Mesomeric effects benzene substituents

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. 

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). 

Inductive and electric field effects of the substituents may overlap the o-mesomeric effects. However, the m and p shifts of monosubstituted benzenes generally follow the pattern discussed above. 

Steric repulsions between substituents with n bonds and other groups attached to the benzene ring may twist the planes of both n systems as outlined in Sections 3.1.3.8 and 4.7.2 for o, o -dialkylated acetophenones. This sterically induced hindrance of conjugation is not only monitored by an increase in the carbonyl shifts but also by a shielding of the para benzenoid carbon due to an attenuated mesomeric effect of the carbonyl group. This... 

The competition of ortho and meta photocycloaddition is much more expressed when the mesomeric effects of the substituents are weak [30,31]. A more precise analysis of the products revealed that ortho and even para side products are formed in minor amounts in cases were normally the meta cycloaddition should be observed as dominant reaction [32]. Bichro-mophroric substrates carrying electron donor substituents on the benzene ring and any electron active groups on the alkene moiety range in this category [15,31,33]. 

Being electron donors, they donate /7-electrons to the benzene core what causes a-electron density in the ortho- and /7 ra-positions in the benzene core to increase. —N02, -CN, -CHO etc. fall in the second group. These substituents have a negative mesomeric effect (-M) (Fig. lb) and draw off Ji-electrons from the benzene core with the result that carbon atoms in the ortho- and /7 ra-positions become positively charged and electron density is localized at carbon atoms in the meta-positions. 

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. 

Because the transmissions of the former and the latter effects through the benzene and pyrimidine systems are different (Section IV,C), the contributions of the inductive and mesomeric effects of the substituents to the electronic effects of the respectively substituted phenyl and pyrimidinyl groups have also been assumed to be different. The transmissions of electronic effects of substituents through heteroaromatic systems are numerically expressed by means of the transmission factors Vi and Vr (Section IV,B). The products Viflphff (Y) and VR Ph RlY) can therefore serve as numerical characteristics of the contributions of the inductive and mesomeric effects of the substituent Y to the electronic effects of a substituted heteroaromatic group. Thus, for substituted pyrimidinyl groups, Eqs. (38) and (39) take the form ... 

In general, substituents are neither purely inductive nor purely mesomeric rather they possess some inductive as well as some mesomeric character. From Equations (2.19) and (2.24) it is seen that inductive as well as mesomeric effects on the HOMO and LUMO are proportional to the square of the LCAO coefficient at the substituted center g. It is therefore difficult to differentiate between these two effects from the absorption spectra. This is possible, however, by means of the MCD spectra that depend essentially on the difference AHOMO - ALUMO. (Cf. Section 3.3.) In the case of uncharged (4N-l-2)-eIectron perimeters and especially benzene, perturbations due to purely inductive substituents always yield AHOMO = ALUMO to first order in perturbation theory. Due to the energy difference in the denominator of Equation (2.24), however, AHOMO and ALUMO can be quite different in the case of mesomeric substituents, depending on the energy... 

Substituent Effects. - The C NMR substituent-induced chemical shifts (SCS) of aromatic carbons in 1,4-disubstituted benzenes were correlated with inductive and mesomeric effects, expressed in ai and qr (NMR) scales, and with charge alterations (Aq) provoked by introducing a new substituent (X) at position Cl. 105 different compounds with groups attached to Cl (X) and C4 (Y) in a sequence NMe2, NH2, OMe, F, Cl, Br, Me, H, CF3, CN, C(0)0Et, C(0)Me, NO2 and CHO were studied. ... 

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