Steric Constants Research Articles

Thermal Decomposition Kinetics of Bis-Gem-Dinitroethyl Derivatives of N-Nitraminoacetic Acid

Thermal decomposition kinetics of bis-gem-dinitroethyl substituted compounds based on N-nitraminoacetic acid is studied. Structural and kinetic patterns are revealed and a chemical mechanism of decomposition reaction is established, including the primary homolysis of C-NO2 bond in the gem-dinitroethyl fragment. The activation parameters are defined for the initial reaction. The reactivity of the studied compounds was analyzed and quantitative correlations between the logarithms of the rate constant, the activation energies and the steric constants of substituents in the gem-dinitroethyl group were found

Kinetics and Mechanism of Thermal Decomposition of 1-R-Substituted bis(1,1-Dinitromethyl-3-Nitro- 1,2,4-Triazole-5-yl)

Thermal decomposition kinetics of 1-substituted bis(1,1-dinitromethyl-3-Nitro-1,2,4-triazole- 5-yl) in solution of 1,3-dinitrobenzene is studied with manometric method under isothermal conditions. The limiting stage of thermal decomposition is homolytic break of C-NO2 bond in gem-dinitromethyl group; activation parameters of this stage are calculated. The reactivity of investigated compounds is analyzed. Correlation dependences between logarithm of rate constant, activation energy and steric constant of substituent R are obtained

Ring-Chain Tautomerism of the Condensation Products of 2-Mercaptobenzohydrazide with Aliphatic and Aromatic Aldehydes: Influence of Electronic and Steric Factors

It has been shown by 1H and 13C NMR spectroscopy that 2-mercaptobenzoyl-hydrazones of aliphatic and aromatic aldehydes exist in solution as tautomeric mixtures of linear and cyclic benzo-1,3,4-thiadiazepine forms. The linear hydrazone form is represented by (E,E')-and (E,Z')-conformational isomers, differing in the disposition relative to the amide C–N bond. Growing bulk of the alkyl substituent at the C=N bond of the aliphatic aldehydes derivatives decreases the fraction of the cyclic tautomer; therewith the logarithms of the constants of the chain-ring tautomeric equilibrium correlate with the ES steric constants of the alkyl substituents. In the series of the aromatic aldehydes 2-mercaptobenzoylhydrazones the linear tautomer prevails, and the logarithms of the tautomeric equilibrium constants KT correlate with the σ-constants of the substituents in the aromatic nucleus.

Spectroscopic studies on the formation of charge transfer complexes of l-phenylalanine with 2,3,5-trichloro-6-alkoxy-1,4-benzoquinones in aqueous medium

UV–Vis, FT-IR, LC–MS and fluorescence spectral techniques were employed to investigate the mechanism of interaction of l-phenylalanine with new π-acceptors, 6-alkoxy-2,3,5-trichloro-1,4-benzoquinones. The interaction of these quinones with l-phenylalanine (LPA) yielding radical ion pair was found to proceed through the formation of donor–acceptor complex. The stoichiometry of the complexes was determined by Job’s continuous variation method and was found to be 1:1 in all the cases. Kinetic and thermodynamic properties of the complexes were determined in aqueous medium at physiological conditions (pH=7). Fluorescence quenching studies indicated that the interaction between the donors and the acceptor is spontaneous. Correlation of association constants of the CT complexes with Taft’s polar and steric constants indicated that the electronic effects of the substitutions play a significant role in governing the reactivity of the quinones when compared to steric factors.

Effect of additive polyfunctional substituents on the thermal decomposition rate of R1,R2,R3-nitromethanes

Kinetics of thermal decomposition of polyfunctionally substituted nitromethanes has been studied by manometric method as well as IR and chromato-mass spectrometry. Activation parameters of the ratelimiting stage of C-NO2 homolysis have been determined. The rate constant and the activation energy are correlated with additive steric constants of the substituents; this enables prediction of reactivity (thermal stability) of the unstudied mononitrocompounds which decompose with the break of C-NO2 bond.

