bluuebunny/crossref_metadata_2025_split huggingface dataset:
Solvent and secondary substrate isotope effects on the acid‐catalyzed ketonization of acetophenone enol in aqueous solution
Authors: Andraos J. , Kresge A. J. , Obraztsov P. A. 
Published: January 2005

AbstractThe enol of acetophenone was generated flash photolytically in aqueous solution by photosolvolysis of PhCBrCH2 and photohydration of PhCCH and PhCCD, and rates of its ketonization were measured in dilute perchloric acid solutions in H2O and D2O at 25°C. The rate constants so obtained provide the solvent isotope effects, kH+/kD+ = 5.02 ± 0.08, and the secondary isotope effect for deuterium substitution at the β‐position of the enol double bond, (kH/kD)β = 0.999 ± 0.014. Arguments are presented which show that these isotope effects requre a stepwise rather than a concerted mechanism for the ketonization reaction.
Reactions of molecules with two equivalent functional groups. 3. Nucleophilic substitution reactions of α, ω‐dibromoalkanes [Br(CH2)nBr, n = 3,4,5] with potassium cyanide
Authors: Constantinides Ioannis , Macomber Roger. S. 
Published: January 2005

AbstractThe substitution reactions of Br(CH2)nBr(1, n = 3–5) with KCN in methanol were studied in detail. Second‐order rate constants k1 [formation of the mononitrile (2) from 1] and k2 (formation of the dinitrile (3) from 2) were determined, as were the rates for the reaction of H(CH2)nBr (4, n = 3‐5) with KCN under the same conditions. The ratios k2/k1 ( = x) of the three homologs of 1 were found to be 1.15, 0.77 and 0.61 for n = 3, 4 and 5, respectively; a x value of 0.5 indicates that the functional groups behave independently. The second‐order rate constants k1 (statistically corrected) and k2 exhibit modest enhancements compared with model compound 4 when any of the following substituents are present: δ or ϵ‐bromo, γ‐, δ‐, or ϵ‐cyano. By contrast, a γ‐bromo substituent slightly retards the rate. These results indicate that the functional groups do influence each other to a modest degree by field effects which accelerate the reactions. Steric effects do not appear to play a significant role.
New kinetic methods. 2.. An indirect measurement of ester formation and hydrolysis rates
Authors: Chandler W. David , Yan Yongfei , Lee Donald G. 
Published: January 2005

AbstractThe use of an assistant reagent to monitor spectroscopically the concentration of compounds that lack chromophores is described. It is demonstrated, for example, that the concentration of an aliphatic alcohol (present during acidcatalyzed hydrolysis or esterification reactions) can be monitored continuously by addition of a small amount of chromium trioxide. From a knowledge of the rate law and the rate constants for chromic acid oxidations, − d [CrO3]/dt = k [alcohol] [CrO3], the concentration of alcohol can be determined at any time by monitoring the absorbance of chromic acid at 363 nm. The rate at which the concentration of the alcohol is changing can then be used to calculate rate constants for the corresponding esterification or hydrolysis reactions. Rate constants obtained in this way are compared with those previously obtained by use of direct methods, and the application of this approach to the study of kinetics under conditions not accessible by other methods is illustrated.
Quantum mechanical and molecular mechanical studies of the hydrolysis of methyl nitrate and the solvent effect
Authors: Wang Daxi , Xiao Heming , Li Shusen 
Published: January 2005

AbstractThe alkaline hydrolysis reaction of methyl nitrate was simulated by quantum mechanical and molecular mechanical methods. The gas‐phase reaction was calculated to proceed with no barrier when ab initio calculations at the STO‐3G level were utilized, and with a very small barrier (5.70 kJ mol−1) when the MINDO/3 method was applied. In solution, the solvation energy was estimated to be 70.33 kJ mol−1 by molecular mechanical calculations. Thus the activation energy (76.03 kJ mol−1) of alkaline hydrolysis of methyl nitrate was found to be in good agreement with the available experimental value (82.42 kJ mol−1). The results indicated that the substantial activation barrier in alkaline hydrolysis was essentially solvent‐induced.
Catalysis in nucleophilic aromatic substitution reactions. A reinvestigation of the reaction between 1‐fluoro‐2,4‐dinitrobenzene and n‐butylamine
Authors: Forlani Luciano , Bosi Michele 
Published: January 2005

