Perchlorate concentration, effect rate constant

For nitrations in sulphuric and perchloric acids an increase in the reactivity of the aromatic compound being nitrated beyond the level of about 38 times the reactivity of benzene cannot be detected. At this level, and with compounds which might be expected to surpass it, a roughly constant value of the second-order rate constant is found (table 2.6) because aromatic molecules and nitronium ions are reacting upon encounter. The encounter rate is measurable, and recognisable, because the concentration of the effective electrophile is so small. 

With a view to determining the equilibrium constant for the isomerisation, the rates of reduction of an equilibrium mixture of cis- and rra/i5-Co(NH3)4(OH2)N3 with Fe have been measured by Haim S . At Fe concentrations above 1.5 X 10 M the reaction with Fe is too rapid for equilibrium to be established between cis and trans isomers, and two rates are observed. For Fe concentrations below 1 X lO M, however, equilibrium between cis and trans forms is maintained and only one rate is observed. Detailed analysis of the rate data yields the individual rate coefficients for the reduction of the trans and cis isomers by Fe (24 l.mole sec and 0.355 l.mole .sec ) as well as the rate coefficient and equilibrium constant for the cw to trans isomerisation (1.42 x 10 sec and 0.22, respectively). All these results apply at perchlorate concentrations of 0.50 M and at 25 °C. Rate coefficients for the reduction of various azidoammine-cobalt(lll) complexes are collected in Table 12. Haim discusses the implications of these results on the basis that all these systems make use of azide bridges. The effect of substitution in Co(III) by a non-bridging ligand is remarkable in terms of reactivity towards Fe . The order of reactivity, trans-Co(NH3)4(OH2)N3 + > rra/is-Co(NH3)4(N3)2" > Co(NH3)sN3 +, is at va-... 

The difference between A obsd and caic might be due to a specific salt effect on the rate constant for solvolysis. However, this is unlikely because perchlorate ion acts to stabilize carbocations relative to neutral substrates.At high concentrations of sodium bromide, the rate-limiting step for solvolysis of 1-Br is the capture of 1 by solvent (ks Scheme 5A). Substitution of Br for CIO4 should destabilize the carbocation-like transition state for this step relative to the starting neutral substrate, and this would lead to a negative, rather than positive deviation of obsd for equations (3A) and (3B). 

The electrochemical rate constants of the Zn(II)/Zn(Hg) system obtained in propylene carbonate (PC), acetonitrile (AN), and HMPA with different concentrations of tetraethylammonium perchlorate (TEAP) decreased with increasing concentration of the electrolyte and were always lower in AN than in PC solution [72]. The mechanism of Zn(II) electroreduction was proposed in PC and AN the electroreduction process proceeds in one step. In HMPA, the Zn(II) electroreduction on the mercury electrode is very slow and proceeds according to the mechanism in which a chemical reaction was followed by charge transfer in two steps (CEE). The linear dependence of logarithm of heterogeneous standard rate constant on solvent DN was observed only for values corrected for the double-layer effect. 

The condensation reactions of silanols are catalyzed by acids [19, 25-27,63—68, 72], Grubb measured the hydrogen chloride catalyzed silanoi condensation reaction of trimethylsilanol in methanol [19]. Lasocki and Michalska studied the effect of acid type on the condensation of dialkylsilanediols in dioxane [68]. Under anhydrous conditions, the rate of acid catalysis by strong acids (such as hydrogen bromide and perchloric acid) was directly related to the acid concentrations. The catalytic effects of weaker acids, such as hydrogen chloride, were not linearly related to the concentration. They postulated that in anhydrous dioxane, the strong acids were completely ionized while the weaker acids were not [68]. When small amounts of water were added to the solvent, all the acids behaved in a similar manner. Lasocki [64-67] extended the studies to examine the effects of alkyl or aryl substitution of silanediols on the condensation rate in aqueous dioxane [64-67]. The rate constants for acid catalyzed condensation of... 

Rate constants for the hydrolysis of 2-exo-norbomyl phenyl ether and several metaox /> ra-substituted derivatives have been measured in concentrated perchloric acid solutions.17 The effect of substituents on the rate constant and activation parameters is small. The mechanism appears to be of the A-1 type. 

Rate constants for deuterium or tritium exchange of benzene, naphthalene, and other aromatic molecules have been measured in concentrated solutions of sulfuric or perchloric acid. Conveniently, Cox has extrapolated values to aqueous solution from plots of log/cx against Xa and corrected them for isotope effects (e.g., k /kp) to yield kA.72... 

TimelP investigated the effect of the acid used. Rate constants were determined for hydrochloric, perchloric, sulfuric, and phosphoric acids at concentrations ranging from 0.5 M to 9 M. Log k was found to vary linearly with Ho, as required by the Zucker-Hammett hypothesis, but a slope of unity is only approximately given by the first three acids and not at all for phosphoric acid and, instead of coinciding, the plots are displaced with respect to each other. These observations led TimeU to conclude that data obtained with one of these four acids cannot accordingly be recalculated to be valid for another acid by referring to the same value for Ho, as has often been done in the past. ... 

Special consideration should be given to the results in perchloric acid solutions. These were chosen because C104" anions do not chemically interact with molecular and radical products of water radiolysis (2). It excluded accessory effects and let us investigate reaction kinetics with the H30+ ion. The results which were obtained were unexpected. As shown in Table I, the value of observed rate constant is constant and independent of hydronium ion concentration over two orders of magnitude. 

