bluuebunny/crossref_metadata_2025_split huggingface dataset:
Selectivity of nitration reactions of aromatic compounds on zeolites H‐Y and H‐ZSM‐11
Authors: Nagy Sándor M. , Yarovoy Konstantin A. , Shubin Vyacheslav G. , Vostrikova Lidiya A. 
Published: January 2005

AbstractSubstrate and positional selectivity in the nitration reactions of benzene and its derivatives (halobenzenes, toluene and o‐xylene) by nitric acid and acyl nitrates supported on zeolites H‐Y and H‐ZSM‐11 were studied. The reaction mechanism and effects governing selectivity of the process are discussed.
Mechanism of reaction of azobenzene formation from aniline and nitrosobenzene in basic conditions. General base catalysis by hydroxyide ion
Authors: Dalmagro Jacir , Yunes Rosendo A. , Simionatto Edesio Luiz 
Published: January 2005

AbstractThe reaction of nitrosobenzene with aniline, to give azobenzene, in basic conditions was studied. It was shown that the reaction exhibits general base catalysis by different buffers giving a Brønsted coefficient β = 0·318. As in previous studies, a two‐step process with a first step of attack of aniline on nitrosobenzene to give an addition intermediate and a second step of dehydration of this intermediate is proposed to interpret the mechanism of the reaction. The analysis of the Brønsted relationship and of the intermediate of the reaction led to the suggestion that hydoxide ion catalyses the reaction by a mechanism of general base catalysis in the dehydration step.
σ–π and η–π Stabilization energies in vinyl and phenyl compounds
Authors: Luo Yu‐Ran , Holmes John L. 
Published: January 2005

AbstractA means for determining the stabilization energies, Ec(X), resulting from σ–π and n–π interactions in vinyl and phenyl compounds is described. It is based on extensions of the additivity principle for thermochemical data for organic compounds. The results also provide a method for predicting heats of formation of unsaturated compounds from those of (known) saturated analogues. It is shown that there is a linear relationship between the stabilization energies Ec(X) and the Hammett substituent constants σ(X). The new results have been used to predict the heats of formation of a number of saturated and unsaturated Si‐, Ge‐, Sn‐, P‐, As‐, Se‐, Zn‐, Cd‐ and Hg‐containing species.
Probing onium ions and carbocations by desorption/ionization mass spectrometry: Crossing the threshold between fragile salts and gaseous intact cations and subsequent guest–host and cation–molecule cluster chemistry
Authors: Laali Kenneth K. 
Published: January 2005

AbstractThe utility of desorption/ionization mass spectrometric (DI/MS) techniques in dealing with physical organic problems involving fragile onium salts and carbocation salts has been explored. The efficacy of these methods, especially when coupled to tandem mass spectrometry, goes much beyond the analytical aspects dealing with identity/purity determinations. For example, insight can be gained regarding the role of the counterion in subsequent cation decomposition pathways. Guest–host chemistry of onium ions and various hosts (crowns, calixarenes) can be probed. The decomposition chemistry of the resulting ion–molecule clusters can be studied. Examples of onium salts and carbocation salts that have been studied are few, probably because research in this area has been mainly a domain for analytical chemists. This paper calls attention to the possibilities for dealing with topical physical organic problems.
Acidity and basicity of thiocarboxamides in the gas phase
Authors: Decouzon Michèle , Exner Otto , Gal Jean‐françois , Maria Pierre‐Charles , Waisser Karel 
Published: January 2005

AbstractThe gas‐phase acidity and basicity of thioacetamide and the basicity of N,N‐dimethylthioformamide were measured by Fourier transform ion cyclotron resonance (FT‐ICR) mass sectrometry under conditions which minimized the extent of their decomposition. Thiocarboxamides are both much stronger acids and stronger bases than carboxamides. The relative stabilities of individual neutral and ionic species were assessed in terms of isodesmic reactions, using the published or estimated enthalpies of formation. The neutral molecules of carboxamides and thiocarboxamides are stabilized by interaction between the C = X and NH2 functional groups. This interaction is of a similar magnitude in the corresponding protonated forms but it is of greater strength in the deprotonated forms. With regard to the difference between thiocarboxamides and carboxamides, the most significant factor is probably the Ione pair–Ione pair repulsion operating in the anions.
Kinetic studies of the reactions of phthalimide with ammonia and pyrrolidine
Authors: Khan M. Niyaz , Ohayagha J. E. 
Published: January 2005

AbstractThe kinetics of the nucleophilic cleavage of phthalimide (PTH) in buffer solutions of ammonia and pyrrolidine were determined. The reaction rates for ammonolysis of PTH revealed a buffer‐catalysed second‐order term in the rate law, but the reaction rates for pyrrolidinolysis of PTH showed buffer‐catalysed second‐ and third‐order terms in the rate law. Both ammonia and pyrrolidine revealed nucleophilic reactivity towards ionized PTH (S−) only within the pH range of the present study. This is attributed to the occurrence of intramolecular general base–acid catalysis. General base catalysis is detected in the reactions of pyrrolidine with both non‐ionized PTH (SH) and S−. The general base‐catalysed third‐order rate constant for pyrrolidinolysis of SH is nearly 28 times larger than that of S−.
Synthesis and photophysics of diaza‐crown ether‐based bisporphyrins
Authors: Mårtensson Jerker , Sandros Kjell , Wennerström Olof 
Published: January 2005

AbstractThe bisporphyria N,N′‐bis[4″‐(meso‐triphenylporphyrinyl)benzyl]‐4,13‐diaza‐18‐crown‐6 and its mono‐ and dizinc derivatives were synthesized in 66%, 46% and 53% yields, respectively, from 5‐(4′‐bromomethylphenyl)‐10,15,20‐triphenylporphyrin or its zinc derivative and 4,13‐diaza‐18‐crown‐6. The zinc‐containing bisporphyrins form dimers in solution at low temperature or at high concentration. The unsymmetrical bisporphyrin; monozinc N,N′‐bis[4″‐(meso‐triphenylporphyrinyl)benzyl]‐4,13‐diaza‐18‐crown‐6 shows singlet–singlet energy transfer from the zinc porphyrin moiety to the free base moiety in both the monomeric and the dimeric form. The energy transfer rates were determined using time‐resolved fluorescence spectroscopy and were found to be 1·26 × 109 and 2·29 × 109 s−1 for the monomeric and dimeric form, respectively. The difference in energy transfer rates between the two forms can be rationalized by the difference in overlap between the donor fluorescence spectrum and acceptor absorption spectrum, donor–acceptor distance and donor–acceptor orientation.
Kinetics and mechanism of aminolysis of propargyl and 1‐methyl‐propargyl arenesulphonates
Authors: Oh Hyuck Keun , Cho In Ho , Jin Min Jeong , Lee Ikchoon 
Published: January 2005

AbstractKinetic studies were carried out on the aminolysis of propargly and 1‐methylpropargyl arenesulphonates in acetonitrile at 45·0°C. The cross‐interaction constants, ρxz and βxz, are similar to, but smaller than, those for the SN2 processes at other primary and secondary carbon centers. Compared with the allyl series, the smaller magnitude of ρxz and βxz reflects a looser transition state, which in turn leads to a lower rate despite the greater Taft's σ* value and the lower intrinsic (ΔE) and thermodynamic barriers (ΔE0).
Effect of cyclic acetal‐type cationic surfactants on the basic dehydrobromination reaction
Authors: Wilk Kazimiera A. , Bieniecki Albert , Matuszewska Barbara 
Published: January 2005

AbstractThe reaction of 2‐(p‐nitrophenyl)ethyl bromide with hydroxide ion was studied at 50 and 25°C in the presence of cationic chemodegradable surfactants, i.e. [2‐alkyl(1,3‐dioxolan‐4‐yl)methyl] trimethylammonium bromides Ia–c (alkyl: a = n‐C9H19, b = n‐C11H23, c = n‐C13H27). The kinetic data were interpreted with the pseudo‐phase ion‐exchange (PIE) formalizm at both temperatures. The results indicate that the major source of rate enhancement is the increased reactant concentration in the small micellar reaction volume. The surfactant stability in micellar conditions was probed by means of a hydrolysis reaction of the surfactant 1,3‐dioxolane ring. The kinetics of acid hydrolysis of Ia–c micelles, as a result of specific hydronium ion concentration, is one order of magnitude smaller than that of unaggregated systems.
Hydrogen bonding. 39. The partition of solutes between water and various alcohols
Authors: Abraham Michael H. , Chadha Harpreet S. , Dixon Julian P. , Leo Albert J. 
Published: January 2005

