyl

A photometric and fluorimetric study of the acid-base behavior of 2,2'-diquinolyl and 2,2',2″-terpyridyl was performed. In sulfuric acid medium, the doubly charged 2,2'-diquinolynium ion undergoes the first dissociation atH 0=0.20±0.09, as determined by fluorimetry (λex=336 nm, λem=424 nm). Photometric titration is less accurate because of the overlapping of the absorption spectra. The second dissociation constant of 2,2'-diquinolyl was determined by fluorimetric titration (λex=336 nm, λem=420 nm), obtaining a value of 3.67±0.03. The triply charged 2,2',2″-terpyridyl molecule was found to undergo the first dissociation atH 0=-7.17±0.04, as determined by fluorimetric titration (λex=316 nm, λem=350 nm), in aqueous sulfuric acid medium. Photometric titration (λ=335 nm) was performed in the presence of 6.5% ethanol because of the low solubility of the compound in water. In this ethanolic∶water medium, a value of the dissociation constant atH 0=-7.39±0.03 was calculated. The second dissociation constant was determined to be 2.81±0.12 by photometric titration at 285 nm, and values of 4.03±0.26 and 4.16±0.20 were found for the third dissociation constant by photometric titrations at 320 and 295 nm, in 10% ethanol, in close agreement with previously reported values. The fluorimetric titration profile obtained by exciting at 274 nm and measuring the fluorescence emission at 350 nm, in the zone betweenH 0=-3 and pH=10, is complicated by the several equilibria involved.

Graft Copolymerization of Acrylic Acid onto Biomedical Polyvinyl Alcohol Initiated by Ce(IV)-Glucose Redox System: Part 2∗

Abstract Graft copolymerization of acrylic acid onto polyvinyl alcohol was carried out using a Ce(IV)-glucose redox system in an aqueous sulfuric acid medium under nitrogen atmosphere. The optimum conditions for grafting were determined by studying the effects of concentrations of metal ion, glucose, monomer, and acid on the percentage of grafting. The graft yield was influenced by reaction time, temperature, the amount of polyvinyl alcohol, some organic solvents, and inorganic salts. More than 80% graft yield could be achieved with the. present system. A suitable kinetic scheme is proposed on the basis of experimental findings.

Mechanism of oxidation of iron(II) chelates by hexavalent chromium and heptavalent manganese in aqueous sulfuric acid and micellar media - a kinetic approach

Oxidation of the FeII chelates [FeL] (L = phen or bipy) by CrVI and MnVII in H2SO4 medium was found to proceed through the formation of a bimetallic insertion complex which decomposes in the slow step, followed by electron transfer from [FeL] to the oxidant. The reactions are catalysed by both anionic and non-ionic micelles [SDS and triton-x (Tx), respectively]. A mechanism is suggested involving electrostatic stabilization of the cationic forms of the FeII chelates by anionic SDS and the partial anionic character of polyoxyethylene moiety of Tx, respectively. The marginal catalysis of cationic micelles (CTAB) is attributed to co-anion-micellar interactions.

Oxidation of a Dipeptide by Electrolytically Generated Manganese(III) in Aqueous Sulfuric Acid Medium: A Kinetic and Mechanistic Study

Abstract The kinetics of oxidation of the simplest dipeptide, glycylglycine (GG) by electrolytically generated manganeseIII-sulfate in aqueous sulfuric acid has been studied. The rate shows first order dependence on [MnIII] and a fractional order each on [GG] and [H+]. The reduced product, MnII has no significant effect on the rate. An increase in ionic strength or decrease in dielectric constant of the medium increases the rate, while addition of complexing agents such as Cl−, F−, and P2O74− decreases the rate. A Michaelis–Menten type mechanism has been suggested to explain the results. The coefficient of the rate-determining step has been evaluated as a function of temperature, by measuring the rates at different temperatures. The corresponding activation parameters have also been computed. Evidence for transient existence of a free radical reaction intermediate has been given. The kinetics of oxidation of GG have also been compared with those of the monomer amino acid, glycine.

Extraction of U(VI) by phosphonic acid in the presence of neutral donors

Extraction of U(VI) by mixtures of 2-ethylhexyl 2-ethylhexylphosphonic acid (PC88A) and TOPO in cyclohexane and xylene from aqueous sulfuric acid medium has been investigated. Synergism was observed and the species responsible was identified as UO2H2Y4.TOPO. From the data equilibrium constants for the various reaction equilibria at different temperatures and the thermodynamic parameters were calculated. Antagonism was observed when alkylamide or TBP was used as the neutral donor.

Kinetics and mechanism of ruthenium (III) catalyzed oxidation of ethanol by cerium (IV) in aqueous sulfuric acid media

AbstractThe kinetics of oxidation of ethanol by cerium(IV) in presence of ruthenium(III) (in the order of 10−7 mol dm−3) in aqueous sulfuric acid media have been followed at different temperatures (25–40°C). The rate of disappearance of cerium(IV) in the title reaction increases sharply with increasing [C2H5OH] to a value independent of [C2H5OH] over a large range (0.2–1.0 mol dm−3) in which the rate law conforms to: where [Ru]T gives the total ruthenium (III) concentration. The values of 10−3kc and 10−3kd are 3.6 ± 0.1 dm3 mol−1 s−1 and 3.9 ± 0.2 s−1, respectively, at 40°C, I = 3.0 mol dm−3. The proposed mechanism involves the formation of ruthenium(III)− substrate complex which undergoes oxidation at the rate determining step by cerium(IV) to form ruthenium(IV)− substrate complex followed by the rapid red‐ox decomposition giving rise to the catalyst and ethoxide radical which is oxidized by cerium(IV) rapidly. The mechanism is consistent with the existence of the complexes RuIII · (C2H5OH) and RuIII · (C2H5O−) and both are kinetically active. The overall bisulphate dependence conforms to: kobsd = A[Ru]T/{1 + C[HSO4−]} where A = 2.2 × 104 dm3 mol−1 s−1, C = 1.3 at 40°C, [H+] = 0.5 mol dm−3, and I = 3.0 mol dm−3. The observations are consistent with the Ce(SO4)2 as the kinetically active species. © 1995 John Wiley & Sons, Inc.

Block copolymerization initiated by Mn(III)-poly(ethylene glycol) redox system ? general features and kinetics

The kinetics and mechanism of thermal polymerization of acrylonitrile initiated by Mn(III) pyrophosphate — poly(ethylene glycol) (PEG, molecular weight 6000) redox system in aqueous sulfuric acid medium was studied in the temperature range 30–60°C. The overall rates of polymerization and the disappearance of Mn3+ were determined. The polymerization was initiated by the organic free radical produced from the Mn3+-PEG reaction and the termination was by the metal ions. The rate of polymerization of acrylonitrile was found to be directly proportional to the square of the monomer concentration and first power of PEG concentration, and inversely proportional to the concentration of Mn3+. The rate of manganic ion disappearance was found to be directly proportional to manganic ion concentration and PEG concentration, and independent of the monomer concentration. Based on these observations, a plausible reaction scheme was suggested and suitable kinetic expressions were evaluated.

Kinetics and mechanism CF oxidation of DL-methionine by mangenese(III) in sulfuric acid medium

The kinetics of oxidation of DL-methionine by manganesei(III) have been studied at 30°C in aqueous sulfuric acid medium. The reaction is first order each in managanese (III) and methionine. A suitable mechanism is proposed which leads to the rarte law. $$ - \frac{{d[Mn(III)]}}{{dt}} = \frac{{k_l [H^ + ][Mn(III)][Met]}}{{[H^ + ] + K_h }}$$ The oxidation of methione, a sulfur containing, optically active essential amino acid, has been studied from the mechanistic point of view only by a few oxidants like iodine [1, 2], chloramine-B [3], bromamine-B [4], chromium(VI) [5, 6], cerium(IV) [7] vanadium(V) [7] and hexacyanoferrate(III) (Os(VIII) catalyzed and uncatalyzed) [8, 9]. The present kinetic study on the oxidation of methionine by manganese (III) has been under-taken in aqueous sulfuric acid medium, since the reaction is very fast in aqueous perchloric acid medium.

Kinetics and mechanism of ruthenium(III) catalysed oxidation of formic acid by cerium(IV) in aqueous sulfuric acid media