Synthesis and characterization of molecular complexes of cimetidine with water soluble 1,4-benzoquinones

The molecular complexes of novel water soluble 1,4-benzoquinones possessing different alkoxy substituents, with cimetidine drug have been investigated using various spectral techniques such as UV–Vis, 1H NMR, FT-IR. The stoichiometry of the CT complexes was found to be 1:1, in all the cases. The Density Functional Theory calculations supported the experimental observations. Correlation of formation constants (K) of the CT complexes with Taft’s polar (σ∗) and steric (Es) constants indicated that an increase in electron releasing property of the alkoxy group makes these acceptors increasingly weaker while an increase in steric property of the substituent decreased the formation constant.

Spectroscopic studies on the formation of molecular complexes of sulfamethoxazole with novel 2,3,5-trichloro-6-alkoxy-1,4-benzoquinones

UV–Vis, 1H NMR, FT-IR, LC–MS and fluorescence spectral techniques were employed to investigate the mechanism of interaction of sulfamethoxazole with alkoxy substituted 2,3,5-trichloro-1,4-benzoquinones and to characterize the reaction products. The interactions of these quinones with sulfamethoxazole (SULF) were found to proceed through the formation of donor–acceptor complex, containing radical anion and its conversion to the product. Fluorescence quenching studies indicated that the interaction between the donor and the acceptors are spontaneous. Correlation of association constants of the CT complexes with Taft’s polar and steric constants indicated that polar factor plays a significant role in governing the reactivity. The results indicated that the electronic effects of the substituents play significant role in governing the reactivity of the quinones when compared to steric factor.

Spectral and theoretical studies on the molecular complexes of azacyclonol with new π-acceptors, alkoxysubstituted 1,4-benzoquinones

The molecular complexes of a series of new electron acceptors, 1,4-benzoquinones possessing variety of alkoxy substituents, with azacyclonol have been investigated using various spectral techniques such as UV–Vis, 1H NMR, FT-IR, fluorescence and LC–MS. The stoichiometry of the complexes was determined by Job’s continuous variation method and was found to be 1:1, in all the cases. The results of equilibrium, and kinetic studies were well supported by ab initio DFT calculations. Correlation of formation constants of the complexes with Taft’s polar and steric constants indicated that both these factors play significant role in governing the reactivity. Also, the results indicated that an increase in electron releasing property of the alkoxy group makes these acceptors increasingly weaker while an increase in steric property of the substituent decreased the formation constant.

Systematic study on the thermal cycloreversion reactivity of diarylethenes with alkoxy and alkyl groups at the reactive carbons

The relationship between the thermal cycloreversion reactivity of diarylethenes and the bulkiness of the substituents at the reactive carbons was systematically investigated. Two photochromic diarylethenes, 1,2-bis(2-isobutoxy-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-neopentoxy-5-phenyl-3-thienyl)perfluorocyclopentene (2a), were newly synthesized and their optical properties and thermal cycloreversion reactivity were examined, because there is insufficient data for diarylethenes with alkoxy groups at the reactive carbons. The steric substituent constant was employed to correlate the relationship between the thermal cycloreversion reactivity of diarylethenes with alkyl and alkoxy groups at the reactive carbons and the bulkiness of the substituent. A good correlation was obtained for the substituent constant using CH2 instead of oxygen in the alkoxy groups. The results indicate that this is a very useful strategy for the design of novel diarylethenes with desired thermal cycloreversion reactivity.

Structure-reactivity relationships in organosilicon chemistry revisited

Abstract Available quantitative data of the reactivity of organosilicon compounds were subjected to correlation analysis. As an alternative to the carbon chemistry values, the scale of steric constants E S (Si) can be used for organosilicon compounds and the inductive effect is best expressed by two terms, involving the parameter of electronegativity of substituents. Alkyl substituents contribute to the reactivity exclusively through their steric effects, steric and polar effects in silicon compounds are additive and the resonance effect is insignificant. The correlation analysis can be employed in investigations of organosilicon reactions along with other methods.

