Kinetics of interaction of 3-aminopropyltriethoxysilane with silica gel using elemental analysis and 29Si NMR spectra

 Three silica gel sample systems, modified with 3-amino-propyltriethoxy silane (APTS), were prepared by sequentially sampling the reaction mixture at various time intervals. The concentrations of 3-aminopropylsilyl groups (APS) bound on the silica surface were determined by elemental analysis. For the same sample systems, ²⁹Si NMR intensities of an (–O)₄Si species belonging only to the silica gel particles and corrected by a cross-polarization correction factor were also measured. Both the APS-concentrations and the correc-ted ²⁹Si NMR intensities depended upon reaction time, reflecting the rate of the APTS–silica gel reaction. Kinetic analysis of these data was made by use of the Gauss–Newton method, and the overall reaction was found to consist of three reaction processes (an initial fast reaction, a slower second reaction and a much slower third reaction). In particular, the conversion of (–O)₃SiOH to (–O)₄Si is predominant in the second reaction process and the pore size of a silica gel particle affects the reaction mechanism. Received: 1 November 1996 Accepted: 24 January 1997     

Ab initio study on the mechanism of reaction HNCO+NH2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH₂+ HNCO?NH₃+NCO (1) and NH₂+HNCO?N₂H₃+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300–2700 K, transition theory rate constant for reaction (1) is 1.68×10¹¹–3.29×10¹¹ mL·mol^-1·s^-1, which is close to the experimental one of 5.0×10¹¹mL·mol^-1·s^-1or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes,cis andtrans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (forcis-mode) and 147.43 kJ/mol (fortrans-mode), respectively, which is much higher than reaction (1). So reaction (1) is the main channel for the titled reaction.     

Catalytic Kinetics and Mechanism Transformation of Fe(acac)3 on the Urethane Reaction of 1,2‐Propanediol with Phenyl Isocyanate

The urethane reaction of 1,2‐propanediol with phenyl isocyanate was investigated with ferric acetylacetonate (Fe(acac)₃) as a catalyst. In situ Fourier transform infrared spectroscopy was used to monitor the reaction, and catalytic kinetics of Fe(acac)₃ was studied. The reaction rates of both hydroxyl groups were described with a second‐order equation, from which the influence of the Fe(acac)₃ concentration and reaction temperature was discussed. It was very surprising that the relationship between 1/C and t became constant when reaction temperature increased, which indicated that there was no reactive distinction between the two hydroxyl groups. Although the phenomenon differed with the variation of temperature, it was unaffected by the Fe(acac)₃ concentration. It was attributed to the transformation of the reaction mechanism with the increase in temperature. Furthermore, activation energy (E_a), enthalpy (ΔH*), and entropy (ΔS*) for the catalyzed reaction were determined from Arrhenius and Eyring equations, which testified to the transformation of the reaction mechanism.     

Kinetics and Mechanism of Catalytic Reduction of U(VI) with Hydrazine on Platinum Catalysts in Nitric Acid Media

The kinetics of U(IV) produced by hydrazine reduction of U(VI) with platinum as a catalyst in nitric acid media was studied to reveal the reaction mechanism and optimize the reaction process. Electron spin resonance (ESR) was used to determine the influence of nitric acid oxidation. The effects of nitric acid, hydrazine, U(VI) concentration, catalyst dosage and temperature on the reaction rate were also studied. In addition, the simulation of the reaction process was performed using density functional theory. The results show that the influence of oxidation on the main reaction is limited when the concentration of nitric acid is below 0.5 mol/L. The reaction kinetics equation below the concentration of 0.5 mol/L is found as: -dc(UO₂²⁺)/dt)=kc^0.5323(UO₂²⁺)c^0.2074(N₂H₅⁺)c^-0.2009(H⁺). When the temperature is 50 ℃, and the solid/liquid ratio r is 0.0667 g/mL, the reaction kinetics constant is k=0.00199 (mol/L)^0.4712/min. Between 20 ℃ and 80 ℃, the reaction rate gradually increases with the increase of temperature, and changes from chemically controlled to diffusion-controlled. The simulations of density functional theory give further insight into the influence of various factors on the reaction process, with which the reaction mechanisms are determined according to the reaction kinetics and the simulation results.     

