Dissociation first-order

Cr(Hedta)(0H2)] is the main product of the reaction between [Ca(edta)] and [Cr(H20)6]. The reaction was studied at pH 3.5 and 297.2-318.2 K in the presence of excess [Ca(edta)] ". Under these conditions, below pH 4.1 a dissociative, first-order process was observed, with some evidence for an associative pathway above pH 4.1. ... 

Second-order effects include experiments designed to clock chemical reactions, pioneered by Zewail and coworkers [25]. The experiments are shown schematically in figure Al.6.10. An initial 100-150 fs pulse moves population from the bound ground state to the dissociative first excited state in ICN. A second pulse, time delayed from the first then moves population from the first excited state to the second excited state, which is also dissociative. By noting the frequency of light absorbed from tlie second pulse, Zewail can estimate the distance between the two excited-state surfaces and thus infer the motion of the initially prepared wavepacket on the first excited state (figure Al.6.10 ). 

With M = He, experimeuts were carried out between 255 K aud 273 K with a few millibar NO2 at total pressures between 300 mbar aud 200 bar. Temperature jumps on the order of 1 K were effected by pulsed irradiation (< 1 pS) with a CO2 laser at 9.2- 9.6pm aud with SiF or perfluorocyclobutaue as primary IR absorbers (< 1 mbar). Under these conditions, the dissociation of N2O4 occurs within the irradiated volume on a time scale of a few hundred microseconds. NO2 aud N2O4 were monitored simultaneously by recording the time-dependent UV absorption signal at 420 run aud 253 run, respectively. The recombination rate constant can be obtained from the effective first-order relaxation time, A derivation analogous to (equation (B2.5.9). equation (B2.5.10). equation (B2.5.11) and equation (B2.5.12)) yield... 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

Retardation of the reaction rate by the addition of dimethyl sulfide is in accord with this mechanism. Borane—amine complexes and the dibromoborane—dimethyl sulfide complex react similarly (43). Dimeric diaLkylboranes initially dissociate (at rate to the monomers subsequentiy reacting with an olefin at rate (44). For highly reactive olefins > k - (recombination) and the reaction is first-order in the dimer. For slowly reacting olefins k - > and the reaction shows 0.5 order in the dimer. 

A well-known example of non-prototropic tautomerism is that of azolides (acylotropy). The acyl group migrates between the different heteroatoms and the most stable isomer (annular or functional) is obtained after equilibration. In indazoles both isomers are formed, but 2-acyl derivatives readily isomerize to the 1-substituted isomer. The first order kinetics of this isomerization have been studied by NMR spectroscopy (74TL4421). The same publication described an experiment (Scheme 8) that demonstrated the intermolecular character of the process, which has been called a dissociation-recombination process. 

In the gas phase, N2O5 decomposes according to a first-order rate law which can be explained by a dissociative equilibrium followed by rapid reaction according to the scheme... 

Much evidence has been obtained in support of the El mechanism. For example, El reactions show first-order kinetics, consistent with a rate-limiting spontaneous dissociation process, l- urthermore, El reactions show- no deuterium isotope effect because rupture of the C—H (or C—D) bond occurs after the rate-limiting step rather than during it. Thus, we can t measure a rate difference between a deuterated and nondeuterated substrate. 

The S il reaction occurs when the substrate spontaneously dissociates to a carbocation in a slow rate-limiting step, followed by a rapid reaction with the nucleophile. As a result, SN1 reactions are kinetically first-order and take place with racemization of configuration at the carbon atom. They are most favored for tertiary substrates. Both S l and S 2 reactions occur in biological pathways, although the leaving group is typically a diphosphate ion rather than a halide. 

In the El reaction, C-X bond-breaking occurs first. The substrate dissociates to yield a carbocation in the slow rate-limiting step before losing H+ from an adjacent carbon in a second step. The reaction shows first-order kinetics and no deuterium isotope effect and occurs when a tertiary substrate reacts in polar, nonbasic solution. 

The best understood compounds are cis- and fra s-RuX2(DMSO)4 (X = Cl, Br). The fra s-isomers are thermodynamically less stable and isomerize in DMSO solution to the m-isomer, with first-order kinetics, probably via a dissociative mechanism. The reverse process, cis to trans, is catalysed by light. Syntheses for these and other DMSO complexes are shown in Figure 1.37 [108],... 

The first step was found to be a fast pre-equilibrium (Scheme 12-8). The dependence of the measured azo coupling rate constants on the acidity function and the effect of electron-withdrawing substituents in the benzenediazo methyl ether resulting in reduced rate constants are consistent with a mechanism in which the slow step is a first-order dissociation of the protonated diazo ether to give the diazonium ion (Scheme 12-9). The azo coupling proper (Scheme 12-10) is faster than the dissociation, since the overall rate constant is found to be independent of the naphthol con-... 