AbstractThe reaction between 1‐fluoro‐2,4‐dinitrobenzene and n‐butylamine in toluene shows a two‐step plot of kobs, values vs the initial values of the concentration of the amine. The usual base‐catalysis mechanism for HF elimination from the zwitterionic intermediate hardly explains this kinetic behaviour and the kinetic effect of addition of salts (and of 2‐hydroxypyridine) to the reaction mixtures at different initial values of the concentration of n‐butylamine. In contrast, the kinetic behaviours are easily explained by the presence of substrate–amine (or catalyst) interactions on the pathway of the substitution reaction.
Potentiometric evaluation of the ion‐selective characteristics of 2‐borono‐1,3‐XYLYL crown ethers
Authors: Williams Gwyn , Tuladhar Sudersan M. , D'silva Claudius 
Published: January 2005

AbstractPoly(vinyl chloride) (PVC) membranes impregnated with 2‐borono‐1,3‐xylyl crown ethers were evaluated in ion‐selective electrodes. The response pattern obtained with these crown ethers appears to be essentially independent of ring size but there is a linear correlation between selectivity and hydration enthalpy of the cation which is similar to that obtained for electrodes prepared only with the lipophilic anionic site KTCIPB. The similarities in correlation and mechanism between these two types of electrodes has been attributed to the presence of anionic sites in both cases dominating the observed selectivity. For the four ionophores considered the pH‐dependent ionization of the boronate group to the boronate anion is proposed to be responsible for the domination of anionic sites on the properties of the ionophores. The ionophores proved to be good K+ neutral carriers, showing responses to this ion in the range 10−1−10−4 M. The compounds tested showed a fast response to changes in K+ concentration which was stable to within < 0.2 mV h−1.
Modulation of the cation complexing properties in the ‘lower rim’ chemically modified calixarene series
Authors: Schwing‐weill M. J. , Arnaud‐neu F. , McKervey M. A. 
Published: January 2005

AbstractThe effect of chemical modifications on the lower rim of calix [n] arenes is analysed with respect to the cation binding ability of the receptor. Extraction data and stability constants of the complexes are discussed. Three main factors are investigated: the size of the calixarene, the conformation of the calixarene and the nature of the ligating group attached to the phenolic oxygen. The work concentrates on esters, ketones, amides, thioamides and carboxylic acids. Some data concern chemically modified tetrahomodioxacalix [4] arene esters.
Solvent effects in aromatic nucleophilic substitution reactions in non‐polar aprotic solvents. Inhibition by electron‐donor–acceptor (EDA) complexation of the substrate by aromatic solvents
Authors: Durantini E. , Zingaretti L. , Anunziata J. D. , Silber Juana J. 
Published: January 2005

AbstractThe reactions of n‐butylamine and piperidine with 1‐chloro‐2, 4‐dinitrobenzene and 4‐chloro‐3‐nitrotrifluoromethylbenzene were studied in n‐hexane, benzene and mesitylene. The reactions seem to follow a bimolecular SNAr mechanism for both substrates and amines. However, whereas in n‐hexane they are base catalysed, in the aromatic solvents only mild acceleration is observed. The different behaviour of these reactions in the aromatic solvents in comparison with hexane is explained by the specific solvent effect exerted by the aromatic compounds, which solvates the aromatic reactant preferentially in view of its electron‐donor properties. These conclusions were confirmed by kinetic studies in hexane‐aromatic solvent binary mixtures. It can be concluded that the presence of electron‐donor solvents may inhibit SNAr reactions when association constant of the solvent with the substrate is greater than the EDA constants between the reactants. These specific solvent effects could also explain why only mild acceleration is observed in aromatic solvents.
Interaction of formamide with stilbazolium betaines: Steric effects in amides
Authors: Catalan Javier , Pérez Pilar , Elguero José 
Published: January 2005