Regioselectivity in the amination of azines: Reaction of pyrazine derivatives with O-mesitylenesulfonylhydroxylamine

Treatment of 2-X-substituted pyrazines (X = H, Me, Et, Pr, i-Pr, t-Bu, MeCH(OH), H2N, AcNH) with O-mesitylenesulfonylhydroxylamine gave the corresponding 2-X- and 3-X-(1-amino)pyrazin-1-ium mesitylenesulfonates. 2-Alkylpyrazines (X = Me, Et, Pr, i-Pr) displayed a correlation between the logarithms of the concentration ratio of 2- and 3-substituted cations and substituent steric constants. Wider series of sub- stituted pyrazines (X = H, Me, Et, Pr, i-Pr, MeCH(OH), H2N, AcNH) conformed to a multiparameter correla- tion between the logarithms of the concentration ratio of 2- and 3-substituted cations, on the one hand, and substituent constants σI, σR o , and Es o , on the other. The obtained data on the regioselectivity of amination of pyrazines were interpreted in terms of DFT/PBE/3Z quantum-chemical calculations.

Kinetics and Mechanism of the Oxidation of Some Secondary Alcohols by Benzimidazolium Dichromate

Benzimidazolium dichromate (BIDC), in dimethylsulfoxide, oxidizes secondary alcohols to the corresponding ketones. The reaction is first order with respect to BIDC. The order with respect to alcohol is more than one but less than two. Michaelis - Menten type kinetics were observed with respect to alcohol. The formation constants of the alcohol-BIDC complexes and the rates of their decomposition were determined at different temperatures. The oxidation of deuterated 2-propanol indicated the presence of a substantial primary kinetic isotope effect. The reaction exhibited a second order dependence on hydrogen-ion concentration. The reaction was studied in nineteen organic solvents. The solvent effect was analyzed using multiparametric equations. It was found that the cation-solvating power of the solvent played the major role. The rates of oxidation exhibited an excellent correlation with polar and steric substituent constants. A suitable mechanism has been proposed.

ChemInform Abstract: Catalytic Hydrogenation of Persubstituted p‐Nitrosophenols.

Catalytic hydrogenation of dialkyl 2-hydroxy-4,6-dimethyl-5-nitrosobenzene-1,3-dicarboxylates over Pd/C gave the corresponding previously unknown dialkyl 5-amino-2-hydroxy-4,6-dimethylbenzene-1,3-dicarboxylates. The first-order rate constants for the hydrogenation process were found to be linearly related to steric constants of the alkyl groups.

Catalytic hydrogenation of persubstituted p-nitrosophenols

Catalytic hydrogenation of dialkyl 2-hydroxy-4,6-dimethyl-5-nitrosobenzene-1,3-dicarboxylates over Pd/C gave the corresponding previously unknown dialkyl 5-amino-2-hydroxy-4,6-dimethylbenzene-1,3-dicarboxylates. The first-order rate constants for the hydrogenation process were found to be linearly related to steric constants of the alkyl groups.

Tautomerism and conformational isomerism of mercaptoacetylhydrazones of aliphatic and aromatic aldehydes

Mercaptoacetylhydrazones of aliphatic and aromatic aldehydes exist in the solutions as tautomeric mixtures of open-chain and cyclic 1,3,4-thiadiazine forms. The linear hydrazone form consists of a set of isomers due to the configurational and conformational isomerism. At growing bulk of the alkyl substituent at the C=N bond of the aliphatic aldehydes derivatives decreases the fraction of the cyclic tautomer; therewith the logarithms of the constants of the chain-ring tautomeric equilibrium correlate with the steric constants of the alkyl substituents. In the series of the aromatic aldehydes mercaptoacetylhydrazones the linear tautomer prevails, and the equilibrium position is insignificantlyt affected at variation of the electronic characteristics of the substituents in the aromatic ring.

