Kinetics and mechanism of diels-alder additions of tetracyanoethylene to anthracene derivatives—I : Substituent effects

The rates of addition of tetracyanoethylene to a number of 9- and 9,10-substituted anthracenes have been measured spectrophotometrically in solvent CCl₄. Substituent effects correlated well using the extended form of the Hammett equation. The importance of steric effects on the reaction was assessed by a systematic variation of the components of the data used in the above correlation.Activation parameters (ΔG_exp^≠, etc.) and the corresponding overall thermodynamic parameters for adduct formation (ΔG_ad°, etc.) were evaluated. ΔG_exp^≠ was found to be linearly related to ΔG_c°, the free energy of formation of the intermediate complex which confirms the role of the latter as a true reaction intermediate. From correlations between ΔG_exp^≠ and ΔG_ad°, an “early” transition state is suggested. The above thermodynamic and activation data enable detailed reaction profiles to be drawn.     

Kinetics and mechanisms of diels-alder addition of tetracyanoethylene to anthracene derivatives—II : Solvent effects

The effect of change of solvent on the rate of Diels-Alder addition of tetracyanoethylene (TCNE) to anthracene has been investigated using solvents CCl₄, CHCl₃ and CH₂Cl₂. Solvent effects were measured on the intermediate complex and on the starting materials from solubility measurements. From this, solvent effects on the transition state alone can be evaluated. These effects were remarkable similar to those measured for both the initial state and the intermediate complex suggesting an “early” transition state having a structure similar to that of the intermediate. From the correlation of ΔG_t^≠ (the free energy of transfer from CCl₄ to another solvent for the transition state alone) with the solubility parameter δ², the molar volume of the transition state can be estimated. The result again suggests that the transition state is more “factor” than product-like.     

Kinetics and mechanism of reaction of p-methoxystyrene and tetracyanoethylene. II. Effect of pressure

The reaction of p-methoxystyrene and TCNE is studied in 50% CHCl₃–50% CCl₄ (v/v) at 25°C at high pressures up to 1000 bar by following the disappearance of the absorbance at 600 nm due to the EDA complex. By using the mixed solvent, equations required for the high-pressure kinetics are simplified. The volume of activation is ?42 to ?46 cm³/mol for the 1,4-cycloaddition and ?55 cm³/mol for the 1,2-cycloaddition. The activation volume for the 1,4-cycloreversion is assumed to be ?8 to ?12 cm³/mol based on a previous study on a similar system. The rate of the cycloreversion process is affected by solvent polarity. The rate of the 1,2-cycloaddition is influenced by solvent more significantly than that of the 1,4-cycloaddition. It is concluded that the transition state is polar in both cycloadditions and that its zwitterionic character is much stronger in the 1,2-cycloaddition.     

Kinetics and mechanism of benzylamine additions to ethyl alpha-acetyl-beta-phenylacrylates in acetonitrile

Kinetic studies of the addition of benzylamines to a noncyclic dicarbonyl group activated olefin, ethyl alpha-acetyl-beta-phenylacrylate (EAP), in acetonitrile at 25.0 degrees C are reported. The rates are lower than those for the cyclic dicarbonyl group activated olefins. The addition occurs in a single step with concurrent formation of the Calpha-N and Cbeta-H bonds through a four-center hydrogen bonded transition state.The kinetic isotope effects (kH/kD > 1.0) measured with deuterated benzylamines (XC6H4CH2ND2) increase with a stronger electron acceptor substituent (deltasigmaX > 0) which is the same trend as those found for other dicarbonyl group activated series (1-4), but is in contrast to those for other (noncarbonyl) group activated series (5-9). For the dicarbonyl series, the reactivity-selectivity principle (RSP) holds, but for others the anti-RSP applies. These are interpreted to indicate an insignificant imbalance for the former, but substantial lag in the resonance delocalization in the transition state for the latter series.     

Kinetics and mechanism of organocatalytic aza-Michael additions: direct observation of enamine intermediates

The imidazoles 1a-g add to the CC-double bond of the iminium ion 2 with rate constants as predicted by the equation log k = s(N)(N + E). Unfavourable proton shifts from the imidazolium unit to the enamine fragment in the adduct 3 account for the failure of imidazoles to take part in iminium-activated aza-Michael additions to enals.     

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).     

UV-spectroscopic study of complex formation by certain cycloolefins with tetracyanoethylene and molecular iodine

UV spectroscopy of charge-transfer complexes (CTCs) with tetracyanoethylene (TCE) and iodine has been used to study the relative donor ability of mono- and bicycloolefins and the stability of the CTCs. The donor ability of bicycloolefins, characterized by _CT(TCE), increases with increase in ring strain. The equilibrium constants for complex formation of bicycloolefins with TCE are linearly related to the rate constants of the reaction of epoxidation by tert-butyl hydroperoxide.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1314–1318, June, 1990.     