AbstractThe visible spectrum of di‐tert‐butylstilbazolium betaine (DTBSB) was recorded in eight simple amides. The appearance of a ‘coarse’ structure in the main absorption band is due to the shielding effect of the tert‐butyl groups, which hinder the approach of the solvent. Since this structure disappears in formamide, it is concluded that this solvent associates through its NH to the carbonyl group of DTBSB. For the other amides, there is a clear relationship between the steric effect, for both the N‐ and C‐substituents, of the solvent–solute hydrogen bond association and the structure of the visible band.
Comparison of MNDO, AM1 and PM3 rotational barriers in branched alkanes
Authors: Burke L. A. , Krishnan P. N. , Morris R. E. , Famini G. R. 
Published: January 2005

AbstractThe MNDO, AM1 and PM3 semi‐empirical methods were used to calculate the equilibrium points and transition‐state structures of the rotational barriers of ethane and all methyl‐substituted propanes and butanes. Force constant methods were used to ensure proper geometries. In general, AM1 barriers tend tobe lower than MNDO calculated ones, which are themselves generally lower than experiment. The PM3 values are closest to experiment, ranging from slightly above to slightly below the experimental values. The complexity of various possible rotational minima makes the development of empirical rules to predict barrier heights difficult.
Allosteric effects in a synthetic receptor for alkali metal ions and dinitrobenzene
Authors: Sijbesma Rint P. , Nolte Roeland J. M. 
Published: January 2005

AbstractA novel host is described which contains three potential binding sites, viz. a cleft and two crown ether rings. The molecule can adopt three different conformations. One of these is able to form a complex with two alkali metal ions. The binding of potassium ions is shown to be a cooperative process. The association constant of the second ion is 128 times larger than that of the first. The compound also binds 1,3‐dinitrobenzene at the third site by π–π interactions. The complex with two potassium ions binds 1,3‐dinitrobezene more strongly than does the free compound by a factor of 2–6, depending on the solvent used. This allosteric effect is caused by the conformational change which is induced by the binding of the potassium ions.
Kinetics and mechanisms of oxidation of thiosemicarbazide, its metal complex and hydrazones by N‐chlorobenzamide in water methanol medium
Authors: Nayak Suresh P. , Gowda B. Thimme 
Published: January 2005

AbstractThe kinetics of oxidation of thiosemicarbazide (TSC) its zinc metal complex and hydrazones by N‐chlorobenzamide (NCB) were investigated in water methanol (1 : 1, v/v) medium in the presence of perchloric acid. The hydrazones studied were benzaldehyde, propionaldehyde, acetone and acetophenone thiosemicarbazones. The reactions show first‐order kinetics in [NCB], a fractional order dependence in [substrate] and an inverse fractional to inverse first order in [H+]. Addition of benzamide has no significant effect on the rates of oxidations. Variation in the ionic strength of the medium has little effect on the rates of reactions, but a decrease in the dielectric constant of the medium by increasing the methanol concentration in the solvent increases the rates. The rate‐limiting steps were identified in all cases and the rate coefficients of these steps and the related activation parameters were also evaluated. The consistency of the deduced rate laws was checked by recalculating the rate constants as the substrate and H+ concentrations were varied.
Structure of transition states in forbidden pericyclic reactions. The second‐order similarity approach
Authors: Ponec R. , Strnad M. 
Published: January 2005

