Ionic radius reaction rates

Alkali metal alkoxides, r-butyl acetate neat, 45°, 30 min, 98% yield of r-butyl ester from methyl benzoate. The rate constant for the reaction increases with increasing ionic radius of the metal and with decreasing polarity of the solvent. Equilibrium for the reaction is achieved in <10 sec. Other examples eire presented. " ... 

The type of catalyst influences the rate and reaction mechanism. Reactions catalyzed with both monovalent and divalent metal hydroxides, KOH, NaOH, LiOH and Ba(OH)2, Ca(OH)2, and Mg(OH)2, showed that both valence and ionic radius of hydrated cations affect the formation rate and final concentrations of various reaction intermediates and products.61 For the same valence, a linear relationship was observed between the formaldehyde disappearance rate and ionic radius of hydrated cations where larger cation radii gave rise to higher rate constants. In addition, irrespective of the ionic radii, divalent cations lead to faster formaldehyde disappearance rates titan monovalent cations. For the proposed mechanism where an intermediate chelate participates in the reaction (Fig. 7.30), an increase in positive charge density in smaller cations was suggested to improve the stability of the chelate complex and, therefore, decrease the rate of the reaction. The radii and valence also affect the formation and disappearance of various hydrox-ymethylated phenolic compounds which dictate the composition of final products. 

The increase in ionic radius from Be2+ to Mg2+, which is accompanied by an increase in coordination number from 4 to 6, is responsible for a substantial increase in lability (Table III, (37-43)). The two activation volumes measured are positive as well as all the activation entropies. The rate laws determined for non-aqueous solvents in inert diluent are first order, showing a limiting D mechanism for all solvent exchange reactions on [MgS6]2+. 

An Arrhenius type equation is obtained for the apparent reaction rate constant. Equations for the apparent activation energy and for the frequency factor are established as functions of Hamaker s Constant, ionic strength, surface potentials and particle radius. 

In this paper it is shown that the rate of deposition of Brownian particles on the collector can be calculated by solving the convective diffusion equation subject to a virtual first order chemical reaction as a boundary condition at the surface. The boundary condition concentrates the surface-particle interaction forces. When the interaction potential between the particle and the collector experiences a sufficiently high maximum (see f ig. 2) the apparent rate constant of the boundary condition has the Arrhenius form. Equations for the apparent activation energy and the apparent frequency factor are established for this case as functions of Hamaker s constant, dielectric constant, ionic strength, surface potentials and particle radius. The rate... 

Initial studies focused on lanthanocene-based catalyst systems that proofed to be efficient in the exo-specific cyclization of terminal aminoalkenes to form five-, six-, and seven-membered azacycles (Scheme 2). The reactions are predictably faster for the formation of smaller five-membered rings and in the presence of em-dialkyl substituents [117]. An increasing metal ionic radius and a more open coordination sphere, for example, in onra-lanthanocenes, are also beneficial for higher cyclization rates [103]. A further increase in catalytic activity is observed when sterically more open and more electrophilic CGC 17 (Fig. 15) are applied [118]. 

Interestingly, the reactivity pattern in rare-earth metal-catalyzed hydroamination/cyclization reactions of aminoalkynes with respect to ionic radius size and steric demand of the ancillary ligand follows the opposite trend to that observed for aminoalkenes, namely decreasing rates of cyclization with increasing ionic radius of the rare-earth metal and more open coordination sphere around the metal. This phenomenon can be explained by a negligible sterical sensitivity of a sterically less encumbered triple bond, as sterically less open complexes and smaller metal ions provide more efficient reagent approach distances and charge buildup patterns in the transition state [110]. 

The rate dependence on the ionic radius of the center metal was investigated. As previously observed for the hydroamination reaction, the reaction rate increased with increasing ionic radius, so that the lanthanum complex 144 was proved to be... 

The catalytic activity of the chiral complexes [Ln(L)Z2] shown in Scheme 70 was investigated in NMR-scale intramolecular hydroamination/cyclization reactions [135]. The rate dependence on the ionic radii of the center metal was studied by using 5 mol% bisoxazoline L32 and [Ln N(SiMe3)2 3] as precatalysts and 2,2-dimethyl-4-penten-l-amine as substrate (Scheme 71). The reaction rate as well as the enantioselectivities increased with increasing radius of the center metal. Therefore, the lanthanum compound 184 was the most active catalyst among the investigated complexes. 

To summarize, the proton transfer reaction can be broken into three distinct parts Diffusion of the reactants to within the radius of the ionic atmosphere accelerated diffusion to within the encounter distance and subsequent conversion of the encoimter complex to products. For reactions in which the equilibrium is rapidly established within the encounter complex, the rate equations are dominated by the diffusion process. This results in the loss of information about the dynamics of the encounter complex. For such a reaction some information can be obtained about the ionic radius by varying the ionic strength and using an electrostatic theory (such as is done for Deby-Hiickel activity coefficients) to calculate the effect of shielding by the ions. ... 

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