摘要:

Kinetic results are reported for reaction of aniline and six of itsN- and ring-substituted derivatives with 4,6-dinitrobenzofuroxan (DNBF) in water - dimethyl sulfoxide mixtures. In acidic solution &sgr;-adducts, the result of electrophilic substitution in the anilines, are formed with bonding between a ring-carbon atom of the anilines and the 7-position of DNBF. Reaction normally occurs at the 4-position of the aniline unless this carries a substituent, when reaction occurs at the 2-position. A value of 2.0 forkH/kD, the kinetic isotope effect, indicates that in the reaction with aniline bond formation is largely rate determining in the substitution pathway. The results allow estimates to be made for the pKavalues relating to carbon protonation of the anilines.Key words: electrophilic substitution, &sgr;-adducts, 4,6-dinitrobenzofuroxan, ambident reactivity of anilines.

ISSN:0008-4042    

DOI:10.1139/v99-033

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The lithium (1) and cesium (2) enolates of &agr;-phenylcyclohexanone, PCH, have been studied in THF at 25°C. Both form monomer-dimer equilibria withK1,2= 2.8 × 103M-1for1and 1.8 × 103M-1for2. The ultraviolet-visible (UV-vis) spectra of the monomers and dimers were derived from the spectra of the enolates at a series of concentrations by singular value decomposition. Measurements of the ion pair pKvalues gave pK= 12.7 for1and 19.8 for2. The monomers of1and2are alkylated by benzyl halides faster than are the dimers.Key words: carbon-acid ionization, enolate ions, ion pairs, ion pair acidity, aggregation, dimerization constants, alkylati

ISSN:0008-4042    

DOI:10.1139/v99-025

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Apparent second-order rate constants (kapp) have been measured spectrophotometrically for the reaction of 2,4-dinitrophenyl X-substituted benzoates with a series of alicyclic secondary amines in H2O containing 20 mol% DMSO at 25°C. The microconstants involved in the reaction (k-1/k2,k1, andk1k2/k-1) have also been calculated. The magnitude ofkapp,k1, andk1k2/k-1values increases with increasing amine basicity and with increasing acid strengthening ability of the acyl substituent X. Thek-1/k2value decreases from ca. 6.5 to 0.3 with increasing the amine basicity, but remains almost constant upon changing the acyl substituent X for a given amine, indicating that the rate-determining step is governed by the basicity of amine but not by the electronic nature of the acyl substituent X. The Brønsted-type plots forkappshow a break at pKa= 9.1, supporting the assumption that a change in the rate-determining step occurs from rate-limiting breakdown to formation of the addition intermediate as amine basicity increases. The corresponding Brønsted-type plots fork-1/k2,k1, andk1k2/k-1are linear but their &bgr; values are different. &sgr;+constants show better correlation with logkapp, logk1and logk1k2/k-1for the reaction with low basic amines (pKa< 9.1), while &sgr; constants exhibit better correlation for the reaction with highly basic amines (pKa> 9.1). The magnitude of &rgr;1is identical to that of &rgr;appand &rgr;eqfor a given amine.Key words: aminolysis, Brønsted-type plot, structure-reactivity correlations, rate-determining s

ISSN:0008-4042    

DOI:10.1139/v99-046

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Peroxide bleaching is significantly affected by transition and alkaline earth metals. Isolating the effects of different transition and alkaline earth metals on the reactions of peroxide with different representative lignin structures allows the separation of the positive from the negative contributions of these metal ions. In this work, five monomeric or dimeric phenolic lignin model compounds were treated with alkaline hydrogen peroxide in the absence or presence of Mn2+, Cu2+, Fe3+, and Mg2+. We followed the disappearance of the starting material and the progress of demethylation, radical coupling and oxalic acid formation were followed. Transition metals increased the reactivities of all the lignin model compounds with hydrogen peroxide in the order Mn2+> Cu2+> Fe3+, which is the same as the order of activity toward peroxide decomposition while Mg2+stabilized the system. Demethylation, radical coupling, and oxalic acid formation were all increased by the presence of transition metals in the system and decreased by the addition of Mg2+. The acceleration of the total degree of reaction and of the demethoxylation reactions improves peroxide bleaching, but the increase in the radical coupling reactions can affect the further bleachability of pulp while the increase in the formation of oxalic acid could lead to a greater probability of scaling.Key words: lignins, hydrogen peroxide, peroxide bleaching, reactivity, chemical pulps, metal compounds, alkali treatment, transition metals, delignification.