AbstractThe recently proposed similarity approach to the Evans/Dewar concept of aromaticity of transition states was generalized by introducing the second‐order similarity index. This generalization is especially convenient for the characterization of antiaromatic transition states in forbidden reactions since, owing to the partial incusion of electron correlation in the second‐order index, the detailed specification of their structure in terms of the electron states of these biradicaloid species becomes possible. The results obtained are in complete agreement with the conclusions of independent quantum chemical analyses.
Significance of the cis–trans isomerization of early intermediates in the carotene biosynthetic pathway
Authors: Paneth Piotr , Madhavan S. , O'Leary Marion H. 
Published: January 2005

AbstractUsing semi‐empirical self‐consistent field molecular orbital (SCF‐MO) quantum chemical calculations, structures of putative intermediates of the desaturation pathway for both cis‐ and trans‐ carotene biosynthesis have been optimized. It was observed that the cis isomers of the early biosynthetic intermediates are more stable than corresponding trans isomers. Both desaturation and cyclization steps confer increased stability on these carotenes. The results also argue for phytofluene, rather than the earlier suggested phytoene or ζ‐carotene, as the energetically favored branch point for poly‐cis‐carotene biosynthesis.
Micelle‐induced change in the rate‐limiting step of substituted benzoate ester thiolysis
Authors: Correia Valdir Rosa , Cuccovia Iolanda Midea , Chaimovich Hernan 
Published: January 2005

AbstractThe effect of hexadecyltrimethylammonium bromide (CTAB) micelles on the acid dissociation constant of para‐substituted benzenethiols, α‐toluenethiol and n‐heptanethiol was determined. The effect of CTAB on the rate of thiolysis by the thiols was measured using para‐substituted p‐nitrophenyl benzoates. The effects of micelles were analysed using a pseudophase ion‐exchange (PIE) model. The Brønsted plot for ester thiolysis showed discontinuity at the pKa of the leaving group in water and was linear in micelles. Micelles increased the Hammett ρ value for thiolysis of esters by α‐toluenethiol from 2·08 to 2·68. The Hammett plot for the rection of benzenethiol with esters in water is linear with σ− and displays a ρ value of 0·87, whereas in micelles the plot is linear with σ and presents a ρ of 2·83. Taken together, these data indicate that micelles produce a change in the rate‐limiting step of thiolysis of substituted benzoate esters, leading exclusively to rate‐limiting thiolate attack. In micelles thiolysis may be concerted and the effect of the aggregate on the reaction mechanism can be ascribed to the interaction of the thiolate ions, and the transition states, with the head groups as well as a medium effect.
Zn2+ complexes with N‐(2‐pyridyl)cinnamides: Characterization and UV studies
Authors: Duran H. , Duran E. , Gorrichon L. , Perry M. 
Published: January 2005

AbstractZinc(II) complexes of N‐pyridylcinnamides (H, m‐OCH3, p‐OCH3 and p‐OH derivatives) were studied by spectrophotometric methods in aprotic media, and represent a chemical model inhibition by cinnamides of coniferyl alcohol dehydrogenase (CAD), a zinc enzyme involved in the lignification process. The complexation of N‐pyridylcinnamide and m‐methoxy‐N‐pyridylcinnamide with zinc ion is effected according to a 1 : 1 stoichiometry (ML), whereas a two‐step equilibrium (M + L ⇌ ML \documentclass{article}\pagestyle{empty}\begin{document}$ V_{\rm m} = 4 \cdot 45C_0 + 6 \cdot 44C_1 - 3 \cdot 33C_2 + 0 \cdot 46C_4 + 52 \cdot 9{\rm (}n = 300,r = 0 \cdot 999,s = 0 \cdot 81) $\end{document} ML2) is preferred with p‐methoxy and p‐hydroxy compounds. These ligands are mainly bonded through the carbonyl oxygen atom and the nitrogen of the pyridyl ring. Molar absorptivities for these complexes, not directly available, were calculated from analysis of the experiemental data. The UV complexation results are also supported by the stoichiometry of the complexes, which were synthesized and characterized.
Carbon and hydride migrations of cyclobutylhalocarbenes
Authors: Moss Robert A. , Ho Guo‐Jie 
Published: January 2005

