Primary kinetic data

Most of the primary kinetic data that have been obtained from bench-scale model systems suggest that reaction with Fe° is first order in the concentration of solution phase contaminant, C. Thus, we can write the following rate law in differential and integrated forms ... 

Figure 10 Polymerization of 8-caprolactone (CL) initiated with Et2AI0Et (THE, 25 °C). Test of eqn [17] for the degree of aggregation m. Only for /77=3, a straight line was obtained as required. Primary kinetic data taken from Duda, A. Penczek, S. Macromol. Rapid Commun. 1994, 15, 559. ...

The observed or primary kinetic data are described as those kinetic data determined directly by experiment, and the treatment of such data with an empirical or theoretical kinetic equation involves either a linear (Equation 7.74) or a nonlinear equation (Equation 7.75), which shows the variation of dependent variable Y with independent variable X. The dependent variable may be the concentration or equivalent of any physical property such as absorbance, A bs, of either reactant or product that has been monitored as a function of an independent variable such as reaction time, t. In kinetic studies, the rate constants, under specific reaction conditions, are obtained from primary kinetic data and, therefore, such experimentally determined rate constants may be considered secondary kinetic data. Dependent variables may be experimentally determined rate constants (also called observed rate constants or secondary kinetic data), which have been determined as functions of independent variables such as the concentrations of reaction components (reactants and catalysts), temperature, and reaction medium. The reliability of the calculated kinetic parameters from a kinetic equation using primary and secondary kinetic data may be described as follows. 

Kinetic Equations Involving Primary Kinetic Data (Y vs. X)... 

Figure 1.11 Polymerization of e-caprolactone (CL) initiated with Et2AIOEt (THE, 25°C). Test of Equation 1.37b for degree of aggregation, m. Only for m = 3 was a straight line obtained as required. (The primary kinetic data were taken from Ref [156].)...

It has been noted that results of steady radiolysis experiments provide adequate data for separations related problems. The difficulty is that in the absence of kinetic data for the primary process it becomes necessary to repeat this type of experiment for each particular set of concentrations and times. 

The kinetic data fit a mechanism of successive reactions sequent to only one primary ion equally well, provided that the first step can yield 1.37 methyl radical/100 e.v. and is pressure dependent and that the succeeding pressure independent step yields methyl radicals with a lesser efficiency and leads to a pressure independent yield of 0.58 methyl radicals/100 e.v. If the first step is either Reaction 9a or Reaction 17b, one can once more use the rate constant ratios given earlier to estimate the yields of the possible primary precursor ions. Hence, either G-(C2H2+) = 1.9 ions/100 e.v., or G(C2H4+) = 1.52 ions/100 e.v. The... 

TTigh pressure mass spectrometry has recently provided much detailed kinetic data (5, 12, 13, 14, 15, 17, 22, 24, 26, 29) concerning ionic reactions heretofore unobtainable by other means. This information has led to increased understanding of primary reaction processes and the fate of ionic intermediates formed in these processes but under conditions distinctly different from those which prevail in irradiated gases near room temperature and near atmospheric pressure. Conclusive identification and measurements of the rate constants of ionic reactions under the latter conditions remain as both significant and formidable problems. 

Kinetic data on the carbonylation of vinyl cations have not been obtained so far, but it is likely to be a diffusion-controlled reaction as in the case of primary alkyl cations (Section IV, A). 

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

Kemp and Waters found a primary kinetic isotope effect of 8.7 for oxidation of C-deuterated mandelic acid and noted a large difference in rate between the oxidations of mandelic acid k at 24.4 °C = 1.7 l.mole . sec ) and a-hydroxy-isobutyric acid ( 2 at 24.4 °C = 5.6 x 10 l.mole . sec ) — a difference not reproduced for the oxidation of these compounds by the one-equivalent reagent, manganic sulphate. The various data are fully in accord with a Westheimer-type mechanism, viz. 

Kinetic data exist for all these oxidants and some are given in Table 12. The important features are (i) Ce(IV) perchlorate forms 1 1 complexes with ketones with spectroscopically determined formation constants in good agreement with kinetic values (ii) only Co(III) fails to give an appreciable primary kinetic isotope effect (Ir(IV) has yet to be examined in this respect) (/ ) the acidity dependence for Co(III) oxidation is characteristic of the oxidant and iv) in some cases [Co(III) Ce(IV) perchlorate , Mn(III) sulphate ] the rate of disappearance of ketone considerably exceeds the corresponding rate of enolisation however, with Mn(ril) pyrophosphate and Ir(IV) the rates of the two processes are identical and with Ce(IV) sulphate and V(V) the rate of enolisation of ketone exceeds its rate of oxidation. (The opposite has been stated for Ce(IV) sulphate , but this was based on an erroneous value for k(enolisation) for cyclohexanone The oxidation of acetophenone by Mn(III) acetate in acetic acid is a crucial step in the Mn(II)-catalysed autoxidation of this substrate. The rate of autoxidation equals that of enolisation, determined by isotopic exchange , under these conditions, and evidently Mn(III) attacks the enolic form. 