Electropolymerization : direct film formation on metal substrates-I: Kinetics and mechanism

Attempts have been made to produce, in situ, polymer films on tinplate cathodes by the electrolysis of conducting solutions of vinyl monomers for use in the can-lacquering industry. Study of a range of vinyl monomers revealed that film formation occurs at low monomer conversion only in the electrolysis of acrylonitrile and methacrylonitrile in NN'-dimethyl formamide. The highest rates of film formation were obtained by constant current electrolysis when tetraethyl ammonium p-toluene sulphonate (McKee Salt) was used as electrolyte. The rate of film formation increases with monomer concentration to a maximum and then falls rapidly. Chain propagation occurs by an anionic mechanism with ion pair formation favoured at high monomer concentrations. The physical properties of the coloured films produced rarely approach those required industrially and the method does not represent an alternative approach to the lacquering of food and beverage cans.     

Kinetics and mechanism of zinc ferrite formation

The kinetics and mechanism of zinc ferrite formation are studied for pure oxide mixtures at temperatures up to 1000°C by X-ray analysis, DTA and chemical analysis. Formation of ferrite obeys a random nucleation equation with an activation energy of 29.4 kcal mole^?1 in the temperature range 635–780°C. The nucleation process is followed by formation of spinel in an ordered lattice at temperatures /s> 800°C. The possibility of ZnO dissolution in the ferrite is also considered for ZnO at ratios higher than equimolecular.     

Resolution of the non-steady-state kinetics of the two-step mechanism for the Diels-Alder reaction between anthracene and tetracyanoethylene in acetonitrile

The Diels-Alder reaction between anthracene and tetracyanoethylene in acetonitrile does not reach a steady-state during the first half-life. The reaction follows the reversible consecutive second-order mechanism accompanied by the formation of a kinetically significant intermediate. The experimental observations consistent with this mechanism include extent of reaction-time profiles which deviate markedly from those expected for the irreversible second-order mechanism and initial pseudo first-order rate constants which differ significantly from those measured at longer times. It is concluded that the reaction intermediate giving rise to these deviations cannot be the charge-transfer (CT) complex, which is formed during the time of mixing, but rather a more intimate complex with a geometry favorable to the formation of the Diels-Alder adduct. The kinetics of the reaction were resolved into the microscopic rate constants for the individual steps. The rate constants, as shown in equation 1, at 293 K were observed to be 5.46 M(-)(1) s(-)(1) (k(f)), 14.8 s(-)(1) (k(b)), and 12.4 s(-)(1) (k(p)). Concentration profiles calculated under all conditions show that intermediate concentrations increase to maximum values early in the reaction and then continually decay during the first half-life. It is concluded that the charge-transfer complex may be an intermediate preceding the formation of the reactant complex, but due to its rapid formation and dissociation it is not detected by the kinetic measurements.     

Kinetics and mechanism of the formation of nitroprusside from aquapentacyanoferrate(III) and NO: complex formation controlled by outer-sphere electron transfer

The kinetics and mechanism of the reaction between nitric oxide and aquapentacyanoferrate(III) were studied in detail. Pentacyanonitrosylferrate (nitroprusside, NP) was produced quantitatively in a pseudo-first-order process. The complex-formation rate constant was found to be 0.252 +/- 0.004 M(-1) s(-1) at 25.5 degrees C, pH 3.0 (HClO(4)), and I = 0.1 M (NaClO(4)), for which the activation parameters are DeltaH++ = 52 +/- 1 kJ mol(-1), DeltaS++ = -82 +/- 4 J K(-1) mol(-1), and DeltaV++ = -13.9 + 0.5 cm(3) mol(-1). These data disagree with earlier studies on complex-formation reactions of aquapentacyanoferrate(III), for which a dissociative interchange (I(d)) mechanism was suggested. The aquapentacyanoferrate(II) ion was detected as a reactive intermediate in the reaction of aquapentacyanoferrate(III) with NO, by using pyrazine and thiocyanate as scavengers for this intermediate. In addition, the reactions of other [Fe(III)(CN)(5)L](n-) complexes (L = NCS(-), py, NO(2)(-), and CN(-)) with NO were studied. These experiments also pointed to the formation of Fe(II) species as intermediates. It is proposed that aquapentacyanoferrate(III) is reduced by NO to the corresponding Fe(II) complex through a rate-determining outer-sphere electron-transfer reaction controlling the overall processes. The Fe(II) complex rapidly reacts with nitrite producing [Fe(II)(CN)(5)NO(2)](4)(-), followed by the fast and irreversible conversion to NP.     

Kinetics and mechanism of the stepwise complex formation of Cu(II) with tren-centered tris-macrocycles

The stepwise complexation kinetics of Cu2+ with three tetratopic ligands L1, L2 and L3, tren-centred macrocycles with different bridges connecting the 14-membered macrocycles with the tren unit, have been measured by stopped-flow photodiode array techniques at 25 degrees C, I= 0.5 M (KNO3), and pH = 4.96. The reaction between the first Cu2+ and the ligand consists of several steps. In a rapid reaction Cu2+ first binds to the flexible and more reactive tren-unit. In this intermediate a translocation from the tren unit to the macrocyclic ring, which forms the thermodynamic more stable complex, takes place. This species can react further with a second Cu2+ to give a heterotopic dinuclear species with one Cu2+ bound by the tren-unit and the other coordinated by the macrocycle. A further translocation occurs to give the homoditopic species with two Cu2+ in the macrocycles. Finally a slow rearrangement of the dinuclear complex gives the final species. The rates of the translocation are dependent on the length and rigidity of the bridge, whereas the complexation rates with the tren unit are little affected by it. VIS spectra of the species obtained by fitting the kinetic results, EPR-spectra taken during the reaction, and ES mass spectra of the products confirm the proposed mechanism. The addition of a second, third and fourth equivalent of Cu2+ proceeds in an analogous way, but is complicated by the fact that we start and end with a mixture of species. These steps were evaluated in a qualitative way only.     