AbstractThe photolysis of cyclobutylchlorodiazirine affords chlorocyclopentene and chloromethylenecyclobutane by direct rearrangements from the diazirine's excited state (65%) and by competitive formation and subsequent rearrangement of cyclobutylchlorocarbene. The involvement of excited diazirine in the analogous cyclobutylfluorodiazirine photolysis is much reduced (12%).
Study of initial‐ and transition‐state solvation in the solvolysis of tert‐butyl halides in alcohols from infinite dilution activity coefficients
Authors: Gonçlalves Raquel M. C. , Simões Ana M. N. , Albuqerque Lídia M. P. C. , Macedo Euǵania A. 
Published: January 2005

AbstractThe Gibbs energies of activation of the solvolytic reactions of tert‐butyl halides (chloride, bromide and iodide) in eight mono‐alcohols and eight di‐alcohols were dissected into contributions from the initial state and transition state. The unified approach proposed by Abraham was applied and the various factors making up the overall solvent effect in both states were identified and compared. To perform this study, the Gibbs energies of transfer of the initial state were calculated using the infinite dilution activity coefficient values of the solutes in the solvents, γ∞. These values were determined from the UNIFAC group‐contribution method, with the modified Flory–Huggins equation in the combinatorial term and with group interaction parameters from the Parameter Table exclusively based on experimental γ∞ data reported in the literature. It is concluded that solvent polarizability and polarity–HBD acidity and cavity effects during the activated process are mainly due to transition‐state contributions.
Theoretical studies of substituent effects on thermal eliminations of aryl ethyl carbonates andS‐arylO‐ethyl thiocarbonates
Authors: Kim Chang Kon , Lee Bon‐Su , Lee Ikchoon 
Published: January 2005

AbstractSubstituent effects on the thermal elimination reactions of carbonates and thiocarbonates were investigated MO theoretically using the AM1 method. For both substrates, an electron‐withdrawing group is found to facilitate the decomposition. A two‐step process is favoured by carbonates, whereas one‐ and two‐step processes can compete in the pyrolysis of thiocarbonates. The ability to transmit substituent effect is greater with oxygen (carbonates) than with sulphur (thicarbonates) in contrast to the experimental results of Taylor and co‐workers. Modelling studies with replacement of the phenyl ring with a vinyl group are found to be justified in this type of work.
Ab initio studies of the reaction between diazonium ions and water
Authors: Sapse Anne‐Marie , Jain Duli C. 
Published: January 2005

AbstractAb initio calculations, using the 6–31G* basis set, were performed in order to study the energetics of the electrophilic attack on water by such diazonium ions as methyl‐, ethyl‐ and fluroethyldiazonium. For the first case, an indication of a transition state appears at the Hartree–Fock level, whereas for the other two, the energy is continuously decreasing from reactants to products. When correlation energy was taken into account via single‐point MP2/6–31G* calculations, a deep ion–dipole energy minimum was observed for the methyldiazonium ion followed by a low transition state, whereas the other two did not change their energy pattern. On performing the MP2/6–31G* optimization for the methyl species, the Hartree–Fock results were confirmed, with a slight displacement of the transition state toward a shorter CO distance.
Homoaromaticity of decamethyl[5]pericyclyne
Authors: Schaad L. J. , Hess B. A. , Scott L. T. 
Published: January 2005

AbstractIt is shown that a theoretical computation of the Dewar resonance energy of decamethyl [5] pericyclyne should give the same value for the homoaromaticity of this compound as that calculated earlier from heat of hydrogenation measurements. Both methods show this quantity to be small, but the accuracy of the methods appears insufficient for more exact agreement.
Temprature dependence of the deuterium isotope effect in the deprotonation of some nitroalkanes by anionic bases
Authors: Amin Mohammed , Saunders William H. 
Published: January 2005

