Enhanced mesomeric effect

M Group viz., p-OMe (an electron releasing function)—its presence as depicted in (c) above will further lower frequencies due to enhanced mesomeric effect. 

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. 

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]. 

Amides and esters exhibit a different dependence of their rates on the electron-donating capacity of the adjacent groups. In this case electron-withdrawing groups enhance the rate of reaction. A linear correlation between logk and (a + a ) is observed, with a positive slope of 1-2. This behaviour has been attributed to the mesomeric effect of these substituents, which can also be inferred from their effect on v(C=0). The mesomeric forms... 

The term has been deemed obsolescent or even obsolete (see mesomeric effect, resonance effect). Many have used phrases such as enhanced substituent resonance effect that imply the operation of the electromeric effect without using the term, and various modern theoretical treatments parametrize the response of substituents to electronic demand, which amounts to considering the electromeric effect together with the INDUCTOMERIC EFFECT. 

Strictly understood, the mesomeric effect operates in the ground electronic state of the molecule. When the molecule undergoes electronic excitation or its energy is increased on the way to the transition state of a chemical REACTION, the mesomeric effect may be enhanced by the electromeric effect, but this term is not much used, and the mesomeric and electromeric effects tend to be subsumed in the term RESONANCE effect of a substituent. 

The effects of structure on reactivity can be divided into three major types field, resonance (or mesomeric), and steric. In most cases two or all three of these are operating, and it is usually not easy to tell how much of the rate enhancement (or decrease) is caused by each of the three effects. 

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

In the two systems so far discussed it is impossible to obtain a quantitative idea of the relative importance of the inductive and resonance effects because it is impossible to achieve the operation of one of the effects without the other. When nitrogen is the basic centre, this becomes possible by steric fixation of the nitrogen lone pair orbital in the plane of the benzene ring, which virtually eliminates its overlap with the 7r-electron orbital of the ring carbon and hence also the mesomerism. So the enhanced acidity of the anilinium ion (pAT = 4-62) as compared with methylammonium (pAfg = 10-67) has been shown (Wepster, 1952) to be half inductive and half mesomeric in origin by a consideration of the following systems ([10]-[12]) ... 

Conversely, if a. para substituent stabilizes the conjugate base of an acid-base pair rather more than it stabilizes the benzoate ion, more positive substituent constants are required to achieve linearity in Hammett plots. Examples of this are acid dissociations of phenols and anilinium ions, where mesomerically electron-withdrawing substituents (Y = —NO2, —C N) are more effective in enhancing acid strength than they are in benzoic acid, because charge delocalization of the type [15] is not possible in the benzoate anion. 

Introduction of a nitro group into a hydrocarbon considerably enhances the thermodynamic acidity of the C—H bond. For example, nitroethane has pK 8.60 [99]. The effect of the nitro group is comparable to the effect of the two keto groups in acetylacetone (pK 8.9) [17]. The nitrocarbanion (or nitronate ion) is strongly stabilized by inductive and mesomeric electron withdrawal. In solution the ionization (73) is slightly complicated by the presence of the aci-nitro isomer, similar to the enolic form of a ketone, viz. 

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