Meta substituent effects

No similar meta substituent effect exists for ions since resonance between a singlet and triplet (diradical) state is prohibited. The dipolar ion theory would suggest that suitably disubstituted radicals should be especially stable, particularly in polar solvents, because of structures like III. 

These and other studies of the relative substituent effects of X and CH X in nitration were considered in terms of the transmission factor a of the methylene group. To avoid complications from conjugative interactions, attention was focussed mainly on substitution at the meta-position, and ct was defined in terms of partial rate factors by the equation ... 

The nucleophilic reactivity of 2-halogenothiazoles is strongly affected by the substituent effect, depending on the kind of substitution reaction. Positions 4 and 5 can be considered as meta and para , respectively, with regard to carbon 2 and to groups linked to it consequently, it is possible to correlate the reactivity data with Hammett s relationships. 

Our analysis of substituent effects has so far centered on two groups methyl and triflu oromethyl We have seen that a methyl substituent is activating and ortho para directing A trifluoromethyl group is strongly deactivating and meta directing What about other substituents ... 

Table 19 3 lists the ionization constants of some substituted benzoic acids The largest effects are observed when strongly electron withdrawing substituents are ortho to the carboxyl group An o nitro substituent for example increases the acidity of benzoic acid 100 fold Substituent effects are small at positions meta and para to the carboxyl group In those cases the values are clustered m the range 3 5-4 5... 

Since substituent effects in aliphatic systems and in meta positions in aromatic systems are essentially inductive in character, cr and cr values are often related by the expression cr = 0.217cr — 0.106. Substituent effects fall off with increasing distance from the reaction center generally a factor of 0.36 corresponds to the interposition of a —CHj— group, which enables cr values to be estimated for R—CHj— groups not otherwise available. 

Physical and chemical evidence supporting the theory mentioned in the foregoing will be given in the appropriate sections. Some predic-. tions may be mentioned here. From resonance structures (2, 3, 7, 8) it is inferred that the substituent effect in 2-substituted thiophenes should be parallel to that in the corresponding benzenes, the 3- and 5-positions may be considered as ortho and para positions and the 4-position as a meta position. It is, however, obvious that the effect of a —M- and a d-M-substituent are not simply reversed, as reso-... 

In spite of the potential complexity of the general problem, even when restricted to the reagent family of amines, the nucleophilicities of such series as meta- and pom-substituted pyridines and anilines appear to correlate very closely with the expected substituent effects and with the basicities. This has been verified in the following cases (i) The reaction of pyridines (R = H, m- andp-CHs) with 2-chloro-3-nitro-, 2-chloro-5-nitro-, and 4-chloro-3-nitro-pyridines. ... 

Studies on substituent effects are scanty. For the five-membered monocyclic nuclei, the relative positions have been compared to the classic ortho, meta, and para positions of benzene derivatives according to formulae 60 and 61. This comparison is based on the assumption... 

For the uncatalyzed reactions the calculations showed that the ortho approaches were favored over the meta, and the endo selectivity was the energetic most favorable reaction paths for most of the electron-donating substituents studied [29]. The endo-ortho reaction path is under FMO control and the substituent effect on the regios-electivity was explained for by a dominant interaction between LUMOdiene id HOMOdienophUe- The ortho reaction path was investigated with BH3 as the Lewis acid and it was calculated that the presence of Lewis acid decreases the activation... 

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

The effect of meta substituents has been studied much less, but it has been reported that here too, electron-withdrawing groups increase the rate See Nurgatin, V.V. Shamin, G.P. Ginzburg, B.M. J. Org. Chem., USSR, 1983, 19, 343. 

According to earlier considerations, the effects of m-substituents have been considered generally to be of the sigma zero type. Swain and Lupton have questioned this conclusion. We have proceeded in the following manner to obtain evidence on the issue. From all the known data, 11 sets of meta substituent reaction series data were found which meet the minimum substituent criterion for basis sets. These reactions are listed in Table XI, with the pT .Pr. ... 

The ffg parameter scales of Table V appear to be applicable to substituent effects at the ortho and meta positions. For the latter position, however, the data generally are not capable of discriminating between Og scales (cf. earlier results and discussion). For the ortho position, the great difficulty is in obtaining a data set covering the full range of electronic properties without the incursion of substantial proximity effect (2a) contributions. In a following section are reported some of the results of treatment of ortho data sets by eq.(l). 

Vemula, P.K., Mallia, V.A., Bizati, K. and John, G. (2007) Cholesterol Phenoxy Hexanoate Mesogens Effect of meta Substituents on Their Liquid Crystalline Behavior and in Situ Metal Nanoparticle Smtesis. Chemistry of Materials, 19, 5203-5206. 

Figure 5. Effect of side chain substituents on polymer crystallinity. Polymers with two para substituents (second row) are more crystalline than polymers with mixed para and meta substituents.

There also seems to be an unexpected effect of meta substituents. Lichtin and Glazer have suggested, since purely radical structures can not involve meta substituents, that combined radical and dipole structures are important 20... 

A classical Hansch approach and an artificial neural networks approach were applied to a training set of 32 substituted phenylpiperazines characterized by their affinity for the 5-HTiA-R and the generic arAR [91]. The study was aimed at evaluating the structural requirements for the 5-HTiA/ai selectivity. Each chemical structure was described by six physicochemical parameters and three indicator variables. As electronic descriptors, the field and resonance constants of Swain and Lupton were used. Furthermore, the vdW volumes were employed as steric parameters. The hydrophobic effects exerted by the ortho- and meta-substituents were measured by using the Hansch 7t-ortho and n-meta constants [91]. The resulting models provided a significant correlation of electronic, steric and hydro-phobic parameters with the biological affinities. Moreover, it was inferred that the... 

Substituent effects on the 170 chemical shifts in meta- and para-substituted nitrobenzenes are presented in Table 13, showing that these shifts are quite sensitive to the nature of the substituent, and range over nearly 40 ppm. Examination of Table 13 shows that the direction of substituent effects is different for 15N and 170 shifts, with electron-withdrawing substituents (such as NO2, CN or CF3) causing upheld 15N and... 

Photolysis of 4- and 3-nitrophenyl acetates (176 —> 177 178 —> 179) in neutral aqueous solution leads to the corresponding phenols with quantum yields 0.002 and O.OO6105 (equation 84). A greater difference in the photoreactivity (quantum yields of 0.002 and 0.129, respectively) is shown between 2-mcthoxy-4-nitrophenyl acetate 180 and 2-methoxy-5-nitrophenyl acetate 182. The nitro substituent clearly exhibits a meta-activating effect in the hydrolysis of phenyl acetates. 

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