Aromatic stabilization energy internal

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

It will be seen that the aromatic energy of naphthalene (4dp) is double that of the monocyclic systems (2dp) naphthalene should therefore be a bicyclic aromatic system, and its chemistry is consistent with this. Phenyl-butadiene and [10]annulene are predicted to be similar in stability in practice [10]annulene is unstable because it cannot exist in a planar geometry, due to mutual interference by the central hydrogen atoms (18). The internal methylene derivative (19) of (18) is, however, stable, being almost planar. 

This did not need to be so. Quantum-mechanical explanations have sometimes been applied erroneously to systems where solvation is the dominating influence, rather than internal electron delocalization. As a case in point, consider the classical examples of the low basicities [ AGf(H20)] of aniline and pyridines compared to ammonia in aqueous systems. For a generation students have been led to understand that the two aromatic bases are weaker than ammonia because of resonance and hybridization factors. Initial examination of Table 1 leads to the refreshing conclusion that the facts are otherwise in the gas phase both aniline and pyridine are much more basic than ammonia, therefore the elementary interpretation of the basicity order for these three compounds is false. However, further examination of the data shows that both aniline and pyridine really are weakly basic if proper non-aromatic models are used. In view of the enormous additive ability of C—C and C—H bonds to stabilize positive charges it is appropriate to compare aniline and pyridine with their alicyclic cognates of similar carbon number, instead of with ammonia. When this is done we see that the difference in AG between cyclohexyl-amine and aniline is 9.6 in the gas phase and 8.2 in water between N-methylpyrrolidine and pyridine it is 8.3 in the gas phase and 7.2 in water. These results isolate clearly the internal-energy terms which are to be interpreted if the correct model is used, and furthermore we... 

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