And the mesomeric effect

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). 

Complexes of platinum(II) with /J-monothiodiketonates, derived by deprotonation of the parent acid (207), can be prepared from PtCl -. The dark red complex Pt(C3H(Ph)2SO 2 (R = R = Ph) shows IR bands at 1535 cm-1 [v(C=C)], 1410 cm-1 [v(O O)] and 1270 cm-1 [v(O S)].1824 Electronic spectra and dipole moment data for these complexes have been compared with the O.O -diketonate complexes.182s 1826 The structure of the phenyl derivative has been confirmed by X-ray crystallography.1827 Detailed dipole moment measurements using static polarization have been made with fluorinated jS-monothiodiketone complexes. Variations with substituent depend on the magnitude and vector directions of the Ph—X bond moments (aryl substituents), the inductive effect of the meta and para substituent on the phenyl ring, and the mesomeric effect of the substituent X.1828 A useful separation method for bis(monothiotrifluoroacetylacetonates) of platinum(II) is gas chromatography.1829... 

The oxidation potentials of TTF derivatives can be varied in a controlled way over a wide range by the introduction of appropriate substituents. The influence of substituents on oxidation potentials values of TTF has been discussed in many articles. For instance, the oxidation potentials of about 40 different substituted TTF derivatives have been measured under the same conditions [32]. In general, all substituents, with the exception of alkyl groups, increase oxidation potentials and the mesomeric effect of substituents have been shown to have an insignificant influence on the donor ability of TTF, in contrast to the trends observed for aromatic and heteroaromatic series. Some representative examples of oxidation potentials for substituted TTF derivatives are collected in Table 7. In the case of aryl-substituted TTFs, a usual Eox-cr+ correlation has been reported [63],... 

From the Table 48.4 one can follow and distinguish the inductive and the mesomeric effects. 

The increased acidic strength of phenol, as compared to the aliphatic alcohols, is usually attributed to the combined effect of the electron attracting inductive effect (—1) of the benzene. ring, and the mesomeric effect in the anion (—M)-,. shown as A. As both effects have the same sign, the increase in acidic strength is large. ... 

The basicities of the parent azole systems in water are shown in Table 1. When both heteroatoms are nitrogen, the mesomeric effect predominates when the heteroatoms are in the 1,3-positions, whereas the inductive effect predominates when they are in the 1,2-positions. The predominance of the mesomeric effect is illustrated by the pK value of imidazole (82 Z = NH), which is 7.0, whereas that of pyrazole (83 Z = NH) is 2.5 cf. pyridine, 5.2). An fV-methyl group is base-strengthening in imidazole, but base-weakening in pyrazole, probably because of steric hindrance to hydration. When the second heteroatom is oxygen or sulfur the inductive, base-weakening effect increases the pK of thiazole (82 Z = S) is 3.5 and that of isoxazole (83 Z = 0) is 1.3. 

If the reactions of the same substituents on heteroaromatic azoles and on benzene rings are compared, the differences in the reactivities are a measure of the heteroatoms influence. Such influence by the mesomeric effect is smaller when the substituent is /3 to a heteroatom than when it is a or y. The influence by the inductive effect is largest when the substituent is a to a heteroatom. 

The mesomeric effect of the C=S linkage is very pronounced and is responsible for the facile quaternization of heterocyclic N-alkylated thiones (159) this effect is operative even when such a shift does not increase the aromaticity of the ring. Thione derivatives of pyridazine, benzothiazole, quinazoline, 1,3-thiazine, triazole,and isoindole are examples of compounds which readily form quaternary salts. 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

The actual structure is somewhere in between, i.e. (2lab) a hybrid of which (21a) and (21b) are the canonical forms. There will also be an inductive effect, as shown in (2lab) but this will be much smaller than the mesomeric effect as a electrons are much less polarisable, and hence less readily shifted, than tt electrons. 

Amides and esters give a positive slope of about 1.2 on Hart et al. s (1967) plot, due to the reduction of the double bond character of C=0 by the mesomeric effect of NH2 and OH, which reduces the reactivity toward eh. 

Thus the specific and exclusive source of energy which is necessary to determine the independence of the mesomeric effect is identified as perturbation energy... 

As Ingold s version of the electronic theory developed, NH2, NR2 and NHAcyl groups were classified as —/ and +7" in their electronic effects, signifying electron attraction through the Inductive Effect and electron release by the Tautomeric Effect59. Later the latter was subdivided into the Mesomeric Effect M (polarization) and Electromeric Effect E (polarizability)60. The NO group was also classified as —I and +T in its electronic effect, while NO2 was classified as —I and —T. 

As one might expect, the mesomeric effect of an alkoxy group enhances the activity of the C=C to attack by the carbene, but it has been noted that, where there is competition between an alkoxyvinyl group and an inactivated alkene group within the same molecule, an alkyl or aryl group stabilizes the transition state better than does the vinyloxy group (Scheme 7.10) [56]. It is noteworthy that vinyl sulphides are five times more reactive than are the enol ethers [62]. 

Analysis of the results presented in Figure 6.20 (conversion versus time) and Table 6.14 indicates that for both the monometallic and the achiral catalysts the reaction rate was lower than the corresponding values observed in the hydrogenation of acetophenone. This fact has been explained in terms of the mesomeric effect due to the -OCH3 in para position, which causes a higher repulsion between... 

The conformational properties of thioketones of the five-membered heterocycles 148 and 149 appear strictly similar (67SA(A)2605 68BSF703 69BSF831 84JST(112)85 85RTC9) to those of the corresponding ketones, and the X,S-cis orientation is preferred as a general rule. The mesomeric effect... 

This is called the mesomeric effect. The move of electronic doublets due to the mesomeric effect generates charges on the carbon and oxygen atoms of acrolein. 

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