Synthesis and characterization of trans-di(nitrobenzo)- and di(aminobenzo)-18-crown-6 derivatives with high selectivity

The dibenzo-18-crown-6 derivatives such as di(nitrobenzo)-18-crown-6 and di(aminobenzo)-18-crown-6 were synthesized by nitration reaction and catalytic hydrogenation with high selectivity. The chemical structures were determined by FTIR, ¹H NMR, ¹³C NMR, and UV. Regarding the mixture of Ac₂O and HNO₃ as nitrating agent, the reaction exhibited commendable trans-isomer selectivity. Effects of nitrating agent ratio, reaction temperature and reaction time on yield of trans-di(nitrobenzo)-18-crown-6 were investigated. The yield of trans-di(nitrobenzo)-18-crown-6 was 62.9% for nitrating agent ratio of 1/1, reaction temperature of 50?°C and reaction time of 5?h. Moreover, effect of reaction time on trans-di(aminobenzo)-18-crown-6 was also studied.     

纳米Cu2O催化二苯甲烷二氨基甲酸苯酯无溶剂热分解制备二苯甲烷二异氰酸酯

研究了无溶剂条件下纳米Cu₂O催化二苯甲烷二氨基甲酸苯酯(MDPC)热分解制备二苯甲烷二异氰酸酯(MDI),考察了纳米Cu₂O的制备条件与反应条件对MDPC热分解反应性能的影响.结果表明,水解法制备的纳米Cu₂O在Ar中于300℃焙烧2h,其催化性能最佳;最佳的反应条件为Cu₂O用量为原料总重的0.06%,反应温度220℃,反应压力0.6kPa,反应时间12min,此时MDPC转化率达到99.8%,MDI选择性86.2%.     

Mechanistic study of cerium(IV) oxidation of antimony(III) in aqueous sulphuric acid in the presence of micro amounts of manganese(II) by stopped flow technique

The oxidation of antimony(III) by cerium(IV) has been studied spectrometrically (stopped flow technique) in aqueous sulphuric acid medium. A minute amount of manganese(II) (10⁻⁵ mol dm⁻³) is sufficient to enhance the slow reaction between antimony(III) and cerium(IV). The stoichiometry is 1:2, i.e. one mole of antimony(III) requires two moles of cerium(IV). The reaction is first order in both cerium(IV) and manganese(II) concentrations. The order with respect to antimony(III) concentration is less than unity (ca 0.3). Increase in sulphuric acid concentration decreases the reaction rate. The added sulphate and bisulphate decreases the rate of reaction. The added products cerium(III) and antimony(V) did not have any significant effect on the reaction rate. The active species of oxidant, substrate and catalyst are Ce(SO₄)₂, [Sb(OH)(HSO₄)]⁺ and [Mn(H₂O)₄]²⁺, respectively. The activation parameters were determined with respect to the slow step. Possible mechanisms are proposed and reaction constants involved have been determined.     

碳纳米管的聚合物功能化与结构控制:聚合物接枝碳纳米管

碳纳米管高分子化是发展高性能的聚合物基纳米功能材料的重要研究方向,本文从"grafting-to"和"grafting-from"两种方式对聚合物接枝碳纳米管的最新进展进行了系统综述。"Grafting-to"方法主要包括羧基衍生反应(酰化、酯化)、加成反应(大分子自由基加成、叠氮环加成)和硫醇偶联反应。"Grafting-from"方法包括普通自由基聚合、可控/活性自由基聚合、离子聚合、开环聚合和逐步聚合反应,其中碳纳米管表面引发活性自由基聚合进一步分为原子转移自由基聚合、氮氧稳定自由基聚合和可逆加成-断链转移聚合。此外,本文还简述了碳纳米管自身的聚合反应,并探讨了目前聚合物修饰碳纳米管所面临的问题和今后的发展方向。     

The Photokinetics of Electron Transfer Reaction of Methylene Blue with Titanium Trichloride in Aqueous–Alcoholic Solvents

