Kinetic parameters for substitution

Rate laws and kinetic parameters for substitution reactions at complexes Cp2TiX2 in acetonitrile solution at 298.2 K have been reported (X = halide or alkoxide). Reactivities are discussed in terms of the nature of the leaving group, the entering group, and the non-leaving Cp ligand. A volume of activation of —15 cm3 mol-1 has been determined for the reaction with thiocyanate.1124... 

Table 5.12. Kinetic Parameters for Substitution at Pentacyanoferrates(II) Rate Constants and Activation Parameters Referring to Limiting Rates for Loss of L from [Fe(CN)sL]", and in All Cases Referring to 298.2 K and Aqueous Solution...

Table 8.1. Kinetic Parameters for Substitution at Pentacyanoferrate(II) Complexes [Fe(CN) L], in Aqueous Solution at 298.2 K...

Table 5 Selected kinetic parameters for substitution reactions...

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

Kinetic parameters for formation of [Ni(CN)5]3, which strictly is addition to [Ni(CN)4]2 rather than substitution, may be found in Table VI in Section IV.A above. 

The cleavage of phenyl acetates by /3-CD shows the same general features as that by a-CD (Table A5.1), although there are quantitative differences that must arise from the larger cavity size of /3-CD. Generally, the mem-substituted esters are not cleaved as well as by a-CD and the para-substituted esters are cleaved better. Thus, the distinction between the kinetic parameters for two series of esters is less dramatic for /3-CD, presumably because of the looser fit of substituted phenyl groups in /3-CD. This trend is continued with the two entries for y-CD (which has a still larger cavity) where the differences between the meta and para isomers of t-butylphenyl acetate are quite small (Tables A5.1). Nevertheless, the... 

Rates and Kinetic Parameters for the Internal Rotation of Substituted 2-Methylphenyl-di-ferf-butylcarbinol (74) in Dodecane (ap — sp)... 

The relative amounts of all these Pt species vary as a function of the pH and the chloride concentration. Only platinum species with a coordinated water molecule are regarded to be reactive, because, in contrast to coordinated chloride or hydroxide, this ligand can be easily substituted by other donor molecules. Hydroxo species are formed as indicated in Eqs. (3)-(7) (34, 35), with [ds-Pt(NH3)2(OH)2] as the stable end product in basic solution (36). It should be noted, however, that this species very easily dimerizes and trimerizes at higher concentrations, producing ions such as [cis-Pt(NH3)2]2(jU.-OH)2 and [cis-Pt(NH3)2]3(ju,-OH)3, as has been proved with, e.g., 195Pt NMR spectroscopy (36a, b). Very recently, accurate pAa values have been presented for the (de)hy-dronation equilibria (36b) the pKa values have been added to Eqs. (5)—(7). Miller and House (36c) have accurately determined the kinetic parameters for the several hydrolysis reactions. They concluded that acid hydrolysis of cis-Pt in vivo is unlikely to proceed beyond [cis-Pt(NH3)2Cl(H20)]+. 

The thermalization of energetic recoil atoms in excess moderator is a useful tool to measure kinetic parameters for abstraction, substitution, and addition reactions. For thermal experiments, the bulk (>90%) of the sample must consist of a compound that is (1) inert for hot and thermal reactions with the recoil atom and (2) able to supply the radioactive atom. For example, Ne, CF4, C2F6, SF6 + Ar, CF2CI2, and CF3C1 meet these requirements for radioactive recoil F and Cl atoms. 

Table 60 Kinetic and Thermodynamic Parameters for Substitutions at [Rh(TPPS)(H20)2]+ ...

Summary The rich variety of the coordination chemistry of silicon is discussed and some theoretical issues are raised. In an attempt to understand further the underlying chemistry, some thermodynamic and kinetic parameters for the formation and substitution of pentacoordinate silicon compounds have been measured by NMR methods. Values of -31 3 kJ mol for SHand -100 10 J K mor for A5-were measured for the intramolecular coordination of a pyridine ligand to a chlorosilane moiety. A detailed kinetic analysis of a nucleophilic substitution at pentacoordinate silicon in a chelated complex revealed that substitution both with inversion and retention of configuration at silicon are taking place on the NMR time-scale. The substitution with inversion of configuration is zero order in nucleophile but a retentive route is zero order in nucleophile at low temperature but shows an increasing dependence on nucleophile at higher temperatures. These results are analysed and mechanistic hypotheses are proposed. Some tentative conclusions are drawn about the nature of reactivity in pentacoordinate silicon compounds. 

Even though this dipeptide is turned over quite slowly, the complex examined is probably a non-productive one. Furthermore an analogous ester substrate has not been found, and it is known that carboxypeptidase behaves quite differently toward ester and peptide substrates. In particular, the kinetic parameters for peptide hydrolysis for a series of metal substituted carboxypeptidases indicate that fccat values can range from 6000 min for the cobalt enzyme down to 43 min for the cadmium enzyme 66). The values on the other hand are almost totally independent of the particular metal present. The exact opposite is true for ester hydrolysis. Km varies from 3300 M for the cobalt enzyme to 120 M for the cadmium enzyme while k<.at is essentially unchanged. 

The vertical ionization potential for a solvated chemical species can be the measure of its reactivity in the solution phase, especially for a single electron transfer reaction. It has been reported that the ionization potentials of anions in solution are conelated with the kinetic parameter for nucleophilic substitution reaction. This implies that an important aspect of the activation process of the reaction is a single electron transfer from anion to substrate. The ionization potential for solvated species has been available as the threshold energy E by photoeiectron emission spectroscopy for solution (PEES). This spectroscopic technique is able to provide the , values of almost any solvated species, such as organic, inorganic, cations, anions and neutral molecules in aqueous and nonaqueous solutions. 

KINETIC PARAMETERS FOR THE THERMOLYSIS OF SUBSTITUTED PHENYL AZIDES... 

This expresses the volume of the reactor in terms of Tand q when Czj- and the kinetic parameters for are given. Substituting into Eq. (7.4.2) gives... 

These values are very similar to the ones of the H-abstraction from propene to form allyl radical (Tsang, 1992). Successive reactions of aromatics include the substitutive addition reactions that progressively favour the dealkylation reactions of aromatic species. The kinetic parameters for the reference reaction H +Toluene -> CH3 + Benzene... 

An example is the use of Z isomerization as a nucleophilicity probe. This process has practical advantages because it does not require the characterization of different products for each nucleophile, and the thermodynamics of the process is independent of the nucleophile. Table I gives the kinetic parameters for the amine-catalyzed (Z)-5 to ( )-5 isomerization (equation 1) (13, 14). The k2 values are easily interpreted in terms of electronic and steric effects on a rate-determining nucleophilic attack. However, the very low AHl values suggest that the observed rate constant is not kx but a more complex expression, and the internal rotation step krot may be ratedetermining. In view of this, this process should not be used as a probe. Very low activation enthalpies in several vinylic substitutions [e.g., AHx = 0.8-2.0 kcal mol-1 for the reaction of the para position of N,N-dialkylanilines with tricyanovinyl chloride in chloroform (15)] may also indicate a composite rate constant. 

Table 40 Kinetic Parameters for the Substitution [Rh(en)2XLr + Y — [RhtenljXY] + L...

Just as in conventional substitution reactions in Cr(III) complexes, there is considerable controversy as to the mechanism of substitution in Cr(III) meso-substituted porphyrins negative AS values suggest associative activation of the axial sites while positive AV values imply dissociative activation. A thorough study of the kinetic and equilibrium properties of the axial ligation reaction between imidazole and pyridine with [Cr(TPPS)(H20)2] " has now been made. Data were analyzed according to Scheme 1 and the rate and equilibrium constants are reported in Table 6.4. Data for the toluene soluble [CrCl(TPP)X] (X = py, quinoline, or PPhj) with 1-methyl imidazole (Meim) are not quite comparable, as the reaction is not first order in Meim. Kinetic parameters for the dissociation of X from [CrCl(TPP)X] in toluene are, in order X, k (s" ), AH, AS, AV py,... 

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