Activated complex charge development

The effect of charge delocalization en route to the activated complex is the result of the relatively nonpolar micellar environment compared to bulk water, charges in the micellar pseudophase are less stabilized by interactions with their environment (cf. stabilization of developing charges by the electrostatically non-neutral environment for (pseudo) unimolecular reactions). This effect was found for the dehydro-bromination reaction of 2-(p-nitrophenyl) ethyl bromide and the dehydrochlorination of 1,1,1 -trichloro-2,2-bis(p-chlorophenyl)ethane. ... 

We would also predict that the transition state for ionization of the 3° tosylate would be earlier, so there should be less C-O bond breaking and less charge development than in the activated complex for ionization of the 2° tosylate. 

While changes in rate constants in response to changes in structure are extremely valuable for indicating the type of charge development occurring in the activated complex, the actual extent of charge development in the activated complex is an additional structural descriptor that would be very useful. 

These substituent constants can be used with rate data to evaluate the type and extent of charge development in the activated complex of the rate-determining step for a wide variety of chemical reactions. The rates of reaction for a particular transformation are measured using a series of compounds which differ only by the phenyl substituents present for example,... 

Then the differences in rate caused by the electronic effect of the substituent are correlated by the Hammett equation log(kz/kH) = poz, where kz is the rate constant obtained for a compound with a particular meta or para substituent, ku is the rate constant for the unsubstituted phenyl group, and crz is the substituent constant for each substituent used. The proportionality constant p relates the substituent constant (electron donating or wididrawing) and the substituent s effect on rate. It gives information about the type and extent of charge development in the activated complex. It is determined by plotting log(kz/kQ) versus ov for a series of substituents. The slope of the linear plot is p and is termed the reaction constant. For example, the reaction shown above is an elimination reaction in which a proton and the nosy late group are eliminated and a C-N n bond is formed in their place. The reaction is second order overall, first order in substrate, and first order in base. The rate constants were measured for several substituted compounds ... 

For similar reactions, comparison of die p values can be used to determine which reaction has a greater charge development. Comparison of the olefin-forming eliminations below reveals which reaction has greater charge development at die benzylic position and thus which has a greater degree of proton removal in die activated complex. 

Most zeolites have an intrinsic ability to exchange cations [1], This exchange ability is a result of isomorphous substitution of a cation of trivalent (mostly Al) or lower charges for Si as a tetravalent framework cation. As a consequence of this substitution, a net negative charge develops on the framework of the zeolite, which is to be neutralized by cations present within the channels or cages that constitute the microporous part of the crystalline zeolite. These cations may be any of the metals, metal complexes or alkylammonium cations. If these cations are transition metals with redox properties they can act as active sites for oxidation reactions. 

The magnitude of A is less than that for A Ve, indicating that the solvation arrangement and charge neutralization is not so fully developed on forming the activated complex as when the final product is formed. The activated complex does resemble the products somewhat, and is probably half-way between reactants and products. 

Now, some addition reactions will be considered, the solvent dependences of which have been reviewed [77, 78]. Addition of uncharged electrophiles [e.g. Br2, ArS—Cl, NO—Cl, R—CO3H) to carbon-carbon multiple bonds leads to the development of a small, usually dispersed charge in the activated complex. In more polar solvents, this is accompanied by a slight rate acceleration. In reactions with substantial charge development in the activated complex, larger rate accelerations with increasing solvent polarity are observed. 

The partial orders with respect to [OH ] observed for most silicate mineral dissolution reactions can be explained by the surface complexation model (Blum and Lasaga, 1988 Brady and Walther, 1989). Brady -and Walther (1989) showed that slope plots of log R vs. pH for quartz and other silicates at 25 °C is not inconsistent with a value of 0.3. Plots of the log of absorbed OH vs. pH also have slopes of about 0.3, suggesting a first-order dependence on negative charge sites created by OH adsorption. Because of the similarity of quartz with other silicates and difference with the dependence of aluminum oxides and hydroxide dissolution on solution [OH ], Brady and Walther (1989) concluded that at pH >8 the precursor site for development of the activated complex in the dissolution of silicates is Si. This conclusion is supported by the evidence that the rates (mol cm s ) at pH 8 are inversely correlated with the site potential for Si (Smyth, 1989). Thus it seems that at basic pH values, silicate dissolution is dependent on the rate of detachment of H3SiO4 from negative charge sites. 

Scheme V. Proposed mechanism for CCP compound I formation, (a) The native enzymes (b) the activated complex with the distal histidine operating as an acid-base catalyst and the active site arginine stabilizing a developing negative charge on RO-OFe (c) the hypothetical oxene intermediate (d) Compound I after the intramolecular electron rearrangement of (c) to give Fe(IV) and a hee radical, X.

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