The effect of salt concentration (sodium perchlorate) on the decay rate of Cl2" transient in the presence of 10"3M Fe2+ was examined. A marked salt effect was observed, the pseudo first order rate constant being reduced from 3.8 X 104 sec."1 at ionic strength 0.04, to 2.5 X 104 sec."1 at ionic strength 0.25. Reaction 3 would be expected to show a negative salt effect, but the observed decrease in rate constant with increasing ionic strength is somewhat less than expected. 

The reaction is strongly accelerated by the added chloride salt. An equivalent concentration of a perchlorate salt has no effect on the form of the rate expression, and only a minor effect on the observed rate constants. Assuming that the acylation proceeds through a tetrahedral intermediate, indicate how the observed catalytic and kinetic effect of added chloride ion might arise. 

There is little effect of micelles upon the rate of an intramolecular nucleophilic reaction. Micelles of hexadecyltrimethylammonium bromide catalyse, by factors of 10 —10, the arenesulphinate anion-induced hydrolysis of 4-tolylsulphonyl-methyl perchlorate. There is no relationship between the rate acceleration and hydro-phobicity of the sulphinate anion and catalysis is attributed to the concentration of the reactants in the micellar phase.The rate constants for the reaction of nucleophiles with carbonium ions and those for the addition of cyanide ion to the A -alkylpyridinium ions are similar in the micellar and aqueous phases, and the rate enhancement is due to the concentration of reactants in the micellar pseudophase. Similarly, although micellar catalysed dephosphorylation by nucleophiles may show rate enhancements of up to 4 x 10 -fold, the second-order rate constants may be slightly smaller in the micellar pseudophase lowing to its lower polarity. However, the reaction of fluoride ion with 4-nitrophenyldiphenyl phosphate is very rapid in micelles of cetyltrimethylammonium fluoride, but the rate constant continues to increase when the substrate is fully bound with increasing cetyltrimethylammonium fluoride or sodium fluoride. The failure of the micellar pseudophase model is also apparent in the reaction of hydroxide ion with 2,4-dinitrochlorobenzene. It is suggested that reaction occurs between reactants in the aqueous and micellar pseudophases and also between hydroxide ion in water and substrate in the micelle. ... 

FIGURE 1.32 (a) Schematic diagram of the application of SECM in the feedback mode measurement of the kinetics of ET between ZnPor in benzene and RufCN) " in water. Electroneutrality maintained by distribution of perchlorate ions across the interface. (b) Dependence of the effective heterogeneous rate constant on potential drop across the ITIES at 5 mM concentrations of RufCN) ". The value A ( ) is expressed in terms of log[ClCf ]. Potential dependence of an effective bimolecular rate constant k) k f/[Ru(CN)g ] (M cm-s" ). (Tsionsky, M., A. J. Bard, and M. V. Mirkin, 1996, J Phys Chem-US, Vol. 100, p. 17881. Used with permission.)... 

Co(NH3)5C1] + has been the subject of controversy for some years. The effects of added chloride on aquation rates of this complex were first studied up to a concentration of 0.1 M sodium chloride, later up to 0.6M sodium chloride. The results of the latter study, in which sodium perchlorate was used to maintain constant ionic strength, were subsequently reinterpreted in terms of three parallel /d reaction pathways, involving chloride and perchlorate ion pairs. Now the effect of added chloride on the aquation rate of the [Co(NH3)5C1] + cation has been investigated at chloride-ion concentrations up to 0.9M. The conclusion reached from this latest and most extensive study is that neither [Co(NH3)5Cl] + Cl nor [Co(NH3)5Cl] + ClOr ion pairs play a kinetically significant role in the aquation of the [Co(NH3)5Cl] + cation under experimental conditions so far used. Moreover the aquation rate constant for this complex is unaffected by the nature of the added anion, be it perchlorate, tetrafluoroborate, or trifluoromethylsulphonate, when the respective sodium salts are used to maintain the ionic strength of the reaction medium (/ = l.OM). ... 

In a further study, Brubaker et have reported on the effects of the addition of chloride ion to the sulphate exchange system at virtually constant ionic strength (3.68 M sulphate and hydrogen-ion concentrations. For the concentration ratio [C1 ]/[T1(III)] of 9.2x10" to 9.5 at 24.9 °C results analogous to the effect observed in perchlorate media were obtained. The minimum in the rate corresponded to a ratio of 2.5. Results were also presented for the conditions, constant [CI ] and variable [804 ] and [If"] ( = 3.68 M). Brubaker et al have suggested that the exchange paths most likely to occur in sulphate media are... 

The effect of chloride ions was investigated first by Silverman and Dodson. These authors observed an increase in as the concentration of chloride ion was increased from 0 to 0.55 M in perchlorate media of constant acidity. The rate expression found to fit the experimental data was... 

The reaction between Fe(IlI) and Sn(Il) in dilute perchloric acid in the presence of chloride ions is first-order in Fe(lll) concentration . The order is maintained when bromide or iodide is present. The kinetic data seem to point to a fourth-order dependence on chloride ion. A minimum of three Cl ions in the activated complex seems necessary for the reaction to proceed at a measurable rate. Bromide and iodide show third-order dependences. The reaction is retarded by Sn(II) (first-order dependence) due to removal of halide ions from solution by complex formation. Estimates are given for the formation constants of the monochloro and monobromo Sn(II) complexes. In terms of catalytic power 1 > Br > Cl and this is also the order of decreasing ease of oxidation of the halide ion by Fe(IlI). However, the state of complexing of Sn(ll)and Fe(III)is given by Cl > Br > I". Apparently, electrostatic effects are not effective in deciding the rate. For the case of chloride ions, the chief activated complex is likely to have the composition (FeSnC ). The kinetic data cannot resolve the way in which the Cl ions are distributed between Fe(IlI) and Sn(ll). 

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