The Hiickel 4n 2 rule

A7-Phenylsydnone, so-named because it was first studied at the University of Sydney, Australia, behaves like a typical aromatic molecule. Explain, using the Hiickel 4n + 2 rule. 

It is evident from the structure of the oxazol-5(4//)-one, also sometimes called an azlactone or oxazolinone, why it is so susceptible to epimerization. Removal of the a-proton generates a five-membered ring with six n-electrons, an aromatic system according to the Hiickel 4n + 2 rule (Scheme 5). 

C=C)—n or cumulene =( C )=K type. Larger clusters, C12-C20, were also found in which C, Cj5 and Cj9 were prominent this phenomenon was ascribed to the fact that these species might be cyclic as the electron counts in the ions conform to the Hiickel 4n + 2 rule. 

The 18 7r-electron structure has the stability of an aromatic ring according to the Hiickel 4n + 2 rule, while the others may be considered to be active like the semiquinone or quinone. So, when an oxidizing agent can be found to oxidize the aromatic molecule, a structure is formed which may abstract a hydrogen atom, by which it is reduced again. A mixture of nitrobenzene and cyclohexadiene-1,4 suited this purpose, and complexes of phthalocyanine and tetraphenylporphyrin were found to catalyze the oxidative dehydrogenation of cyclohexadiene by nitrobenzene t00> ... 

On reaction with acid, 4-pyrone is protonatecl on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hiickel 4n — 2 rule, explain why the protonated product is so stable. 

Molecular Orbital Description of Benzene 566 Aromaticity and the Hiickel 4n+2 Rule 567... 

In a formal sense, isoindole can be regarded as a IOtt electron system and, as such, complies with the Hiickel (4n + 2) rule for aromatic stabilization, with the usual implicit assumption that the crossing bond (8, 9 in 1) represents a relatively small perturbation of the monocyclic, conjugated system. The question in more explicit terms is whether isoindole possesses aromatic stabilization in excess of that exhibited by pyrrole. 

Let s look at some examples to see how the Hiickel 4n + 2 rule work... 

Ring Energy Content as a quantitative measure of fulfilling the Hiickel 4n+2 rule for derivatives of fuivene and heptafulvene... 

In their extended Hiickel calculations, Thorn and Hoffmann treat the carbon portion of the metallacycle as a monoanionic ligand (C5H5 ). It serves as a 4e donor to the metal, and these electrons form the two M—C a-bonds. In the Thorn/Hoffmann analysis, the metallacycle possesses six -electrons and therefore obeys the Hiickel 4n + 2 rule. Four electrons reside in relatively low-lying carbon fragment. Tr-orbitals In and 2ti, Chart 2). ° The remaining two jr-electrons are contributed by the filled metal Axz orbital (or hybrid of appropriate symmetiy), which interacts in a back-bonding fashion with the empty Zti carbon fragment orbital (Chart 2). 

Using the Hiickel [4n + 2] rule for aromatidly, predict which of the following species might be expected to show aromatic properties ... 

This model is today known as the Hiickel molecular orbital (HMO) method [1]. Less known is that Hiickel adopted the so-called hard ball potential of Bloch, used in a simplified theory of metals [14]. As mentioned, Hiickel was able to explain the fundamental difference between the aromaticity of benzene and the lack of aromatic character in the behavior of eight % electrons of cyclooctatetraene, CgHg, known today as the Hiickel 4n + 2 rule. This has been one of the most outstanding and significant accomplishments for early quantum chemistry that follows from the Hiickel An+ 2 rule ... 

Conjugated circuits play the central role in three rather important topics of polycyclic conjugated hydrocarbons (i) definition and expressions of resonance energy, (ii) generalization of the Hiickel 4n + 2 rule of aromaticity, and (iii) calculation of K electron ring currents. In the next three sections, we will address these three topics. 

Instead of counting jt electrons, chemists should pay attention to conjugated circuits (which were not around in the 1960s). But even just paying attention to cycle in molecules, the difficulties of pyrene and coronene would have been resolved. We can restate the the Hiickel 4n + 2 rule and the general 4n + 2 rule in terms of cycles as the following ... 

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