ISSN:0008-4042    

DOI:10.1139/v99-036

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The electrochemical reduction of &agr;-nitrocumene (1) has been investigated under controlled potential, at a mercury pool cathode and at Raney metal (Raney nickel, Raney cobalt and Devarda copper) and fractal nickel electrodes in basic aqueous ethanol. A comparison of the product distribution from the reduction at Hg (electronation-protonation (EP) mechanism) and that from the reduction at Raney metals and fractal nickel has shown that both the electrocatalytic hydrogenation (ECH) and EP mechanisms can be involved at large surface area (porous) transition metal electrodes in a basic protic medium. Cyclic voltammetry was used to determine the reduction potential of1at Hg and at bright polycrystalline nickel, cobalt, and copper cathodes in the same medium.Key words: electrocatalytic hydrogenation, electroreduction, 2-nitro-2-phenylpropane, Raney metal cathodes, mechanism.

ISSN:0008-4042    

DOI:10.1139/v99-035

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

A general classical variational theory of reaction rates is applied to the F + H2→> FH + H reaction. The variational theory gives the rate as the equilibrium flux of phase points through a trial surface which divides reactants from products and is varied to obtain a least upper bound for the rate. This dividing surface (DS) is defined by a power-series expansion of the H-H internuclear separation (r) in internal coordinatesRand θ, i.e.,r= F(R, θ) whereRis the approach coordinate and θ is the orientation angle. The downhill simplex algorithm is used to search the space of 6 and 10 variational parameters of second- and third-order expansions of the DS and obtain minimum values for the canonical rate constant or, in the microcanonical formulation of the theory, the energy-dependent mean reaction cross section. The presence of angle-dependent terms in the DS makes it possible to describe the dynamical stereochemistry of atom-diatom reactions in a new and useful manner. Portraits of the dynamical stereochemistry are obtained by plotting contours of the density of reaction systems on the DS; such plots are reactivity relief maps of the DS. Reactivity relief maps show how the field of reactivity which surrounds the diatomic reactant molecule expands with increasing temperature and energy. Results are presented here for a new power series formulation of the DS which obeys a condition: &dgr;F(R, θ)/&dgr;θ = 0 at θ = &pgr;/2 which is appropriate for reaction of a homonuclear diatomic molecule. The relationship between reactivity relief maps obtained using quadratic and cubic formulations of the new DS and the locations of angle-dependent energy barriers for reaction is described. Variational and classical mechanical trajectory results are used to show how energy-dependent factors, which correct the variational mean reaction cross section for trajectories which cross and recross the DS, depend on the orientation angle.Key words: variational, transition, rate, dividing, surface.

ISSN:0008-4042    

DOI:10.1139/v99-032

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Rate constants obtained for the hydration of some ring-substituted styrenes, YC6H4CR=CH2(R = H, CH3, CF3) (1-3), and for the isomerization of somecis-stilbenes substituted in both rings, YC6H4CH=CHC6H4Z (4), in aqueous H2SO4, D2SO4, and HClO4media at 25°C and, in some cases, at other temperatures, have been subjected to an excess acidity analysis. Data obtained by different groups and in different media for the same substrates agree very well. Them‡values obtained show that all the substrates react via the A-SE2 mechanism, rate-determining proton transfer to carbon; the transition state is late, with the proton transfer being about three-quarters complete. The reactivities of1-3in the aqueous standard state are obtained by extrapolation; they are 1:103:10-7, respectively. The stilbenes are relatively unreactive; logk0values for compounds with substituents in the ring adjacent to the developing positive charge show a good correlation with &sgr;+with a large negative &rgr;+value, but those for compounds with substituents in the other ring correlate with &sgr; with a small negative &rgr;. Extrapolated logk0values for1and3show a good correlation with &sgr;+, but those for2correlate with neither &sgr;+nor &sgr;, because the &agr;-CH3group twists the molecule and reduces the resonance interaction of ring substituents with the developing positive charge. On the other hand, this effect is not seen in3, when the strongly deactivating &agr;-CF3group is present, meaning that the transition state is forced to be planar in this case. The activation parameters, solvent isotope effects, and excess acidity slope parametersm‡m* obtained in the analysis are tabulated and discussed. The parameters obtained for the parent styrenes1are very similar to those previously obtained for the phenylacetylenes YC6H4Ctriple bondCH, meaning that reactions with vinyl cation intermediates and reactions with benzyl cation intermediates can be quite similar.Key words: styrenes, stilbenes, hydration, excess acidity, LFER, mechani

ISSN:0008-4042    

DOI:10.1139/v99-028

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

2,2-Ditipyl-1-R-ethenols (R = H, Me,t-Bu; tipyl (Tip) = 2,4,6-triisopropylphenyl) were equilibrated in hexane with their carbonyl isomers. The equilibration was accompanied by an oxidative cyclization of the enols to substituted 2-R-3-tipyl-4,6-diisopropylbenzofurans, which became more facile with the increase in the bulk of R. At 77.5°C in hexane theKEnolvalues were 90, 0.11, and 0.0011 for R = H, Me,t-Bu, respectively. Comparison withKEnolvalues for the 2,2-dimesityl analogs indicate a higher value for the 2,2-ditipyl derivative with R = H, but lower values for R = Me andt-Bu. The factors affecting this difference are discussed.Key words: 2,2-ditipyl-1-R-ethenols, keto–><-enol equilibria, enols oxidative cyclization

ISSN:0008-4042    

DOI:10.1139/v99-069

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The dehydration of bicarbonate catalyzed by carbonic anhydrase is accompanied by the transfer of a proton from solution to the zinc-bound hydroxide. We have investigated the properties of proton transfer from donors in solution, mostly derivatives of imidazole and pyridine, to a truncated mutant of carbonic anhydrase V with replacements that render the active site cavity less sterically constrained, Tyr 64 →> Ala and Phe 65 →> Ala. Catalysis was measured by determining the rate of exchange of18O between the CO2-HCO3-system and water, and rate constants for proton transfer were estimated as the rate-limiting step in the release of H218O from the enzyme to solution. Each proton donor enhanced catalytic activity in a saturable manner. The resulting rate constants for proton transfer when compared with the values of pKaof the donor and acceptor gave a Brønsted plot of high curvature. These data could also be described by Marcus theory which showed an intrinsic barrier for intermolecular proton transfer near 0.8 kcal/mol and a work term or thermodynamic contribution to the free energy of reaction near 10 kcal/mol. This low intrinsic kinetic barrier for proton transfer is very similar to nonenzymic bimolecular proton transfer between nitrogen and oxygen acids and bases in solution. However, the significant thermodynamic contribution suggests appreciable involvement of solvent and active-site organization prior to proton transfer. These Marcus parameters are very similar to those describing intramolecular proton transfer from His 64 in carbonic anhydrase, suggesting similarities in the intra- and intermolecular proton transfer processes.Key words: carbonic anhydrase, proton transfer, Marcus theory, carbon dioxide

ISSN:0008-4042    

DOI:10.1139/v99-034

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The molecular susceptibility and the atomic contributions including the basin and surface components calculated at the RHF/6-31+G(d)//6-31+G(d) level of theory along with vector current densities obtained at the 6-31++G(d,p)//6-31+G(d) and 6-31+G(d)//6-31+G(d) levels are reported for benzene (1) and correlated - the agreement is excellent - with the results obtained by Keith and Bader with the 6-311++G(2d,2p) basis. This validates our similar studies on the bicyclo[3.2.1]octa-3,6-dien-2-yl anion (2) - initially considered to be a bishomoaromatic species - its parent hydrocarbon bicyclo[3.2.1]octa-2,5-diene (3), the bicyclo[3.2.1]oct-3-ene-2-yl anion (4), and5, the lithium complex of2. While a magnetic susceptibility exhalation is found in going from4to2, the increase in atomic surface contribution to &Dgr;χ arises predominately (42.6%) from the ethenyl bridge carbons C6 and C7, with C2 + C4 and C3 exhibiting substantially smaller increases (27.4 and 8.3%, respectively). This result, in conjunction with the nature of the vector current density maps of2and4,provides further evidence that2does not possess a ring current as depicted in2-bh, the usual representation of "bishomoaromatic"2.Key words: ab initio calculations, IGAIM, CSGT, magnetic susceptibility, induced current density distributions, bicyclo[3.2.1]octa-3,6-dien-2-yl anion.

ISSN:0008-4042    

DOI:10.1139/v99-049

出版商:NRC Research Press    

年代:1999

数据来源: NRC