Hydration reactivity of ketenes. Non-additive effect of alkyl substituents upon the rate of spontaneous hydrolysis

The kinetics of spontaneous hydrolysis of dialkylsubstituted ketenes in water-acetonitrile mixtures at 20°C has been studied. An increase of the total volume of substituents at the terminal carbon atom of the ketenes leads to a sharp decrease of hydration rate, which is connected with the non-additivity of substituent effects. The possible reasons are discussed in terms of the reaction mechanism.     

Nucleophilicity toward ketenes: rate constants for addition of amines to aryl ketenes in acetonitrile solution.

Second-order rate constants (k(Nu)) have been measured for the addition of amines to ketenes 4-6 in acetonitrile solution by the laser flash photolysis technique. These ketenes are formed from a photochemical Wolff rearrangement of diazoketones 1-3, respectively. For all diazoketones studied, the presence of amines as nucleophiles in the reaction medium results in the formation of an intermediate that later converts to the amide. The rate of formation of these intermediates is linearly dependent on amine concentration. Various classes of amines, such as primary, secondary, and tertiary, aromatic, and aliphatic, have been used to investigate the ketene reactivity, and rate constants in the range 10(4)-10(9) M(-1) s(-1) have been measured. Reaction rates are dependent upon steric effects in both the ketene and the nucleophile, which is consistent with a reaction mechanism involving nucleophilic attack at Calpha in the molecular plane of the ketene. On the basis of these data, a set of N(+) parameters for the reaction of amines with ketenes was determined.     

Influence of monomer activity coefficient upon polymerization rate

The monomer activity has been measured as a function of conversion in the methyl methacrylate polymerization up to 25% conversion. The influence of the monomer activity coefficient upon the polymerization rate is discussed; an equation is derived which allows an estimation of the change in termination rate constant. The values obtained for this change are compared with previous estimates that have neglected the influence of the activity coefficients.     

Influence of the structure of alkyl derivatives of salicylaldehyde oxime upon the extraction rate of copper from diluted acidic solutions

Individual pure alkyl derivatives of salicylaldehyde oxime were used for copper extraction using the short-time phase contacting method. The influence of the oxime structure upon the extraction rate and the surface activity was discussed. It was found that the reaction can proceed at the interface and in the bulk of the aqueous phase. The bulk-phase process is dominant for oximes having less than four carbon atoms in the alkyl group, while the interfacial process is dominant for oximes having more than six carbon atoms.     

Formation of secondary fullerene ozonides in the ozonolysis of C60 solutions and chemiluminescence upon their hydrolysis

The formation of secondary fullerene ozonides (SFOs) in the ozonolysis of C₆₀ solutions in CCl₄ has reliably been determined for the first time; SFOs are accumulated during the whole ozonolysis time as a suspension in CCl₄. Hydrolysis of the SFOs results in chemiluminescence (CL) (I _max = 2.65·10⁸ photon s⁻¹ mL⁻¹), whose spectra contain maxima at 558, 608, and 685 nm. The most probable CL emitters are excited fullerene polyketones. Hydrogen peroxide was identified as a stable hydrolysis product of the SFOs by the color reaction with diphenylcarbazide and CL arisen upon the addition of an aqueous solution of FeSO₄·9H₂O to the hydrolyzate of the SFO. Chemiluminescence upon hydrolysis is a selective test for SFOs and allows one to find them in a complex mixture of the ozonolysis products of C₆₀. The rate constant and activation energy of SFO hydrolysis were determined from the kinetic measurements of CL. For SFO hydrolysis several probable reactions were proposed, including the formation of the CL emitters, and their heat effects were estimated using the PM3/RHF and AM1/RHF semiempirical methods for one-and two-cage model structures of SFOs. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1322–1329, August, 2006.     

Effect of the structure of substituted propargyl and allyl alcohols on the rate of their liquid phase hydrogenation on a Pd-Ru alloy membrane catalyst

The rates of hydrogenation of substituted propargyl and allyl alcohols in the liquid phase on a Pd-Ru alloy membrane catalyst are described by a two-parameter Taft equation which takes into account the inductive and steric effects of the substituents. The values of the parameters at 363 K with H₂ at atmospheric pressure are: ^*=–0.20, =0.10 and ^*=–1.1, =1.3 respectively.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1740–1745, August, 1989.     

