Kinetics,Substrate Selectivity,and Activation Parameters for the Reaction of Methylbenzenes with Hydroxymethyl Cations in Sulfuric Acid Solutions

The kinetics of the hydroxymethylation of benzene and methylbenzenes in a formaldehyde–sulfuric acid (52.5 wt.%) system in the temperature range 14-90 °C has been studied. With [ArH] [CH₂O] the reaction is first order in substrate and less than first order in formaldehyde. The substrate selectivity, entropy and enthalpy of activation in the series of arenes studied correlate with the basicity of ArH. A reaction mechanism is proposed which includes the rapid equilibrium formation of a charge transfer complex between ArH and CH₂OH⁺ and a slow stage in which it is converted to a -complex.     

Solvate shell rearrangements and solvation corrections to the Br?nsted equation

Solvate corrections f^solv for the Brönsted equation have been estimated under the assumption of the minimum work of shell rearrangement (MWSR).

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f^solv ().

     

Modified steady-state free precession pulse sequences for the detection of pure nuclear quadrupole resonance

Modifications of the steady-state free precession multi-pulse technique for the effective detection of the nuclear quadrupole resonance (NQR) signals are proposed. These modifications are based on the use of composite pulses and enable the suppression of the coherent noise signals such as the magneto-acoustic and piezo-electric signals or the ringing signal from the NQR probe. Experimental results of applying the proposed technique to nitrogen-14 NQR in the sample of C6H12N4 are also presented and convincingly demonstrate its effectiveness.     

Convective self-oscillations near an air-bubble surface in a horizontal rectangular channel

The concentration convection in an isothermal fluid near an air bubble clamped between the vertical walls of a horizontal channel with a rectangular cross-section is studied experimentally and numerically. The channel is filled with an aqueous solution of a surfactant with a nonuniform concentration. As a result of the competition between the gravitational convection in the cavity volume and the Marangoni convection near the bubble surface, an oscillation flow regime is established. This regime is observed experimentally over several hours. In the numerical experiment, the oscillations are obtained in the presence of an initial horizontal surfactant concentration gradient. Against the background of gravitational convection, short bursts of Marangoni convection with ten times greater intensity are observed. The convective flow patterns and the oscillation periods obtained experimentally and numerically are in fairly good agreement.     

Kinetics, substrate selectivity, and mechanisms of alkane and alkylbenzene reactions with peroxynitrous acid in the gas phase and solution

Specific features of the kinetics of alkane and alkylbenzene oxidation with HOONO formed in the H₂O₂-NaNO₂ system (pH 4.27) are quantitatively explained assuming the simultaneous occurrence of reactions in the gas and liquid phases. A model of the kinetic distribution method is developed and verified that accounts for the equilibrium distribution of a substrate and a reagent between phases and their interaction in both phases. Relative rate constants for the oxidation ofn-alkanes (C₃-C₈), isobutane, cyclopentane, cyclohexane, benzene, and alkylbenzenes are measured over a wide range of the volume ratios of the gas and liquid phases (λ = V_g/V₁). Relative rate constants for the oxidation of alkanes in the gas phase and alkylbenzenes in gas and solution were determined. Similarity in substrate selectivities and kinetic isotope effects of the gasphase reactions of alkanes and arenes with peroxynitrous acid and^•OH radicals suggest that hydroxyl radical or the ˙OH...NO₂ radical pair is an active species in the gas phase. In solution, alkylbenzenes react nonselectively with HOONO, as well as with ˙OH radicals. In contrast to the liquid-phase oxidation of arenes, the liquidphase oxidation of all alkanes under study insignificantly contribute (5–15%) to the overall rate of the substrate consumption.     

The first step of oxidation of alkylbenzenes by permanganates in acidic aqueous solutions

Data on the kinetics, kinetic isotope effects, substrate selectivety, and activation parameters for the first step of oxidation of alkylbenzenes by permanganante in acidic aqueous solutions are surveyed. The MnO₄ ⁻, HMnO₄, and MnO₃ ⁺ species serve as oxidants at different acidities. The increase in the positive charge in this series enhances the electrophilicity of the reagent, which manifests itself as an increase in the reaction rate and a change in the site of attack on the alkylbenzene molecule (either the aromatic ring or C−H bond in the alkyl group). The oxidation of the alkyl C−H bonds in alkylbenzenes and in alkanes follows similar mechanisms, while the attack on the aromatic ring proceedsvia the electrophilic aromatic substitution mechanism with a transition state intermediate between the charge transfer complex and σ-complex. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 765–780, May, 2000.