Kinetics and mechanism of decomposition of intermediate complex during oxidation of pectate polysaccharide by potassium permanganate in alkaline solutions

The kinetics of decomposition of an [Pect·Mn^VIO₄^2?] intermediate complex have been investigated spectrophotometrically at various temperatures of 15–30°C and a constant ionic strength of 0.1 mol dm^?3. The decomposition reaction was found to be first‐order in the intermediate concentration. The results showed that the rate of reaction was base‐catalyzed. The kinetic parameters have been evaluated and found to be ΔS^≠ = ? 190.06 ± 9.84 J mol^?1 K^?1, ΔH^≠ = 19.75 ± 0.57 kJ mol^?1, and ΔG^≠ = 76.39 ± 3.50 kJ mol^?1, respectively. A reaction mechanism consistent with the results is discussed. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 67–72, 2003     

Kinetics and mechanism of reaction of p-methoxystyrene and tetracyanoethylene. I. Evidence for competitive 1,2- and 1,4-cycloadditions

Time-resolved absorption spectra for a reaction mixture of p-methoxystyrene and tetracyanoethylene (TCNE) are found to have a band maximum at 325 nm which is assigned to the 1,4-cycloadduct. The reaction in chloroform at 15, 20, and 25°C is followed by the charge-transfer band at 600 nm. The 1,4-cycloadduct, besides the so far known 1,2-cycloadduct and EDA complex, is taken into account to derive the rate equation for the EDA complex that is a linear second-order differential equation. The rate constants for the elementary steps involved in the reaction are obtained. The 1,4-cycloaddition has an activation entropy of -63 J/K·mol for the cycloreversion and a reaction constant ρ of -4.7, both of which indicate the polar transition state. On the other hand, activation entropy of the 1,2-cycloaddition is 73 J/K·mol more negative than that of the 1,4-cycloaddition, supporting the zwitterionic mechanism for the 1,2-cycloaddition.     

Kinetics and mechanism of the formation of CO2 hydrate

This article proposes a mechanism of CO₂ hydrate formation taking into account both diffusion and reaction, and gives an analysis of its kinetics. The most important assumptions on the model are that water dissolves into liquid CO₂ and reacts to form CO₂ hydrate, and that the hydrate blocks the dissolution and diffusion of water. Computational simulations were conducted, and the model proposed explains well the many observations on the CO₂ hydrate formation in previous articles. It is concluded that liquid CO₂ disposed in a deep ocean will form a very thin film of CO₂ hydrate, and this will greatly control the CO₂ diffusion in the ocean. © 1993 John Wiley & Sons, Inc.     

Use of naphthalene as a solvent for selective formation of the 1:1 diels-alder adduct of C60 with anthracene

A reaction of C₆₀ with an equimolar amount of anthracene in refluxing naphtlhalene gives the 1:1 Diels-Alder adduct in 67% yield based on consumed C₆₀: the adduct was fully characterized by IR, UV-vis, ¹H and ¹³C NMR, and MS spectroscopy, and exhibited reversible reduction waves at the potential 0.11 to 0.19 V more negative and an irreversible oxidation peak at the potential 0.11 V less positive than those of C₆₀ itself.     

纤维素热裂解反应机理及中间产物生成过程模拟研究

基于改进的B-S机理模型,通过求解物料内部和气相空间两段反应过程,对纤维素热裂解过程中一些化合物(活性纤维素、左旋葡聚糖(LG)、乙醇醛、丙酮醇等组分)的生成和演变情况进行了模拟。结果发现,自由水的脱除过程使物料前期升温速率发生了下降,并未影响热解期间温度分布以及反应过程。热裂解过程中,由于一次反应的强烈吸热,物料在长时间内局限于中温范围,其内部各组分质量浓度分布的区别主要体现出一次反应竞争能力的强弱。物料厚度的增加使热裂解时间延长,并加剧物料内部的二次分解。左旋葡聚糖和其竞争产物乙醇醛的生成出现一个大量生成、快速逃逸的过程,相比于左旋葡聚糖,乙醇醛质量浓度的积累具有更快的速度,体现出较高温度下的竞争优势。对于小尺寸反应物,挥发分二次反应主要发生在气相空间,随着气相停留时间的增加,其二次分解的程度提高,该效果随辐射源温度的提高而加剧。相比于LG产率随反应时间的快速下降趋势,高温下生物油产率的降低略显缓和,其变化主要是组分分布的改变,即从大分子结构降解为小分子结构。