摘要:

AbstractA new couloamperometric apparatus has been designed to extend the range of this kinetic technique to the measurement of very high rate constants, 108M−1s−1, by using TFCR‐EXSEL conditions (TFCR—very low reactant concentration; EXSEL—salt excess), which give half‐lives of a few seconds for very fast second‐order reactions. Very low faradaic currents, in the nanoampere range for halogens, corresponding to very low reactant concentrations of 10−8–10−9M, are measured selectively by compensating the eddy currents, principally the residual and the induced currents. When the electroactive species is bromine, the concentration is demonstrated to be linearly related to the limiting reduction current in the very low concentration range. The upper limit of this technique for bromination is at present 3 × 108M−1s−1. The method is applied to the kinetic study of highly reactive enol ethers EtO‐C(R) = CH‐R′, where R and R′ are H or Me. A value of 2.2 × 108M−1s−1is obtained fork Br 2, the rate constant for free bromine addition to EtO‐CH = CH2, by extrapolating the kinetic bromide ion effects to [Br−] = 0. An α‐methyl effect (kα‐Me/kH)EtOof 15 is found; this is a small decrease in the methyl effect compared to the marked increase in the double bond reactivity. For the enol acetate MeCOO‐CH = CH2, whose rate constant is 6 × 102M−1s−1, (kα‐Me/kH)OCOMeis 21. The dependence of substituent effects on reactivity is discussed in terms of the Hammond effect on the transition state position

ISSN:0538-8066    

DOI:10.1002/kin.550150902

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe kinetics of electron transfer in the redox system containing phenylhydrazine (S) and tris(dimethylglyoximato)nickelate(IV), in the presence of catalytic amounts of added Cu(II)aq, have been studied in aqueous medium at an ionic strength of 0.25Min the pH range of 6.01–9.06. The kinetics exhibit pseudo‐zero‐order disappearance of Ni(IV) when an excess of [S]0and small amounts of Cu(II) are present. While the pseudo‐zero‐order rate constants are almost linearly dependent on [S]0at constant [Cu(II)] and pH tending to become non‐linearly dependent on higher relative [S]0, they are linearly dependent on [Cu(II)] in a 20‐fold range. The pH‐rate profiles with low [S]0and [Cu(II)] show a monotonic decrease in rates with increasing pH, the rates tending to attain limiting values at higher relative pH. Results are interpreted in terms of a probable mechanism involving the formation of precursor complexes of phenylhydrazine and Cu(II) species in the medium, followed by the rate‐determining breakdown of the precursors with concomitant electron transfer. The hydrolyzed species of Cu(II) reacts more slowly than does the aquacopper(II). Ni(IV) does not appear to have any kinetic role in the redox system and is involved only in rapid product formation steps. The oxidation product of phenylhydrazine is 4

ISSN:0538-8066    

DOI:10.1002/kin.550150903

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe kinetics of the oxidation of dimethylsulfoxide by oxohydroxoosmate(VIII) complex ions in alkaline media follow pseudo‐first‐order disappearance in Os(VIII). The values of the observed pseudo‐first‐order rate constant are linearly dependent on initial dimethylsulfoxide concentrations in a fortyfold range, and increase with increasing [OH−], leveling off at higher relative [OH−]. The results are interpreted in terms of outer sphere interactions involving dimethylsulfoxide and various species of the Os(VIII) complex. The more nucleophilic dihydroxotetraoxoosmate(VIII) ion reacts about 50 times faster than the trihydroxotrioxoosmate(VI

ISSN:0538-8066    

DOI:10.1002/kin.550150904

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe title reaction was studied in a standard flow system with F atoms produced by RF discharge in F2‐He mixture. Analysis was by gas chromatography using electron capture detection. There were two major products, identified as CF2BrCF2H and CF2BrCF2Br, plus presumably HF which was not detectable. The overall rate of disappearance of reactant was found to be of mixed one and one‐half order, indicating a complex reaction. A mechanism is proposed comprising six steps and involving two radical species CF2BrċFBr (R1) and CF2BrċF2. The 300 K rate constant for the initial step F + reactant → HF + R1is evaluated to be 2.2 × 10−13cm3/molec·s, which fits in with rates of other saturated hydrocarbon reactants containing one hydrogen atom, thus supporting the view that in this class of reactants the rates of reactions of the type F + saturated hydrocarbon depend mainly on the number of hydrogen atoms in t

