Mesomeric field effect

FIGURE 4.15. Interactions of m-aminobenzoic add due to (a) the field effect and (b) the mesomeric field effect. 

The contribution of the mesomeric field effect can be estimated by combining equations (4.134) and (4.140). The charge produced at position j by our substituent S at position i is, as we have seen, given by - charge exerts a field effect given by equation (4.134). The total mesomeric field effect... 

As suggested by Roberts and Moreland many years ago (1953), the acidity constants of 4-substituted bicyclooctane-l-carboxylic acids provide a very suitable system for defining a field/induction parameter. In this rigid system the substituent X is held firmly in place and there is little possibility for mesomeric delocalization or polarization interactions between X and COOH (or COO-). Therefore, it can be assumed that X influences the deprotonation of COOH only through space (the field effect) and through intervening o-bonds. On this basis Taft (1956, p. 595) and Swain and Lupton (1968) were able to calculate values for o and crR. 

Before we close this section we make reference to an extended form of the Hammett equation in which the substituent constant and the reaction constant are separated into contributions from the field effect (F) and the mesomeric effect (R). This procedure was suggested by Taft in 1957 for 4-substituted benzene derivatives. It is called a dual substituent parameter (DSP) equation (Scheme 7-2). 

Electronic effects Changes in the absorption frequency for a particular group take place when the substituents in the neighbourhood are changed. The frequency shifts, in these cases may be due to inductive, mesomeric or due to field effects and there will be combined effect due to all. The effect of one can only be estimated approximately. 

Inductive and electric field effects of the substituents may overlap the o-mesomeric effects. However, the m and p shifts of monosubstituted benzenes generally follow the pattern discussed above. 

In aromatic compounds carbon-13 shifts are largely determined by mesomeric (resonance) and inductive effects. Field effects arising from through-space polarization of the n system by the electric field of a substituent, and the influences of steric (y) effects on the ortho carbon nuclei should also be considered. Substituted carbon (C-l) shifts are further influenced by the anisotropy effect of triple bonds (alkynyl and cyano groups) and by heavy atom shielding. 

We are left with the problem of substituent effects that do not depend on direct mesomeric interactions between the substituent and the reaction center. Effects of this kind can arise in one of two ways. First, the bond between the substituent and the substrate may be polar, and there may also be polar bonds or charged atoms in the substituent itself the charges set up in this way can influence the reaction center either by altering the effective electronegativity of atoms connected with it (inductive effect) or by direct electrostatic interaction across space (field effect). Secondly, the substituent may be attached to a conjugated system which does not itself take part in the reaction, the case exemplified by the Hammett equation (Eq. (105)) here it... 

The difference in electron influence of five-membered heterocycles which contain 0, S, or Se atoms is explained by the fact that the influence is a sum of the positive conjugation and negative induction thus it operates differently in the static (ground-state mesomeric) or dynamic state. In general, the influence depends on the systems in which it is observed and may be masked by space factors or field effects. 

The ionization of monoprotic acids and bases and hence their solubility and absorption is dependent on their pK, the pH at which the drug is 50% ionized. First developed by Hammett and published in 1940, the relation between the dissociation constants of benzoic acid derivatives and the longer range electronic (inductive, mesomeric and field) effects is linear and additive. 

In order to examine whether the acidities observed in a-C(0)R, a-N02, a-CN and a-CF3 substituted eyelopropanes are correctly explained by a predominating mesomeric effect in the case of the a-C(0)R and a-N02 substituents, and a stronger dipole (field) effect with a-CN and a-CFa substituents Wagner and Boche " investigated this subject by means of STO-3G calculations. It was of interest to determine whether the calculations are able to reproduce the relative acidities of cyclopropyl and isopropyl compounds, as well as the configuration of the corresponding cyclopropyl anions. 

See also field effect mesomeric effect polar effect. 

This indirect interaction, through secondary electrostatic interaction with charges produced by polarization of a tt system, is described as the mesomeric field (MF) effect. 

The effects of substituents on the rates of 5 2 reactions has been barely studied. We would expect them to depend only on the inductive and field effects of substituents, -type substituents being unable to exert mesomeric effects. The SeI reaction should, like its S l counterpart, be of BEP type, i.e., It should be accelerated by substituents that can stabilize... 

Within the context of this book the quantitative relationships between structure and chemical reactivity are very informative. One of the early postulates of Ingold and his school in the 1930s (review see Ingold, 1969, p. 78) was that the electronic effects of substituents are composed of two main parts a field/inductive component and a mesomeric component. Hammett s work indicated clearly from the beginning that his substituent constants am and crp reflect Ingold s postulate in numerical terms. In particular, many observations indicated that the /7-substituent constant ap is the sum of a field/inductive component 0 and a resonance (mesomeric) component (Jr. 

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