Pi-electron density

It is interesting to note in the latter connection that nucleophilic substitution transition states in which there apparently is not strong delocalization of pi electron density into the substituent tend to fall into the <7 type (cf. reactions 18 and 19 of Table VIII). In set 18, there are two ortho nitro groups which apparently take up much of the pi charge (thus it is unavailable to X), whereas in set 19, the positive piperidinium center may cause (perhaps with assistance from the NH hydrogen bonding permitted by the aprotic solvent) the... 

The behavior of pyridine in substitution reactions can be understood on the basis of its resonance structures (la-d) and on the basis of the electron-density distribution at the various ring positions as derived from molecular-orbital-theoretical calculations, An example of the published pi-electron density distribution is shown in II, The resonance energy of pyridine is 35 kcal/mole (versus 39 kcal/mole for benzene). 

Thiophene undergoes a considerable number of electrophilic substitutions, some with greater ease than with benzene as might be expected from the increased pi-electron density in the ring. Substitution takes place at the 2-position. Some examples of the special conditions found useful for thiophene substitution are listed below. 

Adcock and his associates have examined these results in detail. They conclude that the new observations do not permit an unequivocal decision concerning the origin of the influence of the trifluoromethyl group. However, they do argue for a significant pi electron interaction. Their suggestion is based on several lines of argument. First, they point out that the shift for the fluorine in the 4/8 compound is —0.09, an unusual result, which they infer arises from the induction of alternate pi electron density at the positions meta to the trifluoro-... 

Chemical shifts have been used to evaluate pi electron densities for several 2-, 4-, and 6-alkyl- and 2-, and 6-aminomethyl-3-pyridinols. Reasonable agreement is observed in a comparison of these charge distributions and those obtained by MO LCAO calculations. ... 

Electrostatic potential map of benzene illustrates the pi electron density (red-yellow) of the aromatic ring. 

Figure 14.56 Pi electron density map for benzene (in the plane of the molecular It orbital)... 

Pi Electron density of phase i (i = c crystalline phase, i = a amorphous phase)... 

---