4n + 2 rule, Hiickel

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... 

Hiickel (4n + 2) rule Monocyclic planar (or almost planar) systems of trigonally (or sometimes digonally) hybridized atoms that contain [An + 2) Jt-electrons (where n is a nonnegative integer) will exhibit aromatic character. The rule is generally limited ton = 0-5. 

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... 

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