The kinetics of oxidation of formic acid by cerium(IV) in the presence of ruthenium(III)(ca. 10–6 mol dm–3) in aqueous sulfuric acid media has been followed at different temperatures (30–50 °C). The rate of disappearance of cerium(IV) in the reaction has been found to be zero order with respect to cerium(IV) concentration. At a fixed [H+], under the conditions, [HCO2H]T[CeIV]T[Ru]T the observed zero-order rate constant (k0) conforms to: –d[CeIV]t/dt=k0=[Ru]T[HCO2H]T{kb+kc[HCO2H]T} where [Ru]T and [HCO2H]T represent the total concentration of ruthenium(III) and formic acid respectively. At 40 °C, [H2SO4]= 1.0 mol dm–3 and I= 2.75 mol dm–3 the values of 102kb and 102kc are 6.0 ± 0.1 dm3 mol–1 s–1 and 5.4 ± 0.1 dm6 mol–2 s–1 respectively. Both kb and kc are found to have an inverse hydrogen-ion dependence. Out of the different possible mono- and bis-complexes, RuIII(HCO2–) and RuIII(HCO2–)(HCO2H) have been found to be kinetically active in the slow oxidative steps (through an inner-sphere mechanism) leading to RuIIIH (through hydride transfer from the C–H bond of metal-bound formate) and CO2 followed by the rapid oxidation of RuIIIH to RuIII by cerium(IV). The activation parameters are ΔH‡= 46 ± 3 kJ mol–1, ΔS‡=–125 ± 5 J K–1 mol–1(for the kb path) and ΔH‡= 47 ± 3 kJ mol–1, ΔS‡=–120 ± 5 J K–1 mol–1(for the kc path).

Reactions of azolium cations. I. Kinetics and mechanism of alkylation of 5‐phenyltetrazole with isopropyl alcohol in aqueous sulfuric acid media

AbstractAlkylation of 5‐phenyltetrazole with isopropyl alcohol in 84–99% sulfuric acid medium occurs regioselectively at N2 position of the heterocycle to produce 2‐isopropyl‐5‐phenyltetrazole. The regioselectivity is conditioned by 1H, 4H‐5‐phenyltetrazolium cation being the alkylation substratum. The reaction mechanism is proved to be two‐stage one. Its first and rate‐limiting stage is an interaction between the azolium cation and isopropyl cation generated from protonated isopropyl alcohol. The reaction rate constant was estimated as 0.47 L mol−1 s−1 at 25°C. Water activity scale was used for quantitative description of equilibrium of isopropyl cation formation from the protonated alcohol, and the equilibrium constant pKR+= −5.3 was evaluated from the data of the alkylation kinetics. © 1993 John Wiley & Sons, Inc.

Polymerization of acrylonitrile initiated with citric acid‐ceric ion redox system

AbstractThe polymerization of acrylonitrile was studied in aqueous sulfuric acid medium with the redox initiator system citric acid‐ceric ion. The effect of ceric ion, citric acid and sulfuric acid concentrations and of temperature on the monomer conversion and average molecular weights of the polymer was investigated. A polymerization mechanism is suggested.

Electrolytically Generated Manganese(III) Sulfate for the Oxidation of l-Histidine in Aqueous Sulfuric Acid: a Kinetic Study

Abstract Conditions have been established for the electrogeneration of Mn(III) from Mn(II) in sulfuric acid medium. Mn(III) formed was identified and characterized by UV spectra. Kinetics of Mn(III) oxidation of l-histidine in aqueous sulfuric acid medium has been studied spectrophotometrically. Oxidation followed second order kinetics in [Mn(III)], first order in [histidine] and increase in [H+] decreased the rate. The reduced product of the oxidant, Mn(II), retarded the rate of reaction. Evidence for the transient existence of the free radical reaction intermediate is given. Effects of ionic strength, solvent composition, HSO4− and certain complexing agents like P2O74−, Cl−, F−, have also been studied. Added complexing agents decreased the redox potential of the Mn(III)/Mn(II) couple, thereby reducing the oxidation rate. Dependence of reaction rate on temperature is examined and activation parameters computed from Arrhenius and Eyring plots. A mechanism consistent with kinetic results has been proposed in which a combined role of Mn(III)aq and MnOH2+aq was envisaged.

ChemInform Abstract: Mechanism of Reactions of Some N1‐Substituted Biguanides with Chromium(VI) in Aqueous Sulfuric Acid Media.

AbstractAmong biguanide, some N(1)‐substituted biguanides, and related compounds such as (III) and its N(1)‐phenyl derivative, only (Ia) (and its p‐methoxy derivative (Ib)) reacts with Cr(VI) to give quinone (II) and urea (III).

