Reactivity electrostatic effect

In Chapter 4, we will discuss the relative importance of inductive effects and field effects on reactivity. Generally, field effects appear to be the dominant mechanism for the transmission of electrostatic effects of polar bonds to other parts of a molecule. 

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

Other particular theories are confined to diffusion-controlled reactions (109), to the so called cooperative processes (113), in which the reactivity depends on the previous state, or to resistance of semiconductors (102), while those operating with hydrogen bridges (131), steric factors (132), or electrostatic effects (133, 175) are capable of being generalized less or more. 

None of the other reactions so far discussed involve interaction between a pair of charged species. This is but another instance of the electrostatic effect shown by Kirkwood and Westheimer to be responsible for the disparity between the first and second ionization constants of dibasic acids, for the effect of the carboxylate ion on the basicity of an a-amino acid, and for the difference in reactivity of ionic compounds compared with analogous nonionic species in acid- or base-catalyzed reactions. ... 

The electrostatic effect of an ionic substituent on the reactivity of a functional group subject to attack by another ion persists over a much greater length of intervening chain than is found for the influence of an unionized substituent (compare column B of Table V). This follows of course from Coulomb s law. From the standpoint of polymer reactions, it is important to bear in mind that when charged... 

Pullman, A., and B. Pullman. 1980. Electrostatic Effect of Macromolecular Structure on the Biochemical Reactivity of the Nucleic Acids. Significance for Chemical Carcinogenesis. Int. I. Quant. Chem., Quant. Biol. Symp. 7, 245. 

Experimenters would do well to avoid any unnecessary changes in the ionic composition of reaction samples within a series of experiments. If possible, chose a standard set of reaction conditions, because one cannot readily correct data from one set of experimental conditions in any reliable manner that reveals the reactivity under a different set of conditions. Maintenance of ionic strength and solvent composition is desirable, and correction to constant ionic strength often effectively minimizes or ehminates electrostatic effects. Even so, remember that Debye-Hiickel theory only applies to reasonably dilute electrolyte solutions. Another important fact is that ion effects and solvent effects on the activity coefficients of polar transition states may be more significant than more modest effects on reactants. 

In the reaction between the Co(III) complex and Fe(II)(phen)3+ [Eq. (6)] we observed higher reactivity of the polymer complexes (Table 98s)). This reaction was carried out at very high ionic strength so that the electrostatic effect on the reaction was neglible. Therefore, other attractive interactions caused by the essential properties of the polymers must be considered in the reaction with Fe(II)(phen)3. 

The enhanced reactivity in the cupric ion-catalyzed hydrolysis cannot be due solely to the electrostatic effect of an attack of hydroxyl ion on a positively charged a -amino ester, since the introduction of a positive charge, two atoms from the carbonyl group of an ester, increases the rate constant of alkaline hydrolysis by a factor of 103 (10), whereas there is a difference of approximately 106 between the cupric ion-catalyzed and the alkaline hydrolyses of DL-phenylalanine ethyl ester. The effective charge on the cupric ion-glycine (buffer)-ester complex is +1, so that the factor of 106 cannot be explained by an increase in charge over that present in the case of betaine. Furthermore, the reaction cannot be due to attack by a water molecule on a positively charged a-amino acid ester, since the rate constant of the acidic hydrolysis of phenylalanine ethyl ester is very small. It thus seems... 

The significance of these results for differences in reactivities of nucleophiles is that, despite the unfavorable relative equilibrium constants, Me2S is more reactive toward the quinone methide than chloride ion by a factor of nearly 3000. This mismatch of rate and equilibrium effects is summarized in Scheme 35. It must imply (a) that there is a relatively long partial bond between sulfur and carbon in the transition state so that the unfavorable steric and electrostatic effects are not developed and (b) that the favorable carbon-sulfur bonding interaction is well developed despite the long bonding distance. 

What the PNS cannot deal with is the effect on reactivity by factors that only operate at the transition state level but are not present in either reactant or product. Examples mentioned in this chapter include transition state aromaticity in Diels Alder reactions, steric effects on reactions of the type A + B ty C + D, or hydrogen bonding/electrostatic effects that stabilize the... 

Such charge repulsion effects can operate either in homogeneous solution or in anisotropic media where appreciable centers of charge density develop. Two important classes of media in which these electrostatic effects appreciably influence chemical reactivity (in self-organizing assemblies and polymers) are treated in the following sections. 

Electrostatic effects result from the presence of charged species associated with the Stern-Layer of micelles. The chemical reactivity can be altered by changing the charge on the surfactant. The phenomenon is also observed for biomolecular reactions where one or more solutes are charged, and is observed for photochemical reactions with charged intermediates. 

One of the important differences between molecules in zeolite cages and species isolated in other matrices such as cyclodextrins [22], micelles [23], vesicles [24] and polymers [25] arises from the fact that the zeolite can be an active host and influence the structure and reactivity of the encapsulated molecules. The nature of the intra-zeolitic environment has been extensively studied and the influence of polarity, steric and electrostatic effects on encapsulated molecules is beginning to be well understood. 

In summary, it is dear that the zeolite is a novel host for the entrapment of molecules and the rigidity and the charged nature of the framework allow for steric and electrostatic effects on the encapsulated molecules. Isolation of entrapped molecules can also influence their reactivity. The interest in zeolites as hosts for electron-transfer reactions stems from a combination of properties, including... 

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