Mesomeric effects examples

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-,... 

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. 

The reactions and compound presented in this chapter support the notion that the formation of o-QMs from the parent phenols is a quite complex process. In the case of the oxidation by Ag20 but also likely in other oxidations, a zwitterionic intermediate is involved that can be stabilized intermolecularly, for example, by electrostatic interaction with other suitable zwitterions, or intramolecularly by neighboring groups or inductive/mesomeric effects. By stabilizing the zwitterionic intermediate and destabilizing the o-QM, the energetic gap between these two intermediates is lowered and... 

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 ... 

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. 

The boron-oxygen mesomeric effect described in the previous section explains the lower reactivity of allylic boronates towards carbonyl compounds compared to that of allylic boranes. The use of Lewis acids, however, allows boronate derivatives, including hindered ones, to react at temperatures comparable to the analogous boranes. As described above (see section Mechanism and Stereochemistry ), the most reactive allylic boronates are those with the most electrophilic boron centers.The nucleophilicity of the y-position of an allylic boron reagent (the position that forms the new C-C bond with the aldehyde) is also important to the reactivity of the reagent. For example, allylic boronates with... 

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],... 

The discussion given earlier for weak mesomeric effects offers an explanation for the empirical increment rules proposed by Woodward (1942) for estimating the absorption maxima of dienes and trienes. The distinct values for the homo-and heteroannular arrangement of the double bonds illustrate the configurational influence on the polyene spectra that has been discussed in Example 2.2. From the data collected in Table 2.5 the absorption of methylenecyclo-hexene (27) with an exocyclic double bond and two alkyl substituents is calcu-... 

The polarographic reduction of benzo[h]quinolizinium salts varied with pH. At pH 1.9 to 7 two waves were observed, the first (half-wave —0.807 V) a one-electron reversible reduction, the second (half-wave —1.022 V) irreversible and diffusion controlled. The proposed pathway is shown in Eq. (46). The reduction in basic medium also had two waves (half-wave potentials about — 1.5 and — 2.2 V) due to the reduction of aldehyde 103, a known product of nucleophilic attack at position 6 in benzo[h]quinolizinium salts. The changes in reduction potential produced by substituents on the benzo-[hjquinolizinium ion were examined (19 examples).123 Various explanations based on inductive and mesomeric effects were offered for observed variations. 

The size as well as the electronic properties (i.e. inductive and mesomeric effects) of the surrounding groups affects the stability of carbocations, carbanions and radicals. When bulky substituents surround a cation, for example, this reduces the reactivity of the cation to nucleophilic attack by steric effects. This is because the bulky groups hinder the approach of a nucleophile. 

This can be understood from the inductive and mesomeric effects of the electronegative substituent. For example, the +M effect of Cl is much weaker than that of the NH2 group because the lone pair on chlorine does not interact well with the 2p-orbital on carbon (see Section 13.2.2). Therefore, whereas Cl withdraws electrons from the carbonyl group, the NH2 group mesomerically donates them. 

Unsymmetrically substituted diazines can give rise to two isomeric quaternary salts. Substituents influence the orientation mainly by steric and inductive, rather than mesomeric effects. For example, 3-methylpyridazine alkylates mainly at N-1, even though N-2 is the more electron-rich site. Again, quater-nisation of 3-methoxy-6-methylpyridazine takes place adjacent to the methyl substituent, at N-1, although mesomeric release would have been expected to favour attack at N-2. ... 

The simple two-parameter equations (1) and (2) hold if the substituents influence the reaction center only by a single independent interaction. Such single interaction need not necessarily be formed by a single interaction mechanism but can result from a combination of two or more interaction mechanisms in constant proportions. For example, structural effects of substituents can be classified as inductive, mesomeric, and sterie. The inductive and mesomeric effects operate... 

A similar pattern of results was obtained for complexes containing a variety of substituted pyridines as ligands. However, a number of significant differences have been found which seem to support the above interpretation. For example, for L = isoquinoline the results are consistent with the view that isoquinoline forms weaker o -bonds to the metal than does pyridine but that it is better able to accept back-coordinated electrons in a 7r-bond. Similarly, marked differences in the data obtained for complexes with 3-alkylsubstituted and 4-alky Substituted pyridines suggest that the former amines are poorer ir -acceptors, a conclusion in accord with publShed information on the mesomeric effects of 3- and 4-substituents and the calculated electronic charge densities at different ring positions (2). 

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