The kinetics of photoinduced electron transfer reaction of methylene blue (MB) and titanium trichloride was investigated in water and different aqueous–alcoholic solvents. The reaction is pseudo‐first order, dependent only on the concentration of titanium trichloride at a fixed concentration of MB. The effect of water and aqueous–alcoholic solvents was studied in the acidic pH range (4–7). It was observed that the quantum yield (ϕ ) of the reaction increased with increase in polarity of the reaction medium. The quantum yield was high under acidic conditions and decreased with further increase in acidity. The addition of ions and increase in temperature increased the rate and quantum yield of the reaction. The absence of any reaction intermediate was confirmed by spectroscopic investigations. A mechanism for the reaction has been proposed in accordance with the kinetics of the reaction. The activation energy (E _a) was calculated by the Arrhenius relation. Thermodynamic parameters such as E _a, enthalpy change (ΔH ), free energy change (ΔG ), and entropy change (ΔS ) were also evaluated.     

Synthesis of poly(4-vinylbenzocyclobutene) and its reaction with dienophiles

4-Vinylbenzocyclobutene ( 1 ) was prepared by the nickel-catalyzed coupling reaction of 4-bromobenzocyclobutene with vinylbromide in 70% yield. Radical homopolymerization of 1 at 60°C for 24 h afforded poly(4 vinylbenzocyclobutene) [poly( 1 )] in 89% yield and radical copolymerizations of 1 with styrene (St) or methyl methacrylate (MMA) were carried out to obtain the corresponding copolymers. The Q = 1.07, e = 0.046. As a model reaction of the polymer reaction of the polymer reaction of poly( 1 ) and poly(4-vinylbenzocyclobutene-co-styrene) [copoly( 1 -St)] with dienophiles, the Diels-Alder reaction of benzocyclobutene with N-phenylmaleimide (MI) or maleic anhydride (MANH) was carried out to determine the optimum reaction conditions. Under the optimum condition, the Diels-Alder reaction of poly( 1 ) and copoly( 1 -St) with MI and MANH in the presence of 4-tert-butyl-catechol as an inhibitor were carried out to yield the corresponding polymers in good yields. The properties (solubilities, T_g, and temperature of 10% weight loss) of the products obtained from the polymer reaction were different from these of poly( 1 ). © 1995 John Wiley & Sons, Inc.     

Cu(II) catalyzed reaction between phenyl hydrazine and toluidine blue—dual role of acid

The detailed kinetics of Cu(II) catalyzed reduction of toluidine blue (TB⁺) by phenyl hydrazine (Pz) in aqueous solution is studied. Toluidine white (TBH) and the diazonium ions are the main products of the reaction. The diazonium ion further decomposes to phenol (PhOH) and nitrogen. At low concentrations of acid, H⁺ ion autocatalyzes the uncatalyzed reaction and hampers the Cu(II) catalyzed reaction. At high concentrations, H⁺ hinders both the uncatalyzed and Cu(II) catalyzed reactions. Cu(II) catalyzed had stoichiometry similar to the uncatalyzed reaction, Pz+2 TB⁺+H₂O=PhOH+2 TBH+2 H⁺+N₂. Cu(II) catalyzed reaction occurs possibly through ternary complex formation between the unprotonated toluidine blue and phenyl hydrazine and catalyst. The rate coefficient for the Cu(II) catalyzed reaction is 2.1×10⁴ M⁻² s⁻¹. A detailed 13‐step mechanistic scheme for the Cu(II) catalyzed reaction is proposed, which is supported by simulations. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 271–276, 1999     

Towards Stereoselective Synthesis of the C(31)–C(39) and C(20)–C(27) Fragments of Phorboxazole A

The stereoselective synthesis of the C(31)–C(39) and C(20)–C(27) fragments of phorboxazole A ( 1 ) was achieved from commercially available and inexpensive D ‐mannitol. Crimmins aldol reaction and a decarboxylative Claisen‐type reaction are the key steps for the C(31)–C(39) fragment, and L ‐proline‐catalyzed aldol reaction, Sharpless asymmetric epoxidation, and epoxide ring opening reaction with Gilman's reagent are the key steps for the C(20)–C(27) fragment of phorboxazole.     