AbstractThe temperature dependences of rates and deuterium isotope effects were determined for the deprotonation of 1‐phenyl‐1‐nitroethane (1), its analogue 1‐d, 1‐phenyl‐2‐nitropropane (2) and its analogue 2‐d by methoxide in methanol, ethoxide in ethanol, hydroxide/methoxide in 50% (v/v) methanol–water and hydroxide in water. The isotope effects varied little with substrate or solvent–base combination. kH/kD at 30°C was between 6·0 and 7·1 in all cases. The temperature dependences varied considerably [AaH/AaD = 0·14–1·35 and EaD – EaH 0·95–2·29 kcal mol−1 (1 kcal = 4·184 kJ)]. AaH/AaD was lower with 1‐phenyl‐1‐nitroethane than with 1‐phenyl‐2‐nitropropane, but there was no consistent variation with solvent/base.
Microwave activation of the mutarotation of α‐D‐glucose: An example of an interinsic microwave effect
Authors: Pagnotta M. , Pooley C. L. F. , Gurland B. , Choi M. 
Published: January 2005

AbstractThe mutarotation reaction of α‐D‐glucose in ethanol–water solvent in a modified commercial microwave oven, when compared with control reactions carried out at identical temperatures, shows a non‐thermal microwave effect, evidenced both by a more rapid reaction and by a change in relative amounts of α‐ and β‐D‐glucose over time.
Adsorption at the liquid–liquid interface: An important factor in phase‐transfer catalysis
Authors: Lasek Wojciech , Mąkosza Mieczysław 
Published: January 2005

AbstractA new rationalization of some phenomena in the phase‐transfer catalysed processes promoted by concentrated aqueous alkali solutions, such as alkylations and β‐eliminations, is proposed. It is predicted on the basis of adsorption theory and subsequently evidenced experimentally that the size of the interfacial area, controlled by the rate of stirring, affects the position of the extraction equilibrium of various anions with quaternary ammonium cations supplied by the phase‐transfer catalysts.
Vibrational spectra of C4H isomers in low‐temprature antimony pentafluoride matrices
Authors: Vančik H. , Gabelica V. , Sunko D. E. , Buzek P. , v. R. Schleyer P. 
Published: January 2005

AbstractThe IR spectra of isomeric bicyclobutonium (1), delocalized cyclopropylcarbinyl (2) and 1‐methylally (3) cations were recorded at 180 K in SbF5 matrices. Cations 1 and 2 generated from cyclopropylcarbinyl and cyclobutyl chloride, respectively, rearrange to 3 at temperatures above 230 K. The structures 1, 2 and 3 were confirmed by comparison of the recorded frequencies with the MP2/6–31G*‐calculated values. These results are in accord with prediction that ions 1 and 2 are rapidly equilibrating non‐classical structures.
Solvation and metal ion effects on structure and reactivity of phosphoryl compounds. 2. β,γ‐Substituted alkylphosphonic esters
Authors: Belciug Marie‐P. , Modro Tom A. , Wessels Philippus L. 
Published: January 2005

AbstractNMR spectroscopic and conformation analyses ware carried out for five dimethyl β,γ‐disubstituted propylphosphonates, YCH2CHXCH2PO3Me2, in five solvents and in acetone containing sodium and magnesium ions. Conformational preferences observed for the rotation about the C1C2 bond are determined by the attractive interactions between the oxygen containing substituents X (X = OH, OMe) and the phosphoryl group, and are enhanced by the metal ions, presumably via chelation effects. The rotation about the C2C3 bond yields statistical distribution of the rotamers. For Y = benzoyl, there was no evidence for a competition of the carbonyl group for the intramolecular interactions with X or with a metal ion.