Studies of azo and azoxy dyestuffs. Part 18. Kinetics and mechanism of the hydrolysis of 3- and 4-(p′-methoxyphenylazo)pyridines in aqueous sulfuric acid media

The kinetics of hydrolysis of 4-(p′-methoxyphenylazo)pyridine, 1, and its 3-isomer, 2, have been studied in moderately concentrated sulfuric acid media at 25 °C. In all the acid solutions investigated, 1 reacted faster than 2; rate differences between the two compounds varied from ca. 1000-fold in the dilute region of acidity to ca. 250-fold in the more concentrated acid solutions. The observed first-order rate constants, kψ, for both substrates exhibit a maximum, at ca. 42% H2SO4 and 47% H2SO4 for 1 and 2 respectively. Activation parameters have also been determined. The pKa values for the second protonation equilibria of 1 and 2 have been evaluated and structures of the diprotonated species are discussed. Hydrolysis is shown to occur from the diprotonated substrates and two main mechanisms are operative. The first is an A-2 type mechanism, which involves rate-limiting attack of H2O on the aryl carbon center giving delocalized transition states and intermediates in which the pyridinium and azonium functions are involved in charge delocalization. Subsequent transfer of a proton and detachment of the leaving group are fast processes. In the second A-SE2 type mechanism, nucleophilic attack and transfer of the proton are fast steps preceding the slow general acid catalyzed separation of the leaving group. The difference in reactivity of the two compounds is attributed to differences in extent of charge delocalization in the transition states of the reactions: for 1 both the pyridinium and protonated azonium functions are involved whereas for 2 only the azonium function participates in charge delocalization.

Effect of Glycerol in the Polymerization of Methyl Methacrylate by Mn(III) Acetate

Abstract The kinetics of the mechanism of the polymerization of methyl methacrylate initiated by the glycerol/Mn(III) acetate redox system has been investigated in aqueous sulfuric acid medium in the temperature range of 40 to 50 °C. The effects of glycerol, methyl methacrylate, metal ion, acetic acid, and sulfuric acid on the rates of polymerization have been studied. One striking observation is that the increase in monomer concentration steadily decreases the rate of polymerization, contrary to what was observed in the case of acrylonitrile. On the basis of these observations, an appropriate kinetic scheme and rate expression have been developed.

Polymerization of acrylamide with ceric ion–malic acid redox pair

AbstractThe polymerization of acrylamide (M) in aqueous sulfuric acid medium initiated with ceric ammonium sulfate–malic acid redox pair was investigated at 35 ± 0.2°C under nitrogen atmosphere. The initiation was caused by the free radical generated by the decomposition of the complex formed between ceric ion and malic acid (MA). The rate of monomer disappearance was proportional to the first power of malic acid, ceric ion, and monomer concentrations at lower ceric ion concentrations. However, at higher ceric ion concentrations the rate was independent of [Ce(IV)]. The rate of ceric ion disappearance was proportional to [MA] and [Ce(IV)] but independent of [M] at lower ceric ion concentrations. The activation energy was found to be 57.74 kJ/mol. Sulfuric acid retarded the reaction. Molecular weights increased with increasing [M] and decreasing [Ce(IV)].

Kinetics of Polymerization of Vinyl Monomers Initiated by Manganese(III) Acetate. II.

The kinetics of the thermal polymerization of methyl acrylate (MA) and acrylamide (AAM) initiated by manganic ion in aqueous sulfuric acid medium has been studied. The rate of disappearance of [monomer] (Rp) was found to bear a square dependence on monomer and to be independent of [Mn2+] in the case of both the systems. Rp for acrylamide increased with the increase in [H+] but that for methyl acrylate remained almost constant. The degree of polymerization of the polymers (Xn) determined viscometrically was directly proportional to the [monomer] and inversely proportional to [Mn3+], for both systems. Xn increased with an increase in [H+] in the case of acrylamide and decreased in the case of methyl acrylate. Suitable reaction schemes to explain these experimentally observed facts have been proposed. Kinetic and thermodynamic parameters are also evaluated.

Polymerization of acrylamide initiated by the Ce4+/thiourea redox system

AbstractThe polymerization of acrylamide (M) initiated by the Ce4+/thiourea (TU) redox system has been studied in an aqueous sulfuric acid medium at 35 ± 0.2°C under nitrogen atmosphere. The rate of polymerization is governed by the expression The activation energy is 5.9 kcal deg−1 mol−1 in the investigated temperature range 30–50°C. The molecular weight is directly proportional to the concentration of monomer and inversely proportional to the catalyst concentration. With increasing concentration of DMF molecular weight decreases. The range of concentrations for which these observations hold at sulfuric acid concentration of 2.5 × 10−2 mol/L are [monomer] = 5.0 × 10−2–3.0 × 10−1, [catalyst] = (5.0–15.0) × 10−4, and [activator] = (1.0–6.0) × 10−3 mol/L.