The only really different case is the azo coupling reaction of nitroethane investigated by Sterba and coworkers (Machacek et al., 1968a, 1968b). With the 4-nitrobenzenediazonium ion the reaction is zero-order with respect to diazonium ion and first-order in both nitroethane and base. Obviously the rate-limiting step is the dissociation of nitroethane the formation of the anion is slower than its subsequent reaction with this diazonium ion. For reactions with diazonium ions of lower reactivity it was found necessary to use the reaction system of Scheme 12-64 with the nitroethane anion as steady state intermediate (Machacek et al., 1968b). 

Kinetic studies have been carried out using the 1 1-complex iodobenzene dichloride as a source of molecular chlorine. In acetic acid solutions, the dissociation of this complex is slower than the rate of halogenation of reactive aromatics such as mesitylene or pentamethylbenzene, consequently the rate of chlorination of these is independent of the aromatic concentration. Thus at 25.2 °C first-order chlorination rate coefficients were obtained, being approximately 0.2 x 10-3 whilst the first-order dissociation rate coefficient was 0.16 xlO-3 from measurements at 25.2 and 45.6 °C the corresponding activation energies... 

This peculiar form applies when a dimeric molecule dissociates to a reactive monomer that then undergoes a first-order or pseudo-first-order reaction. This scheme is considered in Section 4.3. Unless one can work at either of the limits, the form is such that a numerical solution or the method of initial rates will be needed, since the integrated equation has no solution for [A]r. 

Ethane, C2H , dissociates into methyl radicals by a first-order reaction at elevated temperatures. If 250. mg of ethane is confined to a 500.-mL reaction vessel and heated to 700°C, what is the initial rate of ethane decomposition if k = 5.5 X 10 4 s-1 in the rate law (for the rate of dissociation of C2H6) ... 

The exceptionally low propagation constants of t-butyl and of phenyl methacrylate are notable. The polymerization of the former monomer was thoroughly examinedS5). At temperatures even as high as 25 °C this reaction, when performed in THF in the presence of salts depressing dissociation of ion-pairs, yields polymers of highly uniform size. The reaction is strictly first order in growing polymers and in monomer, and no... 

For each of these reactions kinetic data were obtained. The reactions were first order in complex concentration, and zero order in isocyanide, as expected. The complex Ni(CNBu )4, and presumably other Ni(CNR)4 complexes as well, undergo ligand dissociation in solution. In benzene solution, a molecular weight determination for this compound gives a low value (110). This is in accord with the presumed mechanism of substitution. 

The inactivation is normally a first-order process, provided that the inhibitor is in large excess over the enzyme and is not depleted by spontaneous or enzyme-catalyzed side-reactions. The observed rate-constant for loss of activity in the presence of inhibitor at concentration [I] follows Michaelis-Menten kinetics and is given by kj(obs) = ki(max) [I]/(Ki + [1]), where Kj is the dissociation constant of an initially formed, non-covalent, enzyme-inhibitor complex which is converted into the covalent reaction product with the rate constant kj(max). For rapidly reacting inhibitors, it may not be possible to work at inhibitor concentrations near Kj. In this case, only the second-order rate-constant kj(max)/Kj can be obtained from the experiment. Evidence for a reaction of the inhibitor at the active site can be obtained from protection experiments with substrate [S] or a reversible, competitive inhibitor [I(rev)]. In the presence of these compounds, the inactivation rate Kj(obs) should be diminished by an increase of Kj by the factor (1 + [S]/K, ) or (1 + [I(rev)]/I (rev)). From the dependence of kj(obs) on the inhibitor concentration [I] in the presence of a protecting agent, it may sometimes be possible to determine Kj for inhibitors that react too rapidly in the accessible range of concentration. ... 

Let us take the following first-order reaction (obviously an isomerization, although a dissociation of R in two products, Pj and P2 would yield similar expressions)... 

While first-order kinetics are observed with most diatomic gases that adsorb in molecular form, dissociative adsorption of gases such as H2 and N2 follows second order kinetics. In the limit of the empty surface the rate of adsorption is... 

The rapid first-order exchange rate ° for Ni(CO)4 is to be contrasted with the much slower rates for the carbonylate anions Co(CO)4 and Fe(CO)4 , and this is quite in accordance with a higher M-C bond order in these two latter compounds, provided that the primary reaction step is a dissociative one. The group VI hexacarbonyls exchange CO several orders of magnitude more slowly than Ni(C0)4 by a first-order process in the gas phase There is very good... 

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