AbstractThe general solvation equation

was applied to the partition of solutes between water and isobutanol, pentanol, hexanol, octanol, decanol and oleyl alcohol. It is shown that the two main factors that influence partitioning are solute hydrogen‐bond basicity Σβ and solute volume Vx. The b coefficient becomes steadily more negative along the above series of alcohols, showing that the alcoholic phases, which are all less acidic than water, become less and less acidic as the chain length increases, and the water content of the alcoholic phase decreases. The v coefficient, on the other hand, becomes gradually more positive, indicating that as the chain length increases and the water content decreases, the alcoholic phase becomes more and more hydrophobic. Of great significance is that for all six alcohols, the a coefficient is effectively zero, so that all alcoholic phases have the same basicity as bulk water, no matter what their water content is. It is suggested that, contrary to results of solvatochromic measurements, the alcohols have similar hydrogen‐bond basicity to water.
Equilibrium acidities and homolytic bond dissociation enthalpies of the acidic CH bonds in dialkyl malonates and related compounds
Authors: Zhang Xian‐Man , Bordwell Frederick G. 
Published: January 2005

AbstractThe equilibrium acidities, pKHA, of 18 dialkyl malonates, five alkyl 2‐cyanoacetates and nine malononitriles and the oxidation potentials of their conjugate anions, EOX(A−), were measured in dimethyl sulfoxide solution. The homolytic bond dissociation enthalpies (BDEs) of their acidic C–H were estimated by combining their pKHA and EOX(A−) values. The pKHA values of the dialkyl malonates were found to increase from 15·9 to 16·4 to 18·4 as the dialkyl groups were changed from dimethyl to diethyl to di‐tert‐butyl, but the BDEs of the acidic C–H bonds remained constant [95·3±0·3 kcal mol−1] (1 kcal = 4·184 kJ). Introduction of methyl, ethyl, isopropyl and tert‐butyl groups into the 2‐position of diethyl malonate caused the equilibrium acidities to increase by 2·0, 2·4, 3·8 and 8·0 pKHA units, respectively, and the BDE values to decrease by 4·4, 3·7, 2·5 and 0·8 kcal mol−1, respectively. Introduction of a phenyl group into the 2‐position of diethyl malonate had no effect on the acidity, but weakened the acidic C–H bond by 10 kcal mol−1. The effects on acidity and BDE of introducing 3,4,5‐ (MeO)3C6H2CO, CF3, Me3N+, c‐C5H10N, p‐MeC6H4, p‐NO2C6H4, PhO, F and c‐C5H5N+ groups into the 2‐position of diethyl malonate were also examined.
Study of the hydrogen bond donor acidity of binary aqueous mixtures and their role in reversed‐phase liquid chromatography
Authors: Park Jung Hag , Dallas Andrew J. , Chau Phoebe , Carr Peter W. 
Published: January 2005

AbstractThe solvatochromic hydrogen bond donor (HBD) acidity parameter (αmix) of aqueous mixtures of methanol, acetonitrile, propan‐2‐ol and tetrahydrofuran were determined spectrophotometrically. The study was carried out at 25deg;C as a function of composition. The indicators used were 2,6‐diphenyl‐4‐(2,4,6‐triphenyl‐N‐pyridino)phenolate, 2,6‐dichloro‐4‐(2,4,6‐triphenyl‐N‐pyridino)phenolate and Fe(LL)2(CN)2 (LL = N‐(2‐pyridylbenzylidene)‐3,4‐dimethylaniline). The HBD acidity of the aqueous organic mixtures was related to retention in reversed‐phase liquid chromatography.
Relative thermodynamic stabilities of 2‐(methoxymethylene)tetrahydrofuran and 5‐methoxymethyl‐2,3‐dihydrofuran
Authors: Taskinen Esko 
Published: January 2005

AbstractThe relative thermodynamic stabilities of the title compounds were determined by iodine catalyzed chemical equilibration in cyclohexane solution. The main point of interest was a determination of the thermodynamic stability of the OCCO moiety found in the exocyclic isomers, i.e. the stabilizing effect of a MeO group on the olefinic linkage of 2‐methylenetetrahydrofuran. All three isomeric compounds have essentially similar enthalpy values, which, in comparison with some previous thermodynamic data, shows that the double bond stabilization energy of the MeO group in the exo isomers is only ca 3 kJ mol−1. The entropy difference between the geometrical isomers is negligible, whereas the endo isomer is favoured by an entropy contribution of ca 9 J K−1 mol−1.
Hyperbolic Model for the meta–para interrelationship in benzene derivatives
Authors: Reis JoãO Carlos R. , Segurado Manuel A. P. , De Oliveira Jaime D. Gomes 
Published: January 2005

AbstractA new approach to the long‐standing problem of interrelating meta and para substituent constants is presented. An analysis of the unified σ0‐scale shows that the interrelation between σ and σ/σ can be modelled by a pair of conjugate rectangular hyperbolae, one for normal (n) and the other for special (s) substituents. The latter are characterized by a lone electron pair in the first atom. The equations σ (σ − γ0)/(σ − 2γ0) = λ0 σ and σ = γ0 + λ0 σ are derived and discussed in terms of Taft's separation of mesomeric and non‐mesomeric effects. Asymptotic values λ = 0.960 γ = −0.225 were obtained by non‐linear least rectangles fitting. A nonnegligible mesomeric contribution to σ0 constants for normal substituents is predicted by the hyperbolic model. The present results are at variance with Exner's analysis of the meta‐para interrelationship in benzene compounds with normal substituents. This divergence is ascribed to opposite views concerning the role of the π‐inductive effect.
Structural study on quasi‐phosphonium salts containing phosphorus–oxygen, phosphorus–oxygen, phosphorus–nitrogen and phosphorus‐sulphur bonds
Authors: Imrie Christopher , Modro Tomasz A. , Van Rooyen Petrus H. , Wagener Carl C. P. , Wallace Karen , Hudson Harry R. , McPartlin Mary , Nasirun Jusni B. , Powroznyk Luba 
Published: January 2005

AbstractThe crystal and molecular structues of six quasi‐phosphonium salts containing phosphorus‐heteroatom heteroatom=oxygen, sulphur or nitrogen0 bonds were determined. Comparison of the molecular parameters obtained with those available for reference structures demonstrated that the ‘double bond’ character for the P+  Y bond decreases in the order Y = N ≫ O > S.
Reaction of phenoxy radical with nitric oxide
Authors: Yu T. , Mebel A. M. , Lin M. C. 
Published: January 2005

AbstractThe associationof C6H5O with NO was studied with the cavity‐ring‐down method by directly monitoring the decay of C6H5O in the presence of varying, excess amounts of NO. The biomolecular rate constant determined in the temperature range 297‐373 K can be effectively rate constant determined in the temperature range 297–373 K can be effectively represented by k1 = 10 cm3 molecule−1 with a negative activation energy of 0.8 kcal mol−1 (1 kcal = 4.184 kJ). In order to understand better the mechanism of the reaction, ab initio molecular orbital calculations were also carried out at the MP4(SDQ)/6‐31G* level of theory using the HF optimized geometries. The molecular structues and energetics of five C6H5N1O2 isomers were calculated. Among them, the most likely and stable association product, phenyl nitrite (C4H5ONO), was found to be 17 kal mol−1 below the reactants, C6H5O + NO. Combining the measured rate constant and the calculated equilibrium constant for the association reaction, C6H5O + NO = C6H5ONO the rate constant for the unimolecular decomposition of C6H5ONO was obtained as k−1 = 4.6 × 1015E−8580/T s−1. The relatively large frequency factor suggests that a loose transition state was involved in the reaction, akin to those of its alkyl analogs (RONO, R  CH3, C2H5, etc.).
Isotherm of water sorption by human serum albumin in dioxane: Comparison with calorimetric data
Authors: Borisover Mikhail D. , Sirotkin Vladimir A. , Solomonov Boris N. 
Published: January 2005

AbstractThe dependence of the amount of water bound to human serum albumin (HSA) suspended in water–dioxane mixtures vs the equilibrium water concentration in the liquid phase was determined by the Fisher method at 298 K. The Langmuir model was used in order to describe the isotherm of the sorption of water by HSA at low water concentrations in the solvent. The calculated equilibrium constant of water adsorption (3.8 ± 0.6 l mol−1) is in good agreement with the adsorption constant obtained earlier from calorimetric data. The comparison of the determined isotherm of water sorption by HSA with the reported enthalpies of suspension formation showed that at low water concentrations in the solvent, water sorption is the only process contributing to the heat effects of the formation of the ‘protein + liquid’ heterogeneous system. From this comparison, the enthalpy of water adsorption by HSA was evaluated as −11.9 ± 1.7 kJ mol−1. At higher water concentrations in the solvent the amount of water adsorbed by HSA increased considerably. This increase in the amount of water on HSA at water activities above 0.5 is assumed to be due to the enlargement of the protein surface area.
Amines as leaving groups in nucleophilic aromatic substitution reactions. III. Hydrolysis of 1‐amino‐2,4‐dinitrobenzenes
Authors: de Vargas Elba Buján , Remedi M. Virginia , de Rossi Rita H. 
Published: January 2005

AbstractThe kinetic study of the reaction of 1‐pyrrolidino‐2,4‐dinitrobenzene, 1‐piperidino‐2,4‐dinitrobenzene and 1‐morpholino‐2,4‐dinitrobenzene with NaOH in the presence and absence of the amine leaving group was carried out in aqueous solutions at 25°C, giving 2,4‐dinitrophenol as the only product. A mechanism involving the formation of σ complexes by addition of HO− or the amine to the unsubstituted positions of the aromatic ring is proposed. These complexes were found to react faster than the original substrates.
Theoretical studies on the acid‐catalysed aromatization of benzene‐cis‐1,2‐dihydrodiols
Authors: Kim Chang Kon , Chung Dong Soo , Chung Kyoo Hyun , Lee Bon‐Su , Lee Ikchoon 
Published: January 2005