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

Ruasse et al, 1978) is totally regioselective and shows X-dependent chemoselectivity. This is partly in agreement with the kinetic data, which indicate no primary carbocation but rather a competition between the benzylic carbocation and the bromonium ion, depending on X. According to the data of Table 6, bridged intermediates would lead to more dibromide than open ions do. From these results and from those on gem-, cis- or frans-disubstituted alkenes, empirical rules have been inferred for chemoselectivity (i) more solvent-incorporated product is formed from open than from bridged ions (ii) methanol competes with bromide ions more efficiently than acetic acid. 

It should be noted at this point that primary and secondary reaction products can be distinguished not only by kinetic data (13) but also by suppression of the secondary reactions. E.g substitution of 2,2,2-trifluoroethanol for p-dioxane as solvent for HCoCCO) suppresses homologation and methane formation addition of a phosphine to give the less acidic catalyst HCo(CO)3PR3 has the same effect, as has the substitution of the less acidic catalyst HMn(CO)5. 

The primary literature now contains a very large body of kinetic data for the catalysis of enolization and ketonization, not only of ketones and aldehydes but also of )3-diketones, )3-keto esters, and dienones, much of which could be treated by the Kurz approach. Also, data exist for third-order enolization, due to combined general acid and base catalysis, that could also be analysed. Such treatment is beyond the scope the present review. However, one study of metal ion catalysis of enolization is discussed later in this section. 

The important and stimulating contributions of Kebarle and co-workers 119 14 > provide most of the data on gas-phase solvation. Several kinds of high pressure mass spectrometers have been constructed, using a-particles 121>, proton- 123>, and electron beams 144> or thermionic sources 128> as primary high-pressure ion sources. Once the solute A has been produced in the reaction chamber in the presence of solvent vapor (in the torr region), it starts to react with the solvent molecules to yield clusters of different sizes. The equilibrium concentrations of the clusters are reached within a short time, depending on the kinetic data for the... 

The kinetic data for these reactions are numerous, as shown in Table VI. Most of values were obtained by radical clock methods. The ring expansion of radical 7 has been employed as the clock in a study that provided much of the data in Table VI.74 Cyclizations of 5-hexenyl-type radicals also have been used as clocks,75-77 and other competition reactions have been used.78 Hydrogen atom abstraction from n-Bu3GeH by primary alkyl radicals containing a trimethylsilyl group in the a-, >8-, or y-position were obtained by the indirect method in competition with alkyl radical recombi-... 

A simple compound to begin our presentation is acetylsalicylic acid (aspirin, 7.44), the well-known analgesic and anti-inflammatory drug whose primary metabolite, salicylic acid (7.45), is also an anti-inflammatory agent but not an analgesic. Extensive kinetic data have been published on the chemical hydrolysis of acetylsalicylic acid as a function of temperature and... 

However, in the reaction of vitamin BjjS with primary halides and with benzyl chloride, where the 8 2 driving force can be calculated from the standard potential of the Co(ii)/Co(i) couple and from existing Co—C bond energy data, it appears that recasting the kinetic data point against DF Sf 2) instead of DF ET) brings it close to the ET line (Walder, 1989). 

An appraisal has been made of the available kinetic data on the acid hydrolyses of hydroxyamic acids. For A-substituted hydroxamic acids both A-2 and A-1 paths are recognized, but for primary hydroxamic acids there is evidence only for the A-2... 

The kinetic data for the reaction of primary alkyl radicals (RCH2 ) with a variety of silanes are numerous and were obtained by applying the free-radical clock methodology. The term free-radical clock or timing device is used to describe a unimolecular radical reaction in a competitive study [2-4]. Three types of unimolecular reactions are used as clocks for the determination of rate constants for this class of reactions. The neophyl radical rearrangement (Reaction 3.1) has been used for the majority of the kinetic data, but the ring expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical (Reaction 3.3) have also been employed. 

The competitive kinetics of Scheme 3.1 can also be applied to calibrate the unimolecular radical reactions provided that kn is a known rate constant. In particular the reaction of primary alkyl radicals with (Mc3Si)3SiH has been used to obtain kinetic data for some important unimolecular reactions such as the p-elimination of octanethiyl radical from 12 (Reaction 3.5) [12], the ring expansion of radical 13 (Reaction 3.6) [8] and the S-endo-trig cyclization of radical 14 (Reaction 3.7) [13]. The relative Arrhenius expressions shown below for the... 

Since the experimental kinetic data refer to a reaction rate and how this is affected by variables, such as concentration, temperature, nature of the solvent, presence of other solutes, structural variations of the reactants, and so forth, the assignment of a mechanism is always only indirectly derived from primary data. Therefore, it is not surprising that more than one mechanism has often been proposed to explain the same rate law and that reaction mechanisms, which were once consistent with all experimental information available on a system, have later on been considered erroneous and have been disregarded, or drastically modified, as long as new experimental evidence was accumulated. In general, the stoichiometry of the reaction, even when this is a simple one, cannot be directly related with its mechanism, and when the reaction occurs through a series of elementary steps, the possibility that the experimental rate law may be interpreted in terms of alternative mechanism increases. Therefore, to resolve ambiguities as much as possible, one must use aU the physicochemical information available on the system. Particularly useful here is information on the structural relations between the reactants, the intermediate, and the reaction products. 

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