Kinetics of the formation of cyclobutane from ethylene

The rate of the thermal reaction of ethylene to form cyclobutane has been measured over the temperature range 723°–786°K and at pressures between 300 and 1300 torr. The equilibrium constant for the system \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$${\rm 2C}_{\rm 2} {\rm H}_{\rm 4}\mathop {\leftrightharpoons}\limits_{kf}^{kr} c - {\rm C}_{\rm 4} {\rm H}_{\rm 8}$$\end{document} was calculated both from the initial rate data and from measurements of the equilibrium concentration of cyclobutane. Agreement with the reported thermodynamic quantities for cyclobutane was satisfactory. The initial rate data gave the following epxression for k_f: while the measurements of the equilibrium concentration of cyclobutane gave the expression for K, .     

Enantioselective synthesis of pantolactone analogues from an ephedrine-derived morpholine-dione

An efficient enantioselective synthesis of β,β-dialkyl α-hydroxy γ-butyrolactones, analogues of pantolactone, has been developed employing a stereoselective Prins reaction of chiral alkylidene morpholinones, as the key step. Enantioselective synthesis of a naturally occurring pantolactone homologue and a spiro analogue of pantolactone was achieved with this protocol.     

One-pot synthesis of N-substituted pantolactams from pantolactone

Rac-N-substituted pantolactams (5) are readily obtained in medium to good yields by reaction of rac-pantolactone (1) with primary amines under acid catalysis, whether at 250°C in a pressure reactor or under microwave irradiation. It appears that the amine can react with pantolactone at the carbonyl carbon atom to give a hydroxyamide (3) in a reversible way and at the methylene carbon atom to give a γ-amino acid (4). The last one on dehydration would give the corresponding pantolactam (5).     

Kinetics of the formation of benzoguanamine from dicyandiamide and benzonitrile

The rates of the formation of benzoguanamine from dicyandiamide and benzonitrile in the presence of a basic catalyst, have been measured by means of UV spectrophotometry. The rate is expressed as ν = k × [dicyandiamide]·[benzonitrile]·[KOH]. Since the addition of a small amount of cyanamide retards the rate, cyanamide cannot be an intermediate. Application of the rate data observed with p-methyl-, methoxy-, and chlorobenzonitriles to the Hammett's equation gives a positive value of +1·82. A probable mechanism involving a rate-determining attack of the conjugate base of dicyandiamide on benzonitrile is suggested and discussed.     

Kinetics of epoxide resins formation from epichlorohydrin and bisphenol-A

The kinetics of formation of epoxide resins based on epichlorohydrin and bisphenol-A via synthesis with one- or two-step addition of alkali are considered. The kinetics of the reaction were studied by taking into account the consumption of the alkali added and the change of molecular mass and epoxide value of epoxide oligomers. The results obtained are satisfactorily explained within the framework of a model containing the following postulates: all the alkali is bound as phenolate ions; all phenolic groups are of equal reactivity; the reactivity of epichlorohydrin is higher than that of glycidyl ethers; the stage of dehydrochlorination of chlorohydrin ethers in the synthesis with one-step addition of alkali is rapid and does not limit the process; the first stage of the two-step synthesis is the formation of chlorohydrin ethers and the second stage their consumption. The factors leading to differences between the experimentally obtained parameters of the resulting epoxide resins (molecular weight, epoxide value) and their calculated values are discussed. The synthesis of epoxide resins with two-step alkali addition is shown to possess certain advantages over the one-step process.     

A voltammetric method for the quantitative determination of midecamycin compared to its simultaneous HPLC determination

The aim of this study was to present the quantitative determination of midecamycin at a gold electrode in 0.05 M NaHCO₃ using cyclic linear sweep voltammetry. It was found that the value of the oxidative peak of pure midecamycin at 0.85 V vs. SCE at a scan rate of 50 mV s⁻¹ is a linear function of the concentration in the range 0.1693–0.3289 mg cm⁻³. The value of its reductive peak at 0.3 V vs. SCE is a linear function of the concentration in the range 0.11396–0.3802 mg cm⁻³. HPLC analysis of the bulk of electrolyte confirmed the data obtained by analysis of the current peak values concerning the correlation with each investigated concentration of midecamycin. By MS spectrometry no degradation products were found in electrolyte during its quantitative determination.     

Kinetics of bayerite microstructure formation from powdered aluminum

Electron microscopy is employed to study the kinetics of bayerite microstructure formation during the hydration hardening of powdered aluminum at 373 K. It is shown that the formation of splices between Al(OH)₃ crystallites is governed by the diffusion of aluminum hydroxocomplexes through growing porous bayerite layers. Adsorption and textural properties of dehydrated materials are investigated.     

Kinetics and mechanism for oxime formation from 2-quinolinecarboxaldehyde

Oxime formation from 2-quinolinecarboxaldehyde occurs with rate-limiting carbinolamine dehydration under both acidic and neutral conditions. Carbinolamine dehydration occurs via a transition state bearing a single positive charge, similar to the corresponding reaction for 2-pyridinecarboxaldehyde and unlike the same reaction for 2-, 3-, and 4-formyl-1-methylpyridinium ions and for the conjugate acids of 3- and 4-pyridinecarboxaldehydes. © 1996 John Wiley & Sons, Inc.     