AbstractTheoretical studies on the acid‐catalysed dehydration of 15‐substituted dihydrodiols were carried out using the AM1 method. In agreement with the experimental results, the rate‐limiting step is dehydration of the protonated diols, R+, and the o‐phenol‐forming pathway (path 1) is favoured both kinetically and thermodynamically over m‐phenol product formation. The transition state for path 1 (TS 1) is found to be at an early position (ca 23% progress) along the reaction coordinate and a better correlation of the rate constant is obtained with σ+. Three resonance forms can exist for the benzenonium intermediate but the contribution of one form is dominant in the later TS, which resembles the cationic intermediate. The contribution of the other two, however, weakens the σ+ effect of +M subsituents. The σP correlation of the rate constant in acid solution is therefore suggestive of a later TS resembling the cationic intermediate which is stabilized by solution.
First successful correlation of the UV spectra of three kinds of styrenes with spin‐delocalization substituent constants σ.
Authors: Jiang Xi‐Kui , Ji Guo‐Zhen , Wang Daniel Ze‐Rong 
Published: January 2005

AbstractUV spectra of 17 para‐Y‐substituted styrenes, 8 para‐Y‐substituted α‐methylstyrenes and 20 para‐Y‐substituted α,β,β‐trifluorostyrenes were measured and correlated against spin‐delocalization substituent constants (σ.) with confidence levels above 99·9%.
The question of pentaphenylethyl: An AM1 study
Authors: Smith William B. 
Published: January 2005

AbstractSemi‐empirical AM1 theoretical calculations were carried out on the pentaphenylethyl and 9‐trityfluorenyl free radicals, related radicals and their precursor hydrocarbons. Previous reports of the preparation of the former radical have been questioned, although the existence of the second is on secure ground. Based on simple energy considerations, it is concluded that pentaphenylethyl should be as readily prepared. However, the acidity of the precursor hydrocarbon is considerably less than that of 9‐tritylfluorene, and possible radical decomposition via dissociation to triphenylmethyl and diphenylcarbene is calculated to be more facile than the corresponding decomposition of 9‐tritylfluorenyl. A previous postulation of a rapid phenyl equilibration of the pentaphenylethyl radical is shown to be untenable.
The search for the ethynylcation: Nitrosation of N,N‐bis(trimethylsilyl)ynamines
Authors: Alvarez Roberto Martínez , Hanack Michael , Schmid Thomas , Subramanian L. R. 
Published: January 2005

AbstractThe reaction of bis‐silylated ynamines with different nitrosyl reagents affords products derived from an electrophilic attack at the β‐carbon atom and not the expected alkynyldiazonium salts.
Stereochemical control in anionic polymerization of 2‐vinylpyridine and related vinyl monomers
Authors: Hogen‐Esch Thieo E. , Jin Qiao , Dimov Dimo 
Published: January 2005

AbstractThe alkylation, silylation and other electrophilic reactions of 1‐lithio‐1,3‐bis(2‐pyridyl)butane (2a) in THF at −78°C leads to meso‐like products that may be accounted for by intramolecular coordination of Li ion with the penultimate 2‐pyridyl group. This is also demonstrated by proton‐transfer equilibria between 1‐lithio‐2‐ethylpyridine and 2a, allowing the determination of the ΔH and ΔS of intramolecular chelation in 2a. The ΔS of chelation is nearly zero, probably as a result of chelation‐induced desolvation of Li ion. Surprisingly, however, the corresponding addition of 2‐vinylpyridine to 2a and higher P2VPLi homologs in THF is not stereoselective. In toluene at −78°C the t‐BuLi‐initiated polymerization of 2‐VP is actually somewhat syndiotactic (rr = 0·44). The presence in this polymerization of at least one equivalent of t‐BuOLi or other tertiary ROLi surprisingly leads to isotactic P2VP. SEC viscometric and other studies indicate that both the P2VPLi– and P2VPLi–ROLi complexes are monomeric in toluene. The stereochemistry is apparently due to a 1 : 1 P2VPLi–ROLi complex that discriminates between the two pro‐chiral faces of the anion by preferntial coordination of the (ROLi2) moiety to the meso‐prochiral face. Isotactic‐like addition now occurs by electrophilic attack of 2‐VP that is syn with respect to the (ROLi2) ‘counter ion.’