Hiickel 4/ + 2 rule explanation

The annulenes are that series of monocyclic polyolefins (C H ) containing a complete system of contiguous double bonds. While benzene (the best known member of this class of compounds) has been in evidence for some time it is only of late that interest in the higher members has become apparent. This interest has its origins in the LCAO-MO theory of re-elec-tron systems as formulated by E. Hiickel (in particular the "Hiickel rule relating aromatic stability to structure). Although the non-classical chemistry of the benzenoid hydrocarbons had previously been the subject of some conjecture, Httckel s theoretical studies provided the first satisfactory explanation of the peculiar stability of this class of compounds and, incidently, the elusiveness of cyclobutadiene. 

So apparent confusion about what is aromaticity is not due to a lack of explanation of aromaticity in terms of the presence, or lack of the presence, of conjugated circuits of 4 -I- 2 and 4n size in the set of Kekule valence structures of compounds, but due to (i) lack of attention given to articles describing conjugated circuits aud their use for characterization of aromaticity and (ii) not recognizing that these explanations, in fact, represent a generalization of the famous Hiickel rule to polycycUc systems, which Hellwinkel recognized just as the paper was published, almost 40 years ago. 

We do not need to look far to find the explanation. The four 71 electrons of cyclobutadiene do not satisfy the Hiickel rule. Cyclobutadiene is extremely unstable and has been isolated only at very low temperature. Its fleeting existence also has been inferred from the products of its reactions. Cyclooctatetraene has eight 71 electrons. This number that does not satisfy Hiickel rule either. Cyclic polyene that have 4 71 electrons, where is an integer, are unusually unstable they are antiaro-made. 

The first theoretical explanation - based on quantum mechanical approach - of the aromatic character of a molecule was given by physical chemist E. Hiickel in 1931. The Hiickel s rule... 

The basic concepts of this theory were the mutual repulsion consequent upon the interaction of two electro-chemical double layers, and the attraction by the London—Van der Waals forces. The principal facts of stability could be explained by combining these two forces. Among other things, a quantitative explanation of the rule of Schulze and Hardy has been given. For this purpose it was essential to use the unapproximated G o u y—C h a p m a n equations for the double layer. The approximation of Debye and Hiickel, however useful in the theory of electrolytes, appears to have only a very limited applicability in colloid chemistry. 

Another explanation has been proposed by K. Fukuii on the basis of frontier molecular orbitals (HOMO—LUMO) of the substrates this method is known as the frontier molecular orbitals (FMO) method. Alternatively, the PMO theory based on the Woodward—Hoffmann rule and Hiickel-Mobius method is also used to explain the results of pericyclic reactions. 

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