Constant electronic influences

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

Consider a molecule X Y Z, where X is the substituent, Y the constant part of the molecule (the link between X and Z), and Z the reaction center. Now we wish to compare its reactivity with the molecule H Y Z bearing hydrogen as the reference substituent. The electronic influence of the substituent X may depend on whether it comes into contact with the surface or not. In terms of adsorption this means whether the molecule will be attached to the surface horizontally or perpendicularly ... 

Two are the main factors governing the activity of materials (i) electronic factors, related to chemical composition and structure of materials influencing primarily the M-H bond strength and the reaction mechanism, and (ii) geometric factors, related to the extension of the real surface area influencing primarily the reaction rate at constant electronic factors. Only the former result in true electrocatalytic effects, whereas the latter give rise to apparent electrocatalysis. 

To summarize, there is still a need for carefully determining more rate constants for various substances of biological interest in their various charged forms. This phase of the subject will be complete when critically chosen values have passed into the Tables and when theoretical correlations have been sufficiently developed to enable rate constants for unexamined substances to be reliably predicted. There is also still a need to correlate the reactivity of the hydrated electron with the reactivity of free radicals such as H, OH, organic radicals, peroxy radicals, etc., so as to be able to predict the reactivity of unexamined free radicals. Another need is to establish the influence of conditions on the rate constants. The influence of ionic strength is now well known, but other factors, such as the dielectric properties of the medium, have been shown to have an effect in some cases (2, 20). Also, the effect of temperature has been investigated in only a few cases (9). 

In the conventional Hammett treatment, the electronic influences of the substituents, <7, are assumed to be constant. To test the adherence of a reaction to the Hammett eq. (1), it is customary to plot the log (fc/fcH) values against the a-constants. A linear relationship establishes that the reaction obeys the equation. The slope of the straight line defines the reaction constant p for the process. 

The electronic properties of both alkyl [5] and aryl alcohols [6] play a clearly definable role in ester formation, with formation constants decreasing with increase in electron withdrawing ability of the ligand. For both types of ligands, the electronic influences are quite small, but the resonance effects found with the aromatic ligands indicate there are 71-electron contributions to the empty d orbitals of vanadate [6], The influences of the electronic properties of ligands on coordination mode and geometry are discussed in detail in Chapter 9. 

The results of two-parameter correlations of the 13C and 15 N NMR chemical shifts of 2-substituted 5(6)-nitrobenzimidazoles with the induction and resonance constants (a, ak) are given in Table 3.26 [688, 689], The electronic influence of substituents on the chemical shifts of carbons (and protons) in positions 4 and 7 are mainly transmitted by a resonance mechanism, C-4 being more sensitive than C-7 to the substituent effect. For positions 5 and 6 a slightly smaller contribution from the resonance component to the total transmission of substituent effects is observed. However, analysis of correlation between 5 15 N and substituent parameters indicates an approximately equal influence of the induction and resonance substituent effects on the nitro group shielding (see Table 3.26) [688, 689], When other sets of substituent constants (F and R, c, and ck, etc.) are used in the correlation the percentage ratios of resonance and inductive contributions remain unchanged. 

An attempt to estimate the transmission of electronic influence of substituents R from positions 3-5 and from 5-3, using correlations between wave numbers and (ct, ctc) constants of substituents R, was made in [1177]... 

C. Dissociation Constants of Ferrocenoic Acids VI, The Electronic Influence of Ferrocenyl and Related Groups as Sub... 

An example of degenerate 1,2-methyl shifts are arenonium ions of the type A it has been found that the electronic influence of the MCR of the carbocation on the rate of rearrangement is characterized not only by 8C but also by such structural parameters as the <7 -constants of substituents and the infrared frequencies of C = 0, V( of ketones related to the rearranging carbocations . ... 

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