Kinetic study of hydrolysis of benzoates. part xxvii. ortho substituent effect in alkaline hydrolysis of phenyl esters of substituted benzoic acids in aqueous Bu4NBr

The second-order rate constants k (in dm3 mol–1 s–1) for alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C6H4CO2C6H5, have been measured spectrophotometrically in aqueous 0.5 and 2.25 M Bu4NBr at 25 °C. The substituent effects for para and meta derivatives were described using the Hammett relationship. For the ortho derivatives the Charton equation was used. For ortho-substituted esters two steric scales were involved: the EsB and the Charton steric (υ) constants. When going from pure water to aqueous 0.5 and 2.25 M Bu4NBr, the meta and para polar effects, the ortho inductive and resonance effects in alkaline hydrolysis of phenyl esters of substituted benzoic acids, became stronger nearly to the same extent as found for alkaline hydrolysis of C6H5CO2C6H4-X. The steric term of ortho-substituted esters was almost independent of the media considered. The rate constants of alkaline hydrolysis of ortho-, meta- and para-substituted phenyl benzoates (X-C6H4CO2C6H5, C6H5CO2C6H4-X) and alkyl benzoates, C6H5CO2R, in water, 0.5 and 2.25 M Bu4NBr were correlated with the corresponding IR stretching frequencies of carbonyl group, (ΔνCO)X.

Steric effects of polar substituents evaluated in terms of energy by means of isodesmic reactions

Steric effects of various polar and some charged groups were estimated on sterically crowded cyclopropane cis-1,2-bis derivatives 2 or 3, in which the variable substituent is in the proximity of a t-butyl group or of a methyl group. The steric energy was evaluated with reference to the pertinent mono derivatives, that is as reaction energy of an isodesmic reaction, in which the crowded compound is formally synthesized from simple derivatives. Energies were calculated within the framework of the density functional theory at level B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) for 11 dipolar and 5 charged substituents. Interaction of charged substituents is not only steric (destabilizing) but also inductive (stabilizing). The steric effects evaluated in this way differ distinctly from the standard steric constants derived purely from the van der Waals radii of the substituents.

Quantitative substituent effects in the Grignard reaction with silanes

Kinetics of reactions of ethyl- and phenylmagnesium chlorides with chlorosilanes, RMeSiCl 2, were investigated in diethyl ether under pseudo-first order conditions with a great excess of the Grignard reagent. Rate constants for alkyl substituted silanes correlate well with E s(Si) steric parameters. A good linear correlation of rate data for substituted phenyl derivates with σ 0 inductive constants together with correlations of the literature data rule out the resonance effect of substituents at least in nucleophilic displacement reactions at the silicon center. An attempt to calculate the steric constants for polar substituents was made. It appeared that the inductive constants σ ∗ derived from the carbon chemistry are not applicable to the silicon chemistry. New scales of parameters for description of polar and steric effects in the organosilicon chemistry need to be created.

Steric Effects of Alkylmagnesium Chlorides in the Grignard Reaction with Silanes

Rate constants for the reactions of methylvinyldichlorosilane and tetraethoxysilane with alkylmagnesium chlorides RMgCl (R = Et, n-Bu, i-Bu, i-Pr, s-Bu, t-Bu) in diethyl ether were determined. Excellent correlations of rate data with steric constants ES(Si) by Cartledge and v′ by Charton were found for the reaction of methylvinyldichlorosilane. Linear correlations with break points were obtained for the tetraethoxysilane reaction. It was assumed that this could be referred to a change in the reaction mechanism.

Long‐range polar and steric effects in propionate‐SG1‐type alkoxyamines (SG1‐CHMeCOOX): a multiparameter analysis

AbstractThe effects of the substituent X on the homolysis rate constants (kd) of SG1‐propionate type alkoxyamines (SG1‐CHMeCOOX) are analyzed by a multiparametric equation with υ, the steric constant and σI, the polar inductive/field Hammett constant of X. An influence of long‐range polar and steric effects on kd was observed, that is, decrease in kd with increasing size of the X group and increase in kd with increasing polarity of the X group. Copyright © 2006 John Wiley & Sons, Ltd.