Vibrational spectra and structure of methoxysilanes and products of their hydrolysis

Equilibrium geometries and force fields for the series of molecules (MeO)_nSiMe_4−n(I), (OH)_nSiMe_4−n(II), and (MeO)_nSi(OH)_4−n(III) with n = 1–4 are obtained at the DFT/B3LYP level of theory with 6-31G^* and aug-cc-pVDZ basis sets in order to predict the structural parameters and vibrational spectra of these molecules, the larger part of which was not characterized experimentally. The performance of these theoretical methods was assessed on the existing spectral data for series I. The B3LYP/aug-cc-pVDZ method, firstly applied to this class of molecules, demonstrates a fair agreement with experimental vibrational frequencies even without empirical scaling. For molecules of series II and III vibrational spectra are predicted in order to supply spectral data for monitoring the sol–gel processes at the hydrolysis stage. The hyperconjugative strengthening of SiO bonds with the number of oxygen atoms coordinated to silicon leads to the growth of their frequencies, but the larger increase of νSiO (due to kinematic factors) occurs at the SiOMe/SiOH substitution. The predicted distinctive feature of series II and III is the appearance of bands with high IR intensity in the 1000–900 cm⁻¹ spectral region that increase their frequencies with n. In series III it is accompanied with the steady increase of the ν_sSiO₄ frequency in the 700–600 cm⁻¹ range.     

The influence of hydrophobic solvation on the alkaline hydrolysis of ethyl esters of polar substituted 2-methylpropionic acids in water

The rate constants of the alkaline hydrolysis of 2-methylpropionic acid ethyl ester, -hydroxy, -bromo, -cyano, and -nitro-2-methylpropionic acid ethyl ester have been measured in water between 5°C and 45°C with the help of an appropriate conductance bridge developed in our laboratory. The influence of the polar substituents on the rate constant, a powerful probe for discussing two-particle interactions in solutions, is characterized by an intramolecular structure parameter of the activated complex and a dielectric parameter of its surroundings. The dependence of the reaction rate on the structure parameter is compared to that of substituted acetic acid derivatives. The smaller dielectric parameter of 2-methylpropionic acid esters reveals the increasing hydrophobic solvation in comparison to acetic acid derivatives.The 7^th communication of investigations on substituent and solvent effects of solvolysis reactions.     

Influence of temperature on the structure of liquid water

Molecular dynamics simulations have been carried out for liquid water at 7 different temperatures to understand the nature of hydrogen bonding at molecular level through the investigation of the effects of temperature on the geometry of water molecules. The changes in bond length and bond angle of water molecules from gaseous state to liquid state have been observed, and the change in the bond angle of water molecules in liquid against temperature has been revealed, which has not been seen in literature so far. The analysis of the radial distribution functions and the coordinate numbers shows that, on an average, each water molecule in liquid acts as both receptor and donor, and forms at least two hydrogen bonds with its neigbors. The analysis of the results also indicates that the water molecules form clusters in liquid.     

Reactivity of Poly-, Di-, and monosaccharides in their hydrolysis in subcritical water

The reactivity of the water-soluble sodium salt of carboxymethyl cellulose (CMC), sucrose, glucose, and fructose in subcritical water at 120–320°C is reported. A mathematical model is suggested to describe the variation of the CMC molar mass during hydrolysis at various temperatures and reaction times. Kinetic parameters of the conversions of CMC, sucrose, glucose, and fructose in subcritical water are presented. High temperatures and short reaction times are favorable for the formation of monosaccharides from CMC.     

On the rate of hydrolysis of trimethylphenoxysilane

Trimethylphenoxysilane is shown to undergo hydrolysis rapidly in 96% ethanol at ambient temperature; therefore ethanol is unsuitable for taking UV absorption spectra of Me₃SiOPh.