Triphenylarsine as an effective catalyst in a mechanistic investigation of trans- 1, 2-di (methoxycarbonyl)-6, 6-dimethyl-5, 7-dioxaspiro [2, 5] octane-4, 8-dione formation

The kinetics and mechanism of the reaction between dimethyl acetylendicarboxylate (DMAD) and Meldrum's acid (MA) in the presence of triphenylarsine (TPA) as a catalyst were investigated in a methanol environment by the UV/vis spectrophotometry technique. In this work, the reaction followed second- order kinetics and the first and second steps of the reaction mechanism were recognized as the fast and rate-determining step (RDS), respectively. A significant point in this reaction “in comparison with previous work” is related to the change in behavior of the kinetics and reaction mechanism in the presence of triphenylarsine (TPA). Activation energy and parameters (E_a, ΔH^?, ΔS^?, and ΔG^?) were determined for the reaction and a comparison between ΔH^? and TΔS^? values showed that the reaction is entropy-controlled. High values of the activation Gibbs free energy indicated that the reaction was chemically controlled. Also, the large negative value of ΔS^? implied an associative mechanism.     

Model studies on the kinetics and mechanism of polyarylate synthesis by acidolysis

Model reactions were carried out to simulate the acidolysis process for polyarylate synthesis by using p-tert-butylphenyl acetate (ptBuPhOAc) and benzoic acid in diphenyl ether. p-tert-Butylphenol was formed in the reaction mixture and its concentration stayed constant throughout the reaction. Acetic benzoic anhydride and benzoic anhydride were detected by NMR. Based on this experimental evidence, a mechanism for the acidolysis was proposed involving the mixed anhydride. The kinetics of the acidolysis reaction was studied for this model reaction. The overall reaction order is two and the reaction order with respect to each reactant is one. Second-order reaction rate constants were measured at different reaction conditions (200–250°C). The activation energy (E_a), activation enthalpy (ΔH^≠), and activation entropy (ΔS^≠) were calculated from these data. The thermodynamic parameters of the acidolysis reaction were also measured for the analogous reaction of p-tert-butylphenyl pivalate (ptBuPhOPiv) and benzoic acid. The kinetics of two other elementary reactions involved in the acidolysis reaction were also studied: p-tert-butylphenol with acetic anhydride or benzoic anhydride, and p-tert-butylphenyl pivalate with benzoic acid.     

Nucleophilic Addition to Carbonyl Compounds. Competition Between Hard (Amine) and Soft (Phosphite) Nucleophile

Abstract

In the reaction mixture of carbonyl compound, amine and diethyl phosphite several different reactions are observed. The formation of aminophosphonate (Kabachnik- Fields reaction) is frequently accompanied with the formation of hydroxyphosphonate (Pudovik reaction) or product of its rearrangement. ^1–3 This is due to the presence of one electrophile (carbonyl compound) and two nucleophiles (amine and phosphite) in the reaction mixture, which may compete for the electrophilic center.     

Synthesis and characterization of a new fluorinated polyether glycol prepared by radical grafting of hexafluoropropylene onto polytetramethylene glycol

A new fluorinated polyether glycol (PTMG-g-HFP) was prepared by radical grafting of hexafluoropropylene (HFP) onto polytetramethylene glycol (PTMG) in the presence of different initiators. The structure of PTMG-g-HFP was characterized by means of IR, ¹H NMR and ¹³C NMR. The effects of nature and amount of initiator, reaction time and reaction temperature on grafting HFP onto PTMG were investigated. The results showed di-tert-butyl peroxide (DTBP) was the most efficient in the reaction and the optimal reaction conditions were: [DTBP]₀/[PTMG]₀, 0.12; reaction temperature, 140 °C; reaction time, 6 h.     

