摘要:

AbstractThe acid‐catalysed rate constants (kH+) for the photohydration of 18meta‐ andpara‐substituted styrenes and phenylacetylenes were determined in H2SO4solutions by means of fluorescence quenching measurements and the standard Stern–Volmer approach. Singlet lifetimes of the substrates were determined in the reference solution (water at pH 7) by single photon counting. The logk H+values obtained did not correlate well with any previously reported σ scales, and were used to try to establish a σhνscale (for H‐,p‐F,m‐F,p‐Me,m‐Me,p‐Et,m‐Et,p‐OMe andm‐OMe substituents) for possible use in other photochemical reactions. Correlation of all available literature data versus the new σhvscale gave reasonable correlations (r= 0.957–0.966). The problems associated with establishing linear free energy relationships

ISSN:0894-3230    

DOI:10.1002/poc.610040402

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe rates of the reaction ofN‐alkyl‐4‐cyanopyridinium (RCP) ions [alkyl = CH3(MCP),n‐C4H9(BCP),n‐C8H17(OCP),n‐C12H25(DCP)] with OH−ion in water are independent of chain length and RCP concentration up to 0.001 M. The ratios of the concentrations of the reaction products,N‐alkyl‐4‐pyridone (P) andN‐alkyl‐4‐carboxamidopyridinium (A),P/A, are similar for all substrates. In water theP/Aratios increase with pH, reaching a plateau value ofca2at pH ≥ 13. Added salts slightly decrease the reaction rate and do not affect theP/Aratios. In aqueous solutions, addition of dioxane increases both rates andP/Aratios. Micelles of hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide; CTAB) and 3‐(N,N‐dimethyl dodecyl ammonium)propane‐1‐sulphonate (SDP) catalyse the alkaline hydrolysis of OCP and DCP and increase the yield of pyridone with BCP, OCP and DCP. In micellar CTAB and SDP the attack of OH−on RCP occurs almost exclusively at the 4‐position of the pyridinium ring. Sodium dodecyl sulphate micelles inhibit the reaction and produce small changes in theP/Aratio. The regiochemical selectivity produced by micelles, leading to a preference for the attack of OH−at the pyridinium ring, was rationalized in terms of the low effective dielectri

ISSN:0894-3230    

DOI:10.1002/poc.610040403

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractQuinolinium dichromate (QDC) oxidizes hydrocarbons (toluenes and phenanthrenes) smoothly in dimethylformamide in the presence of acid. The rate of the reaction was first order in each substrate, oxidant and acid. The effects of variations in solvent composition and temperature were studied. The rate data obeyed Hammett's relationship and the values of ρ were −0.20 for toluenes and −1.79 for phenanthrenes. Induced polymerization of acrylonitrile and the reduction of mercury(II) chloride were not observed. For the oxidation of toluenes, a kinetic isotope effect,kH/kD= 5.13, was observed. The initial reaction for the oxidation of toluenes involved hydrogen abstraction, forming an intermediate which was rapidly converted to the product, the corresponding aldehyde. For the oxidation of phenanthrene, the experimental data have been rationalized in terms of a hydride ion transfer in the ratedetermining

ISSN:0894-3230    

DOI:10.1002/poc.610040404

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractEquilibrium displacements within the enol–enthiolic tautomeric systems of β‐thioxoketones caused by long‐range isotope effects were monitored by1H and13C NMR spectroscopy. Thioacetylacetone and 1,3‐diphenyl–3‐thioxopropanone deuteriated at various positions were investigated. Both compounds are in fast (Z)‐enol‐(Z)‐enethiol equilibrium on the NMR time scale. The investigations showed that CD3and C6D5groups changes the equilibrium so that the tautomer having a C  X bond next to the CD3or C6D5groups decreases. Deuteriation at the methine position pushes the equilibrium towards the (Z)‐enol form. The study further showed that deuteriation at the phenyl ring next to the C  O group causes the largest effect if the deuterium is at theorthoposition and the smallest if it is at theparaposition. Long‐range effects on equilibrium are discussed in general in order to establish a common pattern. Deuterium scrambling is observed both during the coupling reaction of ethyl thionoacetate with hexadeuterioacetone and during preparation of specifically labelled acetophenone. The latter compounds are synthesized by acetylation of specifically labelled benzenes. The scrambling reaction is suggested to be a transdeuteriation of the deuteriated benzenes rather than i

ISSN:0894-3230    

DOI:10.1002/poc.610040405

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractDeuterium‐ or tritium‐substituted aromatics undergo isotope exchange in fairly concentrated aqueous sulfuric acid media. The rate constant for this process is complex, being composed of that for the slow electrophilic attack, and the isotope effect on the breakup of the Wheland intermediate. Using experimental rate constant data for both deuterium and tritium exchange on the same substrate, the excess acidity method and the Swain–Schaad relationship, true protonation rate constants and isotope effects have been separated out for several positions in several simple aromatic substrates. Most of the electrophilic attack rate constants are described by a Hammett‐type linear free energy relationship with a ρ+of − 6.5. The average observed deuterium isotope effect on Wheland intermediate breakup was 5.3. Using rate constants obtained at different temperatures, enthalpies and entropies of activation were also obtained for benzene, naphthalene and thiophene; the activation entropy averaged − 8 cal mol−1K−1. Some evidence was found for general acid catalysis at the

ISSN:0894-3230    

DOI:10.1002/poc.610040406

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe structures of a series of heterosubstituted methylenecyclopropenes and methylenecyclopropanes were optimized at the HF/6–31G* level. All methylenecyclopropenes are planar except for the silicon analogue, which is bent at both C‐3 and Si. The planar silicon structure is a transition state. The relative aromaticity of these compounds were evaluated using the C‐1C‐3 bond length, the integrated charge on C‐3 and the heteroatom, and delocalization energy. Second‐row systems have slightly larger delocalization energies than their first‐row counterparts, owing to their larger polarizability. Using these criteria, methylenecyclopropene and the silicon analogue are not aromatic and the N, O, P and S analogues are moderately aromatic. The planar silicon analogue is antiaromatic and bends from planarity to reduce this

ISSN:0894-3230    

DOI:10.1002/poc.610040407

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe alcoholyses of ethyl fluorosulphate in methyl, ethyl,n‐propyl, isopropyl,n‐butyl, isobutyl,tert‐butyl andn‐pentyl alcohols in the initial concentrations and temperature ranges of (1–10) 10−3M and − 15.00 to 22.00 °C, respectively, were studied. The reactions follow pseudo‐first‐order kinetic laws up to at leastca75 per cent ester conversions. At 10°C the experimental rate constant for ethanolysis is similar to that of ethyl triflate but much smaller than for its hydrolysis. The reactivity of the ester isca104–105times higher than that of alkyl halides and benzenesulphonates. In alcohol–benzene and alcohol–acetone mixtures the ΔH≠and ΔS≠activation parameters are only slightly affected by the media composition. The effect of the ‘lyate ion’ on the kinetics of these reactions were investigated and the effects of changes in the dielectric constant of the media were evaluated. It is concluded that the alcoholyses of the ester fulfil the requirements for predominantSN2‐type mechanism, where the reaction ground states are more polar tha

ISSN:0894-3230    

DOI:10.1002/poc.610040408

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractOxygen‐17 NMR spectra of someN‐nitrosamines are discussed in terms of steric and conjugative effects. Relationships have been observed between rotational barriers, conformations and17O chemical shifts. A tentative linear correlation has been determined between repulsive Van der Waals interactions and someN‐nitrosopiperi

ISSN:0894-3230    

DOI:10.1002/poc.610040409

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610040401

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY