Mesomeric effects substitution

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-,... 

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). 

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. 

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 ... 

For dinitro-substituted substrates, it has been recently shown that activation in 2,4-dinitrophenyl substrates is mainly due to the mesomeric effect of the 4-nitro group, thus reducing the electron density at the reaction site83. However, another important feature of highly polarizable groups, such as a nitro group in the ortho position, has been considered to be the field effect101. 

A plethora of substituted pentafluorobenzenes has been utilized in substitution reactions and the substitution pattern of the products has been thoroughly investigated.120 In most cases the inductive effect of the attached fluorine atoms is stronger than a possible mesomeric effect... 

The differences in the value of the dipole moments of the complexes [M(RCS=CHCOCF3) ] (R = Ph or X-substituted phenyl) depend upon (a) the magnitude and vector direction of the Ph—X bond moments (b) the inductive effect arising from the difference in electron density of the C-l and C-5 atoms of the monothio-jS-diketone (this is affected by the nature and position of the substituent X on the phenyl ring) (c) the change in moment brought about by the mesomeric effect (56) of X.230-231... 

Thiophenol is an aromatic compound (Table 6). One hydrogen atom in its benzene core is substituted for a SH-group. The SH-group has a weak negative induction and a positive mesomeric effects. In addition, the unshared pair of -electrons of a sulfur atom contributes to the 7i-electron cloud of the benzene ring. 

With respect to a-substituents bearing p- or 7r-electrons which are directly attached to the C—Cl bond (Table 6, Z = CH2=CH to CH3CH20), these may delocalize their electrons through resonance or mesomeric effects with the positively charged carbon atom in the transition state. Because of this, they were not plotted in the Taft figure for a-substituted ethyl chlorides. Furthermore, the rates for these substituents also could not be correlated with the electrophilic substituent constants a+. The o+ parameters have been defined for substituents on the benzene ring which are far from the reaction site. Even though steric effects may interfere with the coplanarity and hence with delocalization, the effect of these substituents was believed to be polar in nature. 

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