Kinetics of styrylquinolinc formation

A comprehensive kinetic investigation of reactions occurring in the formation of styryl-quinolines has been conducted. Specific rate data such as rate equations, rate constants, and thermodynamic activation values have been determined and utilized in a study of which factors are of greatest importance in the reactions forming 2-styrylquinolines. A mechanism has been proposed for the condensation reaction which agrees with rate relationships found. Gas-liquid partition chromatography was used to follow the kinetics of the condensation reactions. A rate constant of 5.41 × 10^?2M^?1min^?1 was found for the reaction of benzaldehyde with 2-methyl-quinoline using zinc chloride as a catalyst at 104.0°. Rate constants of 1.28 × 10^?2 MT^?1 min^?1 and 1.05 × 10^?2 M^?1 min^?1 were found for the reactions of p-methylbenzaldehyde and p-methoxybenzaldehyde with quinaldine to form 2-(p-methylstyryl)quinoline and 2-(p-methoxystyryl)-quinoline, respectively at 92.4°. A linear relationship was found using the Hammett equation. An Arrhenius plot was constructed from rate constants determined at five different temperatures for the reaction of benzaldehyde and quinaldine to form 2-styrylquinoline, using zinc chloride as a catalyst. The energy of activation, E_a, was found to be 22.2 kcal/mole for this reaction. The enthalpy of activation, ΔH?, free energy of activation, ΔF?, and entropy of activation, ΔS?, were found to be 21.4 kcal/mole, 27.7 kcal/mole and -16.7 eu/mole, respectively, at 104.0°. The mechanism proposed in the formation of 2-styrylquinoline involves the fast formation of a carbanion-zinc chloride complex, which then attacks, in the rate determining step, the aldehyde utilized in the reaction. The lack of reaction of certain methylquinolines is attributed to the inadequacy of the carbanion formed and not to the difficulty involved in the initial formation of the carbanion.     

Kinetics of vesicle formation

 The kinetics of vesicle formation from a hydrotrope (sodium xylenesulfonate) solution of a surfactant (Laureth 4) is studied by the use of a stopped-flow apparatus combined with a dynamic light scattering device to determine vesicle size in the system. The hydrotrope system studied presents a system with a high surfactant solubilization combined with vesicle formation simply by dilution with water. The kinetic results show a single exponential decay time. The kinetic analysis indicates that the vesicles are formed from a molecular solution which resulted from the shear in the stopped-flow device and grow by monomeric association. Received: 1 October 1996 Accepted: 22 November 1996     

Kinetics and mechanism of peroxymonocarbonate formation

The kinetics and mechanism of peroxymonocarbonate (HCO(4)(-)) formation in the reaction of hydrogen peroxide with bicarbonate have been investigated for the pH 6-9 range. A double pH jump method was used in which (13)C-labeled bicarbonate solutions are first acidified to produce (13)CO(2) and then brought to higher pH values by addition of base in the presence of hydrogen peroxide. The time evolution of the (13)C NMR spectrum was used to establish the competitive formation and subsequent equilibration of bicarbonate and peroxymonocarbonate following the second pH jump. Kinetic simulations are consistent with a mechanism for the bicarbonate reaction with peroxide in which the initial formation of CO(2) via dehydration of bicarbonate is followed by reaction of CO(2) with H(2)O(2) (perhydration) and its conjugate base HOO(-) (base-catalyzed perhydration). The rate of peroxymonocarbonate formation from bicarbonate increases with decreasing pH because of the increased availability of CO(2) as an intermediate. The selectivity for formation of HCO(4)(-) relative to the hydration product HCO(3)(-) increases with increasing pH as a consequence of the HOO(-) pathway and the slower overall equilibration rate, and this pH dependence allows estimation of rate constants for the reaction of CO(2) with H(2)O(2) and HOO(-) at 25 °C (2 × 10(-2) M(-1) s(-1) and 280 M(-1) s(-1), respectively). The contributions of the HOO(-) and H(2)O(2) pathways are comparable at pH 8. In contrast to the perhydration of many other common inorganic and organic acids, the facile nature of the CO(2)/HCO(3)(-) equilibrium and relatively high equilibrium availability of the acid anhydride (CO(2)) at neutral pH allows for rapid formation of the peroxymonocarbonate ion without strong acid catalysis. Formation of peroxymonocarbonate by the reaction of HCO(3)(-) with H(2)O(2) is significantly accelerated by carbonic anhydrase and the model complex [Zn(II)L(H(2)O)](2+) (L = 1,4,7,10-tetraazacyclododecane).     

Kinetics of the formation of Liesegang rings

The kinetics of the formation of Liesegang rings is considered and they are classified. The relationship between the position of the propagating diffusion front and the moments of the formation of Liesegang rings is shown. In order to describe the formation processes of the Liesegang rings a generalized model based on the diffusion equation hierarchy is proposed. The kinetics of changes in the dispersed phase sizes at the initial stage of the formation of Liesegang rings is studied.     

Kinetics of levulinic acid formation from carbohydrates at moderate temperatures

The kinetic curves of levulinic acid accumulation in the process of saccharide dehydration at 80-98^oC are compared. The structural features of the substrates and 5-hydroxymethylfurfural, which is the intermediate of the process, explain the observed difference in the rates of fructose and glucose conversion. Sulfuric acid can be effectively used under moderate conditions to synthesize levulinic acid with yields exceeding 35 mol % for glucose and 50% for sucrose at 98^oC.