Substituent effects, inductive mesomeric

Electronic effects Changes in the absorption frequency for a particular group take place when the substituents in the neighbourhood are changed. The frequency shifts, in these cases may be due to inductive, mesomeric or due to field effects and there will be combined effect due to all. The effect of one can only be estimated approximately. 

These p/Tg-values show that the inductive effect of the C=C bond accounts for about one half of the reduction in the basicity of the enamine nitrogen atom, the other half being due to the mesomeric effect (Stamhuis et al., 1965). The localization energy of the lone pair on the nitrogen, calculated from the second Ap/iTa, is only about T6-1-7 kcal mole (6 7-7 l kj mol ). There are indications from the substituent effects on the stretching frequency of the C=CH2 bond that this group is an electron sink of the same kind... 

The mechanisms of substituent effects are numerous23 the most prominent being inductive, mesomeric, neighbor anisotropy, ring-current, electric-field and steric effects, as well as van der Waals compression. 

Complexes of platinum(II) with /J-monothiodiketonates, derived by deprotonation of the parent acid (207), can be prepared from PtCl -. The dark red complex Pt(C3H(Ph)2SO 2 (R = R = Ph) shows IR bands at 1535 cm-1 [v(C=C)], 1410 cm-1 [v(O O)] and 1270 cm-1 [v(O S)].1824 Electronic spectra and dipole moment data for these complexes have been compared with the O.O -diketonate complexes.182s 1826 The structure of the phenyl derivative has been confirmed by X-ray crystallography.1827 Detailed dipole moment measurements using static polarization have been made with fluorinated jS-monothiodiketone complexes. Variations with substituent depend on the magnitude and vector directions of the Ph—X bond moments (aryl substituents), the inductive effect of the meta and para substituent on the phenyl ring, and the mesomeric effect of the substituent X.1828 A useful separation method for bis(monothiotrifluoroacetylacetonates) of platinum(II) is gas chromatography.1829... 

We are left with the problem of substituent effects that do not depend on direct mesomeric interactions between the substituent and the reaction center. Effects of this kind can arise in one of two ways. First, the bond between the substituent and the substrate may be polar, and there may also be polar bonds or charged atoms in the substituent itself the charges set up in this way can influence the reaction center either by altering the effective electronegativity of atoms connected with it (inductive effect) or by direct electrostatic interaction across space (field effect). Secondly, the substituent may be attached to a conjugated system which does not itself take part in the reaction, the case exemplified by the Hammett equation (Eq. (105)) here it... 

Most organic compounds that show absorption in the visible or in the near-UV region have a linear or cyclic n system as the chromophoric system. Therefore, the results of the previous sections may be used and extended to discuss light absorption of all those compounds that can be derived from linear and cyclic hydrocarbons by including the influence of substituents in an appropriate way. (Cf. Michl, 1984.) A complete theory of substituent effects comprises all areas of organic chemistry. Here, only the fundamental concepts of the influence of inductive and mesomeric substituents will be considered. In order to simplify the discussion, substituent effects will be called inductive if in the HMO model they can be represented by a variation of the Coulomb integral of the substituted n center p. If they are due to an extension of the n system they will be called mesomeric. 

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. ... 

The additive natures of electronic and steric factors are obvious in the methylation, ethylation, and benzylation of 4-nitroimidazole (see Table 8). It seems that the dominant electronic effects of substituents are inductive since mesomeric effects affect electron density both a and y to the substituent. Inductive effects will mainly be felt on the nitrogen adjacent to the substituent (R ). Hammett correlations are also much better with CTm or u, than Cp constants <94H(37)735>. When alkylations were carried out in DMF with potassium carbonate as base, again electron-attracting... 

In the following few chapters the electronic spectra of various types of molecules have been discussed. The electronic spectra of molecules are modified b> substitution with different groups. Both electronic and stcrie interactions of groups are important in electronic spectra. The term, electronic interaction, includes both inductive and resonance (mesomeric) interactions, although the influence of the latter may be more prominent. The substituent effects on the spectra of different chromophores and molecules will be discussed appropriately in the following chapters. Ster e effects on electronic spectra have been dealt with separately in Chapter S. 

Substituent effects in R02-, where R is benzyl or substituted benzyl, have only small effects on 2kt, largely unrelated in direction to mesomeric or inductive effects of the substituents ring-substituted styrenes [127] have 2kt = (3.88 1.24) X 1071 mole-1 s"1 a-substituted toluenes have 2kt = (10.3 10.1) X 1071 mole"1 s 1 [128], Among f-R02-, increasing bulk in R leads to higher values for 2kt which range from 1.3 X 1031 mole-1 s"1 for t-butyl to 6 X 104 1 mole"1 s 1 for 1,1-diphenylethyl [129],... 

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... 

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