ISSN:0538-8066    

DOI:10.1002/kin.550150905

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe photolysis of azocyclopentane in the presence of cyclopentane–carbon tetrachloride mixtures has been investigated in the gas phase. Product analysis data have been used to determine the Arrhenius parameters for the reactions\documentclass{article}\pagestyle{empty}\begin{document}$$\begin{array}{*{20}c} {(4)_C - {\rm C}_5 {\rm H}_{9.} + {\rm CCl}_4 \to _C - {\rm C}_5 {\rm H}_9 + {\rm CCl}_{3.} } \hfill & {k_4 = 10^{9.0 \pm 0.6} {\rm exp}[- (10.3 \pm 1.0){\rm kcal}/{\rm mol}/{\rm RT}]} \hfill \\ {(6){\rm CCl}_{3.} + _C - {\rm C}_5 {\rm H}_{10} \to {\rm CCl}_3 {\rm H} + _C - {\rm C}_5 {\rm H}_{9.} } \hfill & {k_6 = 10^{8.4 \pm 0.4} {\rm exp}[- (10.0 \pm 0.7){\rm kcal}/{\rm mol}/{\rm RT}]} \hfill \\ \end{array}$$\end{document}The rate data for chlorine atom abstraction from CCl4by the cyclopentyl radical were compared with available data for other alkyl radicals in both the gas and the solution phases. The results indicate that the rate constant for chlorine atom abstraction in the gas phase is fairly insensitive to the nature of the attacking alkyl radical and that the activation energy for a secondary radical is about 4 kcal/mol higher than the corresponding reaction in the solution phas

ISSN:0538-8066    

DOI:10.1002/kin.550150906

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe rate coefficient of the reaction\documentclass{article}\pagestyle{empty}\begin{document}$$(2){\rm H}_2 {\rm CN} \to {\rm H} + {\rm HCN}$$\end{document}has been determined in the temperature range of 2700–3500 K using a shock tube technique. C2N2H2Ar mixtures were heated behind incident shock waves and the early‐time CN history was monitored using broad‐band absorption spectroscopy. The rate coefficient providing the best fit to the data was\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm k = (7}{\rm .5}_{ - 2.0}^{{\rm + 2}{\rm .5}} {\rm)} \times {\rm 10}^{{\rm 13}} {\rm cm}^3 /{\rm mol} \cdot {\rm s} $$\end{document}in good agreement with extrapolations of previously published low‐temperat

ISSN:0538-8066    

DOI:10.1002/kin.550150907

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe Ce(IV) oxidation of the five‐, six‐, and seven‐membered ring α‐hydroxycycloalkanecarboxylic (C5, C6, and C7) acids to the corresponding cyclic ketones has been studied in acidic perchlorate media. The data may be interpreted in terms of a mechanism which involves fast preequilibrium complexation steps between Ce(IV) and the hydroxy acids, yielding two complexes which differ only by a proton. Complexation is followed by rate‐determining decarboxylation to an intermediate (free radical?), which reacts quickly with another Ce(IV) to give products. Of the two proposed complexes, the protonated one is virtually unreactive. The C7 ring acid is oxidized more rapidly than the C6 acid, which, in turn, is oxidized faster than the C5 acid.For comparison, the oxidation of the five‐, six‐, and seven‐membered ring cyclic alcohols to the corresponding cyclic ketones by Ce(IV) in acidic perchlorate was also studied. The order of reactivity is cyclopentanol>cycloheptanol>cyclohexanol. The differences in observed reactivities between the hydroxy acids and the cyclic alcohols are explained in terms of differences in transition state structure.The stepwise hydrolysis constants of Ce(IV) leading to Ce(OH)3Plus;and Ce(OH)22+were determined. In the case of the hydroxy acids, evidence is in favor of Ce(OH)3+as the reactive ceric species in aqueous acidic p

ISSN:0538-8066    

DOI:10.1002/kin.550150908

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

ISSN:0538-8066    

DOI:10.1002/kin.550150909

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

ISSN:0538-8066    

DOI:10.1002/kin.550150901

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY