Pi-electron systems

As a second example of a 4N-pi-electron system, we shall consider the pi dication of 1,2-difluoroethylene which can exist in a cis and tram geometry. The dissec-... 

In aromatic systems, the Lewis acids which activate via coordination are also capable of activating the aromatic system by the formation of a and ir complexes. There are a sufficient number of examples available to indicate that the activation via the latter processes is the more important of these, where all are present. Olivier (52) showed in 1913 that the kinetic behavior of such reactions consists of two portions. When the catalyst, say aluminum chloride, is present in less than the amount required to complex all the functional groups, the reaction is relatively slow and the catalytic activity is due to the small amount of Lewis acid resulting from the dissociation of the complex. As soon as all the functional groups are coordinated, any additional Lewis acid is found to accelerate the rate enormously. In these electrophilic substitutions it seems highly probable that the the activation involves the pi electron system of the benzene ring. Olivier studied the reaction sequence ... 

Waals interaction between chains is unlikely to be larger than that between the pi electron systems of the naphthalene rings occupy considerably smaller limiting areas (21). 

The spectrum of naphthalene is shown above. Its broad bands reflect the absorption of energy by the extended pi-electron system. 

It is known many organic compounds containing pi electron systems exhibit remarkably large nonlinearities. 

Because the unshared electron pair on the nitrogen is not part of the aromatic pi electron system, pyridine is a much stronger base than pyrrole. Recall that pyridine is used as a basic solvent in a number of reactions. 

Proton Magnetic Resonance. The proton resonance shift accompanying H bond formation is toward lower fields (except when the base is an aromatic pi electron system). This implies a shift in the direc-... 

Lack of response to water and carbon disulfide allows these to be used as solvents without interference with the analysis Good for electronegative compounds such as those containing extensively conjugated pi-electron systems, nitro- and highly conjugated aromatic compounds and compounds which can be readily derivatized to respond. " Response depends also upon carrier gas flow rate... 

Mobius aromaticity A monocyclic array of orbitals in which a single out-of-phase overlap (or, more generally, an odd number of out-of-phase overlaps) reveals the opposite pattern of aromatic character to Hiickel systems with 4n electrons it is stabilized (aromatic), whereas with 4n + 2 it is destabilized (antiaromatic). In the excited state 4n + 2, Mobius pi-electron systems are stabilized, and 4n systems are destabilized. No examples of ground-state Mobius pi systems are known, but the concept has been applied to transition states of PERI-CYCLIC REACTIONS (see AROMATIC [3]). 

It is apparent that the methods discussed here are particularly effective for molecules whose MO s are largely determined by their point group symmetry because, in these cases, central AO s are good MO templates. Clearly, there are other types of molecules in which central AO s are less suitable and other simple orbitals are more effective templates. For general pi-electron systems, for example, free-elec-tron MO s would be more useful GO s (14). In its broadest sense, then, the generator orbital approach is based on the principle that major characteristics of complicated molecular orbitals can be semi-quantitatively anticipated by conceptual analogy to simpler orbitals with which they share essential physical features. We feel that there is an elucidating quality in this reduction of the more complex to the less complex, which is of value to theorists and non-theorists alike. 

The resonance stabilization of benzene is commonly estimated to be about 36 kcal/mol (151 kJ/mol), but this is not an experimentally verifiable number. The 4n pi electron systems (4, 8,12, 16,...), with half-filled energy levels, not only are less stable than the 4n + 2 systems but are destabilized relative to their open-chain analog and are called antiaromatic. 

Since tautomer II is favoured, the guanidine group does in fact bear a close structural similarity to the thiourea group. Both groups have a planar pi electron system with similar geometries (equal C-N distances and angles). They are polar and hydrophilic with high dipole moments and low partition coefficients. They are weakly basic and also weakly acidic such that they are un-ionized at pH 7.4. 

The compound predicted to have the second greatest potential for swelling nitrile rubber was 2-naphthol. Its optimized interaction with isobutyronitrile is shown in Figure 3. The position of the -OH group in 2-naphthol precludes the simultaneous occurrence of the phenolic hydrogen-nitrile interaction and the much weaker methyl-hydrogen aromatic pi-electron system interaction. 

The substituted dinitroanlline herbicides absorb well into the visible region due to extended pi-electron systems. The Xmav for each (Table I.) is in the 380-400 nm range where extinction coefficients (logio) are 3.0-3.4 for the pi to pi electronic transition. These electronic spectra afforded excellent chromo-phores for absorption of the emitted black-light radiation. 

Zeise s salt /zy-sez, tsy-/ A complex of platinum and ethane (ethylene) first synthesized by W. C. Zeise in 1827. It has the formula PtCl3(CH2CH2) and was the first complex in which the metal ion coordinated to a pi-electron system rather than to individual atoms. 

When visible light encounters this pi electron system, the colour seen is a result of the degree of absorbance of the light by the pi electron system of the dyestuff. 

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