摘要:

AbstractThe crystal and molecular structure ofN,N‐dimethyl‐N′‐phenylsulphonylformamidine is solved by direct methods and refined to R = 0.041 for 935 reflections, Equalization of CN bond lengths in the amidine fragment is discussed in terms of a push—pull effect operating between theN,N‐dimethyl and SO2Ph groups. A significant role of the substituent at the functional carbon on the degree of π‐electron delocalization on the NCN fragment is interpreted by use of

ISSN:0894-3230    

DOI:10.1002/poc.610040602

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractA new mechanism of substitution betweenn‐hexyl bromide (RBr) and solid potassium chloride in toluene under phasetransfer catalysis conditions is suggested, involving the formation of intermediates, adsorbed on the solid phase, viz., two binary (KCl·QX and KX·QBr) and two ternary (QX·KCl·RBr and QBr·KX·RCl) complexes. According to this mechanism, the catalytic activity of onium salts (QX) in the substitution changes in the order QI>QBr>QCl. The nature of the catalyst cation has little or no effect on the reaction rate. A general rule is proposed according to which in this reaction the substrate is attacked by a stronger nucleophile whereas a weaker nucleophile accepts the leaving group of the substrate. These and earlier data are interpreted in term of theSN2 process occurring on the solid‐phase surface. The structure of the ternary complexes and the role of the phase‐transfer catalyst ar

ISSN:0894-3230    

DOI:10.1002/poc.610040603

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe paper is concerned with the effect of temperature and the nature of a solid ionophore (MCl, where M = Li, Na, K, Rb or Cs) on the rate and equilibrium constants in the process of substitution of chlorine for bromine in hexyl bromide. The non‐linearity of the logkvs 1/T, dependence is shown to be linked with a change in the physical meaning of the constants with temperature variations. This is attributed to the occurrence of a strong exothermic process consisting in the formation of a kinetically independent intermediate, that precedes the limiting stage. A mechanism including the formation of two stable ternary complexes coordinated on the solid‐phase surface, one of which is structurally close to the starting reagents and the other to the final reaction products, is proposed. The observed activation energies, enthalpies of formation of the ternary complex and activation energies of the limiting stages for the forward and reverse substitution processes were calculated. Thermal effects of the substitution in the presence of various solid MCl species were determined from the thermodynamic data and observed activation energies for both reaction types. The substitution is shown to involve a limiting stage represented by a transition between the ternary complexes. A linear relationship between the activation energy for the limiting stage and the solid ionophore crystalline lattice was established, suggesting incorporation of the solid salt molecule into the transition reaction state. Potential energy changes in the course of substitution in the presence of the various MCl solid salts are discussed. The Bell—Evans—Polyani principle is shown to hold for these

ISSN:0894-3230    

DOI:10.1002/poc.610040604

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe orientation of benzoic acid and terephthalic acid on alumina was ascertained by IR measurements. Benzoic acid was chemisorbed as a carboxylate anion with a vertical conformation of the benzene ring (0<θ<1.0) and as a mixture of the anion and the acid (1.0<θ). Terephthalic acid was chemisorbed as a mixture of monocarboxylate anion (hydrogenterephthalate) and dicarboxylate anion (0<θ<0.3), as the monocarboxylate anion with a vertical conformation of benzene ring (0.3<θ<1.0) and a mixture of the monocarboxylate anion and terephthalic acid (1.0<θ). The carboxyl group of expected hydrogenterephthalate adsorbed through the carboxylate anion on alumina (θ = 0.8 : 0.37 mmol g−1alumina) readily reacted with dimethyl sulphate to form monomethyl terephthalate sele

ISSN:0894-3230    

DOI:10.1002/poc.610040605

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractGas‐phase acidities are reported for series of substituted quinuclidinium and bicyclo [2.2.2] octylammonium ions.Ab initiocalculations are also reported for these equilibria at the STO‐3G and 3–21G (for the quinuclidines) bases and can be used to extend the results. pKavalues obtained for the bicyclooctylammonium ions are compared with previously reported results for the quinuclidinium ions. The results, as a whole, are considered together with similar values obtained for other saturated systems, to look at the varying transmission of field effects with distance. The falloff observed between the two series in the gas phase is considerably less than expected from simple electrostatics. In contradication with a recent claim, it is also shown that field effects, as measured by the parameter σF, give a good measure of the effect of the substituents in each series compared with a combination of resonance and electronegativity e

ISSN:0894-3230    

DOI:10.1002/poc.610040606

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractVibrational analysis calculations have been conducted in an effort to understand the origins of apparently anomalous relationships reported betweenkH/kTandkD/kTkinetic isotope effects. The actual isotopic substitutions used in previous work are more complex than a simple comparison between H/T and D/T isotope effects. The relationships between the actual isotope effects determined in these studies amount to tests of two standards of conventional isotope effect theory, the rule of the geometric mean (or the lack of isotope effects on isotope effects) and the Swain—Schaad rule. Model calculations illustrate the importance of violations of the rule of the geometric mean (over Swain—Schaad deviations) arising in models that incorporate both explicit reaction‐coordinate coupling of two isotopic sites and reaction‐coordinate tunneling. Implications for experimental studies of turneling using apparent Swain—Schaad exponents are

ISSN:0894-3230    

DOI:10.1002/poc.610040607

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe reactions of 1‐chloro‐ and 1‐fluoro‐2,4‐dinitrobenzenes with aniline and 4‐methylaniline in dimethyl sulphoxide are not base catalysed. When the nucleophile is 2‐methylaniline, the reaction of the fluoro compound is base catalysed whereas that of the chloro compound is not. The reactions of 1‐bromo‐2‐fluoro‐ and 1‐bromo‐2‐chloro‐3,5‐dinitrobenzenes with both aniline and 2‐methylaniline are not base catalysed. These results indicate that, in certain cases, the incidence or absence of base catalysis in aromatic nucleophilic substitution (SNAr) reactions in dipolar aprotic solvents results from an interplay of steric and electronic factors as they affect the mag

ISSN:0894-3230    

DOI:10.1002/poc.610040608

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractElectrochemical and spectrophotometric studies of the pyridoxal‐5′ ‐phosphate hexylamine Schiff base (PHSB) in strongly basic media were carried out. The equilibrium constant and rate constants of the formation and hydrolysis of the unprotonated imine from the unprotonated pyridoxal‐5′ ‐phosphate and free amine were determined by linearscan cyclic voltammetry. The acid‐base dissociation constant of the imine group was obtained by spectrophotometric measurements. Absorption bands for the free imine and the conjugate acid are characterized by a log‐normal distribution. These studies are for general application to the quantitative characterization of non‐enzymati

ISSN:0894-3230    

DOI:10.1002/poc.610040609

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractAddition (Add.) vs reduction (Red.) products in the reactions of substituted benzophenones, acetophenone and fluorenone with ‘EtMgBr’ were compared in diethyl ether, 1,2‐dimethoxyethane and tetrahydrofuran. Plots of log (Add./Red.) vs σ‐values or the oxidation potentials of ketones showed characteristic upward curves depending on the polarity of the solvents. The results are discussed on the basis of both the combined effects of the stability of the monomeric radical ion pair and the dimeric radical ion pair derived from ketone and ‘EtMgBr’, and the well known Schlenk equilibrium of ‘EtMgBr

ISSN:0894-3230    

DOI:10.1002/poc.610040610

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe tendency of ion‐pair intermediates generated by treatment oftrans‐10‐bromo‐10, 11‐dihydro‐11‐hydroxy‐5H‐dibenzo [a,d] cycloheptene (5) with HBr to release Br2giving 5H‐dibenzo [a,d,] cycloheptene (1) has been evaluated. The observed 9:1 ratio between ion‐pair collapse to thetrans‐dibromide (7) and Br2release to olefin 1, as compared with the 3:7 ratio found with the 5H‐dibenz [b,f] azepine‐5‐carbonyl chloride system, has been interpreted as indicating a much lower propensity for reversibility of ion‐pair formation in the bromination of 1. The structural parameters obtained by x‐ray diffraction of the dibromide 7 show for this compound an angle strain at C‐10 and C‐11 analogous to that found intrans‐10, 11‐dibromo‐10, 11‐dihydro‐5H‐dibenz [b,f,] azepine‐5‐carbonyl chloride. Olefin 1 adds Br2in 1,2‐dichloroethane at 25°C according to a third‐order rate law, withk3= 30(3) M−2s−1. About 10% of 5‐bromo‐5H,‐dibenzo [a,d] cycloheptene, arising from collapse of a dibenzo [a,d] tropylium ion intermediate (9), is formed in this reaction besides the expectedtrans‐dibromide 7. Spectral evidence for the formation of 9, arising by rearrangement of a first formed weakly bridged ionic intermediate in the reaction of 1 with Br2, is presented. the results obtained with the 5H‐dibenzo [a,d,] cycloheptene system have been rationalized on the basis of the formation of very weakly bridged intermediates having a much lower tendency to revert back to olefin and B

ISSN:0894-3230    

DOI:10.1002/poc.610040611

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY