摘要:

AbstractRates of hydrolysis catalyzed by dilute perchloric acid were measured for the vinyl ethers MeOC(SiMe3)CH2, EtOC(SiMe3)CH2,cis‐EtOCHCHSiMe3, MeOC(t‐Bu)CH2and EtOC(t‐Bu)CH2in wholly aqueous solution and for MeOCHCH2, MeOC(SiMe3)CH2and MeOC(t‐Bu)CH2in ethanol—water (4:1), in order to assess the effects of silyl substitution on the stability of the carbocations formed in the rate‐determining step of these reactions. The results for α‐substitution give the reactivity order H

ISSN:0894-3230    

DOI:10.1002/poc.610041002

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractA new approximate model of the solvatochromism of merocyanines is presented. In solution merocyanines exist in the electronic ground state as a mixture of three different structures. Since only one longest wavelength band is observed in the UV—visible absorption spectra of such merocyanines, their lowest excited states are roughly assumed to be represented by a single structure. The molar fraction of each of the three structures of merocyanines in the ground state is a function of the solvent polarity. This (VBHB) model has been verified using merocyanines prepared from 5‐(hydroxyaryl)‐7,7‐dimethyl‐7H‐indolo [1,2‐a] quinolinium perchlorates. The analysis of the experimental data suggests that the three ground‐state structures of merocyanines are the vinylogous amide (V) and the betaine as the free (B) or hydrogen

ISSN:0894-3230    

DOI:10.1002/poc.610041003

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractA kinetic study is reported for the ene reaction between methyl acrylate as the enophile and β‐pinene, with aluminium chloride as catalyst, in a series of solvents varying in polarity. These experiments point to a transition state having pronounced zwitterionic character, in line with earlier suggestions in the literature. An unexpected and significant change of the rate constant with the initial concentrations is also fou

ISSN:0894-3230    

DOI:10.1002/poc.610041004

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractA ‘cone angle’, θR, is defined for an alkyl group (R), which is proposed as a measure of the steric effect exerted by the group. The θRvalues for some 57 groups have been calculated mathematically by constructing the corresponding Corey—Pauling—Koltun molecular models. The variation in θRwith the bulk of a group is satisfactory. However not all types of alkyl groups can be accommodated in this cone angle approach. In the case of the groups for which the cone angle can be measured, correspondences between θRand the two existing steric parameters (Taft'sEsscale and Dubois'E′sscale) have been established. For some 23 alkyl groups θRhas been used to explain the variation in the rate constant of the base‐catalysed hydrolysis of RCOOEt with the nature of R. Our studies independently substantiate the basic assumptions of the Taft—Ingold hypothesis that, while the base‐catalysed ester hydrolysis is mostly controlled by the steric and electronic effects of the R group, the acid‐catalysed one is controlled almost solel

ISSN:0894-3230    

DOI:10.1002/poc.610041005

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractCationic micelles of cetyltrimethylammonium chloride, bromide and mesylate (CTACl, CTABr, CTAOMs) speed reactions of OH−with phosphinate and thiophosphinate esters: Ph2PO.OPh (1a), Ph2PO.OC6H4NO2−p(1b), Ph(i‐Pr)PO.OC6H4NO2−p(1c), Ph2PO.SEt (2a), Ph2PO.SPh (2b), (EtO)2PO.SPh (2c) and (EtO)2PS.OC6H4NO2−p(Parathion, 3). First‐order rate constants go through maxima with increasing [surfactant]. The rate—surfactant profiles are fitted quantitatively in terms of a kinetic model that treats the distribution of OH−between aqueous and micellar pseudo‐phases in terms of coulombic and non‐coulombic ion‐micelle interactions. Second‐order rate constants at the micellar surface are lower than in water by factors that range from 0.035 for 1a to 0.7 for 1c. The thiophenyl derivative (2b) is more reactive than the corresponding phenoxy derivative (1a) and it is more reactive than the corresponding thioethyl derivative (2a). Parathion (3) is the least reactive substrate in

ISSN:0894-3230    

DOI:10.1002/poc.610041006

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractCore‐electron energies for sulphur in a series of 2‐substituted thiophenes have been used to investigate the relationship between electronic substituent parameters as developed by Taft and Topsom and by Charton. Initial‐ and final‐state energies are analysed in terms of familiar substituent effects, such as field, delocalization and polarizability. The results show that the data correlate equally well with both sets of parameters and that there are many common features between the two methods. Both agree that resonance delocalization contributes significantly to the initial state, but has virtually no effect on the final state. In connection with this investigation we have also correlated the parameters for 31 substituents that are parameterized in the two approaches. The results of this more general study indicate some real differences between these and suggest that neither of the parameterizations incorporates all the electronic

ISSN:0894-3230    

DOI:10.1002/poc.610041007

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractCarbon and deuterium kinetic isotope effects (KIEs) on the isomerization of methyl isocyanide were calculated by using the geometries and force fields obtained with MNDO, AM1 and PM3 Hamiltonians. These theoretical KIE values were compared with the corresponding experimental values. None of the hamiltonians used proved to be superior to any of the others. The best agreement for the carbon isotope effect was obtained by using the PM3 method; however, the deuterium isotope effect obtained by this method gave the poorest fit to the experimental data. The opposite is true when the MNDO Hamiltonian was used.

ISSN:0894-3230    

DOI:10.1002/poc.610041008

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractRates of oxidation of cyclohexanol by dimethyl sulphoxide in conjunction both with acetic anhydride and with dicyclohexylcarbodiimide have been measured with variation of both temperature and pressure. Activation volumes were found to be −25 and −34 cm3mol−1, respectively, and the absence of an αd PKIE was noted. The reaction mechanism is discussed in the context of thes

ISSN:0894-3230    

DOI:10.1002/poc.610041009

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

Abstract4‐Cyano‐1‐(7‐carboxyheptadecyl)pyridinium bromide (CCHP) was synthesized using standard procedures. The rates and product distribution of the reaction between OH−ion and CCHP are very similar to those previously obtained with alkyl‐substituted 4‐cyanopyridinium ions. The effect of hexadecyltrimethylammonium bromide micelles on the rate and product distribution of CCHP indicated that the cyanopyridinium ring of this compound is distal to the surface of the micelle. CCHP constitutes a probe for ionic composition in the vicinity of positively cha

ISSN:0894-3230    

DOI:10.1002/poc.610041010

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610041001

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY