Electrons mesomeric effect

The underlying principle of the PEOE method is that the electronic polarization within the tr-bond skeleton as measured by the inductive effect is attenuated with each intervening o -bond. The electronic polarization within /r-bond systems as measured by the resonance or mesomeric effect, on the other hand, extends across an entire nr-system without any attenuation. The simple model of an electron in a box expresses this fact. Thus, in calculating the charge distribution in conjugated i -systems an approach different from the PEOE method has to be taken. 

This qualitative theory still provides the most widely used means for describing reactions in organic chemistry. Two principal modes of electronic interaction in organic molecules are recognised the inductive and mesomeric effects. 

The 3-, 4-, 5- and 6-positions in the pyridazine nucleus are electron deficient due to the negative mesomeric effect of the nitrogen atoms. Therefore, electrophilic substitution in pyridazines is difficult even in the presence of one or two electron-donating groups. The first reported example is nitration of 4-amino-3,6-dimethoxypyridazine to yield the corresponding 5-nitro derivative. Nitration of 3-methoxy-5-methylpyridazine gives the 6-nitro-,... 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

DSP treatments allow one to separate the field and mesomeric effects of substituents on chemical reactivities and physical properties (electronic and NMR spectra, etc.) of organic compounds. In Section 8.3 we will discuss heterolytic dediazoniation of substituted benzenediazonium ions. For this series of reactions the classical Hammett equation completely fails to give useful results (see Fig. 8-1), but the DSP treatment yields a good and mechanistically very meaningful correlation. 

The Taft relation Ej 2 = P 2a + x, which was found to hold for organic compounds and some transition metal complexes can also be of use here (37). Phenyl compounds do not fit the relation. This is probably due to a mesomeric effect that depends on the dihedral angle between the phenyl and the NCS2 planes. For bulky substituents deviations are also found which could be caused by widening of the CNC angle, changing the hybridisation of the N. The low values of p indicate that the M.O. s involved in the electron transfer have little ligand contribution. 

Figure 7.13. Top and center Line structure and ORTEP representations of carbenes 26 and 27. Bottom N,B-heterocyclic carbenes (NBHCs) showing the competition between the N-C Vi. N-B electronic interaction, that is mesomeric effect (the former is preferred). ORTEP representations adapted from references 85 and 86.

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. 

Mesomeric, like inductive, effects are permanent polarisations in the ground state of a molecule, and are therefore manifested in the physical properties of the compounds in which they occur. The essential difference between inductive and mesomeric effects is that while inductive effects can operate in both saturated and unsaturated compounds, mesomeric effects can operate only in unsaturated, especially in conjugated, compounds. The former involve the electrons in a bonds, the latter those in tt bonds and orbitals. Inductive effects are transmitted over only quite short distances in saturated chains before dying away, whereas mesomeric effects may be transmitted from one end to the other of quite large molecules provided that conjugation (i.e. delocalised tt orbitals) is present, through which they can proceed. 

The change is also pronounced with C=0, for not only is the nitrogen atom, with its electron pair, bonded to an electron-withdrawing group through an sp2 hybridised carbon atom (c/ p. 59), but an electron-withdrawing mesomeric effect can also operate ... 

With substituents such as OH and OMe that have unshared electron pairs, an electron-donating, i.e. base-strengthening, mesomeric effect can be exerted from the o- and p-, but not from the m-position, with the result that the p-substituted aniline is a stronger base than the corresponding w-compound. The m-compound is a weaker base than aniline itself, due to the electron-withdrawing inductive effect exerted by the oxygen atom in each case. As so often, the effect of the o-substituent remains somewhat anomalous, due to direct interaction with the NH2 group by both steric and polar effects. The substituted anilines are found to have related pAa values as follows ... 

Significant electron-withdrawal by a substituent to stabilise the anionic intermediate, e.g. (81), only occurs through a mesomeric effect, i.e. when the nitro group, for example, is o- and/or p- to the leaving group. Thus we observe the reactivity sequence ... 

The carbonyl carbon atom of (162) is, however, more reactive towards nucleophiles than that of the original ester (161), because of the electron-donating mesomeric effect, in the latter, of the ester oxygen atom ... 

A polarographic study of 4,4-diacyl triafulvenes290 showed that both oxidative and reductive processes may occur, reduction being somewhat favored over oxidation due to mesomeric effects of the acyl grouping [one-electron reduction —1.2 to —1.3 V (475) one-electron oxidation +1.6 to +1.75 V (476)]. 

Some substituents induce remarkably different electronic behaviors on the same aromatic system (8). Let us consider, for example, the actions of substituents on an aromatic electron system. Some substituents have a tendency to enrich their electronic population (acceptors), while others will give away some of it (donors). Traditionaly, quantum chemists used to distinguish between long range (mesomeric) effects, mainly u in nature, and short range (inductive) effects, mainly a. The nonlinear behavior of a monosubstituted molecule can be accounted for in terms of the x electron dipole moment. Examples of donor and acceptor substituents can be seen on figure 1. 

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. 

M Group viz.,p-N02 (an electron withdrawing function)—its presence as shown in (d) above will further increase frequencies due to decreased mesomeric effect. 

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