Electromeric substituent

Electromeric substituents (i.e. those containing pom electrons and able to interact with adjacent conjugated systems) can be divided into three classes ... 

Substituents which conform to these conditions are called electromeric (E) substituents. This term carries over from earlier theories of organic chemistry in which the n interactions of these substituents were described as the electromeric effect, ( resonance active has also been used to describe these substituents). Vinyl (29), phenyl (30), formyl (31), and amino (32) are typical electromeric substituents. Substituents which do not conform to the conditions indicated above, and which do not therefore alter the topology of an adjacent n system, are called inductive (/) substituents. Such substituents (e.g., —CH3, - p3, —CH2CH2CH3) can influence the reactivity of an adjacent conjugated system only by the Tu-inductive effect or by direct electrostatic interactions across space, a phenomenon discussed later in this chapter. 

These arguments are based on the assumption that the transition state is truly intermediate in structure between the reactants and products. If this is not true, the arguments fail and so does the BEP principle. The main case when this happens is when electromeric substituents are attached to a carbon atom which is tetrahedral and saturated in both reactants and products but has sp hybridization and forms part of a delocalized system in the transition state. Compare, for example, the 5 2 reactions of iodide ion with benzyl chloride and with a saturated alkyl chloride RCH2CI, where R is an alkyl group,... 

Electromeric substituents of all kinds stabilize radicals if they are attached to active positions. All such substituents are consequently ortho and para directing, regardless of whether they are -f , — , or E, The rules for orientation, etc., are identical with those for electrophilic substitution and therefore need not be elaborated. 

For the purpose of light absorption, electromeric substituents can be divided into two types. Those ( , -h ) with both filled and empty n MOs that can interact with n MOs of the substrate, and simple — substituents (NH3, OCH3, etc.) that have a pair of p electrons. 

The second way in which the substituent R affects the charge distribution of the molecule is called the resonance effect (or sometimes the tautomeric or electromeric effect). This results when the molecule resonates among several electronic structures. For example, for aniline the structures... 

The influence of substituents on the catalytic oxidation of toluene was investigated by Trimm and Irshad [330]. Toluene, chlorotoluenes and xylenes were oxidized over a M0O3 catalyst at 350—500° C. Partial oxidation products are aldehydes, acids and phthalic anhydride (in the case of o-xylene). Unexpectedly, both xylenes and chlorotoluenes are oxidized faster than toluene. The authors conclude that apparently the electromeric effect of the chlorosubstituent is more important than its inductive (—I) effect. The activation energies of the xylenes and chlorotoluenes all fall in the same range (17—18 kcal mol"1), while a much higher value is reported for toluene (27 kcal mol 1). 

Thus, one can state that an inductive effect of the substituents is distributed uniformly throughout the whole macrobicyclic system, and, unlike the well-studied substitution in aromatic systems, there is no pronounced ortho, para, and meta effects of the substituents in clathrochelates. A key role in the transfer of electromeric interactions in a clathrochelate molecule is played by the encapsulated central metal ion, rather than a a, r-bond system. Therefore, the electron interactions in the clathrochelate molecule are distributed isotropically rather than alternately [69]. 

Resonance effect is an energy stabilization due to delocalization of electrons in the bond network of the molecule and can be attributed to a mesomeric effect, i.e. the delocalization of Jt electrons on the jr orbital network, a hyperconjugation effect, i.e. a delocalization of a electrons in a ji orbital aligned with the o bond, and secondary mesomeric effects, such as repulsion of the ir electrons by nonbonded electrons on a substituent or solvent, or by time-dependent effects due to polarizabilities (for the last, the term electromeric effect is sometimes used). 

The term has been deemed obsolescent or even obsolete (see mesomeric effect, resonance effect). Many have used phrases such as enhanced substituent resonance effect that imply the operation of the electromeric effect without using the term, and various modern theoretical treatments parametrize the response of substituents to electronic demand, which amounts to considering the electromeric effect together with the INDUCTOMERIC EFFECT. 

Strictly understood, the mesomeric effect operates in the ground electronic state of the molecule. When the molecule undergoes electronic excitation or its energy is increased on the way to the transition state of a chemical REACTION, the mesomeric effect may be enhanced by the electromeric effect, but this term is not much used, and the mesomeric and electromeric effects tend to be subsumed in the term RESONANCE effect of a substituent. 

Having demonstrated the concept in principle, what remained was the selection of a substituent which produces a shift of the desired magnitude. For this one needs ideally a table of appropriate substituent constants for use in the prediction of the magnitude of induced spectral changes. To be generally useful, these constants should be relatively insensitive to the inductive nature of the substituent and reflect only the electromeric contribution of the group. 

From the earlier discussion on the nature of the transition state for E2 reactions, two salient factors affecting reactivity can be recognised, these being polar and steric effects. The polar effect can be divided into inductive and conjugative or electromeric components . The influence of a substituent will depend principally on the nature of the transition state, which to a large extent is determined by the leaving group and the base and solvent. A reaction... 

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