Electromeric effects

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... 

Resonance or electromeric effects. Certain molecular structures are characterized by the possibility of having two or more compatible electronic structures and the molecules exist in a resonance state intermediate between the several extremes. These effects are particularly characteristic of aromatic structures and other molecules containing conjugate double bonds. 

In his 1933 paper on "tautomerism," Ingold began discussion with references to the physics of the electron, citing a 1923 paper by J. J. Thomson and a recent book by John H. Van Vleck.62 He noted Lewis s contributions (1923) to the notion of inductive effect ( ) in which electrons remain bound by their original atomic nuclei Lowry (1923) to the notion of electromeric effect, in which there is a displacement of a duplet, shifting from one pair of atoms to... 

As Ingold s version of the electronic theory developed, NH2, NR2 and NHAcyl groups were classified as —/ and +7" in their electronic effects, signifying electron attraction through the Inductive Effect and electron release by the Tautomeric Effect59. Later the latter was subdivided into the Mesomeric Effect M (polarization) and Electromeric Effect E (polarizability)60. The NO group was also classified as —I and +T in its electronic effect, while NO2 was classified as —I and —T. 

Eclipsed conformation, 7, 254 Electrocyclic reactions, 341, 344-348 Electrol ic oxidation, 307 Electrolytic reduction, 307 Electromeric effect, 24 Electron configuration, 3 Electron density, 21, 26, 29, 393 Electron-donating groups, 23, 26 addition to 0=C and, 183 addition to 0=0 and, 205, 206 aromatic substitution and, 153, 158 pinacol change and, 115 Electronegativity, 21, 22, 95 Electrons, lone pair, 10, 72 Electron spin, paired, 2, 308 Electron-withdrawing groups, 23 acidity and, 59, 61, 62, 272... 

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). 

Spectrophotometry shows that heterocyclic analogs of chalcones101 when protonated by 100% sulfuric acid dissolved in acetic acid form hydrogen bonds and reveals a positive dynamic conjugation (electromeric) effect which decreases in the series a-furyl > a-selenienyl > a-thienyl. 

The symbols K (conjugative effect) I (inductive effect) M (mesomeric effect) and E (electromeric effect) are employed throughout this chapter and follow the notation employed by C. K. Ingold in Structure and Mechanism in Organic Chemistry, 2nd Ed., Bell, London, 1969. 

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). 

Results and discussion. The most effective substance was found to be (among the S-compounds) diphenyl sulphide. On the other side, p-nitro-p-amino-diphenyl sulphide hcw d nearly complete lack of activity. In the group of substituted sulphoxides and sulphides, may be noticed the dependence of sensitizing efficiency on the electromeric effects in the molecule. At present status of the work it is imposible to predict whether this result has as its own basis an I - or M - effect. However, decrease of sensitizing ability of discussed compounds may also be connected with disturbance of coplanarity of photosensitizers structures. 

The sulfonation of halobenzenes is anomalous because although the halogen atom exerts a strong (-1) inductive (electronic withdrawal) effect with deactivation, sulfonation occurs in the ortho-position and para-position owing to the electron donation involving the electromeric effect (+E) from the lone electron pairs on the halogen atom in the presence of the reagent. 

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. 

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