Theoretical study on the reaction mechanism of CN radical with ketene

The bimolecular single collision reaction potential energy surface of CN radical with ketene (CH2CO) was investigated by means of B3LYP and QCISD(T) methods. The calculated results indicate that there are three possible channels in the reaction. The first is an attack reaction by the carbon atom of CN at the carbon atom of the methylene of CH2CO to form the intermediate NCCH2CO followed by a rupture reaction of the C-C bond combined with -CO group to the products CH2CN CO. The second is a direct addition reaction between CN and CH2CO to form the intermediate CH2C(O)CN followed by its isomerization into NCCH2CO via a CN-shift reaction, and subsequently, NCCH2CO dissociates into CH2CN CO through a CO-loss reaction. The last is a direct hydrogen abstraction reaction of CH2CO by CN radical. Because of the existence of a 15.44 kJ/mol reaction barrier and higher energy of reaction products, the path can be ruled out as an important channel in the reaction kinetics. The present theoretical computation results, which give an available suggestion on the reaction mechanism, are in good agreement with previous experimental studies.     

Thermokinetics on the reaction of formation of Dy[(C5H8NS2)3(C12H8N2)]

The enthalpy change of formation of the reaction of hydrous dysprosium chloride with ammonium pyrrolidinedithiocarbamate (APDC) and 1,10-phenanthroline (o-phen•H₂O) in absolute ethanol at 298.15 K has been determined as (-16.12 ± 0.05) kJ•mol^-1 by a microcalormeter. Thermodynamic parameters (the activation enthalpy, the activation entropy and the activation free energy), rate constant and kinetics parameters (the apparent activation energy, the pre-exponential constant and the reaction order) of the reaction have also been calculated. The enthalpy change of the solid-phase reaction at 298.15 K has been obtained as (53.59 ± 0.29) kJ•mol^t-1 by a thermochemistry cycle. The values of the enthalpy change of formation both in liquid-phase and solid-phase reaction indicated that the complex could only be synthesized in liquid-phase reaction.

     

Ferric Acetylacetonate Catalyst on the Urethane Reaction of Phenyl Isocyanate and 1,2-Propyleneglycol: A Study of Kinetics,Thermodynamics, and Mechanism with In Situ FT-IR Spectroscopy

The urethane reaction of phenyl isocyanate and 1,2-propylene glycol was investigated with ferric acetylacetonate (Fe(acac)₃) as catalyst. The effect of the catalytic properties of Fe(acac)₃ on the formation of the urethane bond was evaluated with in situ FT-IR. The influence of the Fe(acac)₃ concentration as well as the reaction temperature is discussed. It was observed that there was a turning point in the reaction rate when the temperature decreases, which remained unchanged with variation in Fe(acac)₃ concentration. Arrhenius and Eyring parameters of the primary hydroxyl group were determined for the catalyzed reaction. The low-temperature and high-temperature values are surprisingly different. A reasonable reaction mechanism is proposed and the possible active species are discussed, followed by a kinetics and thermodynamics discussion.     

Reaction heat-driven CO2 desorption during CO oxidation on Au(997) at low temperatures

Adsorption and reaction of CO and CO₂ were studied on oxygen-covered Au(997) surfaces by means of temperatureprogrammed desorption/reaction spectroscopy. Oxygen atoms (O(a)) on Au(997) enhances the CO₂ adsorption and stabilizes the adsorbed CO₂(a), and the stabilization effect also depends on the CO₂(a) coverage and involved Au sites. CO₂(a) desorption is the rate-limiting step for the CO+O(a) reaction to produce CO₂ on Au(997) at 105 K and exhibits complex behaviors, including the desorption of CO₂(a) upon CO exposures at 105 K and the desorption of O(a)-stabilized CO₂(a) at elevated temperatures. The desorption of CO₂(a) from the surface upon CO exposures at 105 K to produce gaseous CO₂ depends on the surface reaction extent and involves the reaction heat-driven CO₂(a) desorption channel. CO+O(a) reaction proceeds more easily with weakly-bound oxygen adatoms at the (111) terraces than strongly-bound oxygen adatoms at the (111) steps. These results reveal complex rate-limiting CO₂(a) desorption behaviors during CO+O(a) reaction on Au surfaces at low temperatures which provide novel information on the fundamental understanding of Au catalysis.