Huckefs rule

The fact that -hybridized C-C bonds prefer the honeycomb bond angles of 120° contributes to the exceptional stability of benzene and graphite. HuckeFs rule stipulates that aromaticity is associated with a cyclic conjugated system of 4n + 2... 

In Chap. 10 we discussed the structure of aromatic compounds. An aromatic molecule is flat, with cyclic clouds of dclocali/ed n electrons above and below the plane of the molecule. We have just seen, for benzene, the molecular orbitals that permit this delocalization. But delocalization alone is not enough. For that special degree of stability we call aromaticity, the number of n electrons must conform to HuckeFs rule there must he a total of 4n -f 2) rr electrons. 

Huckefs rule has been abundantly verified [17] notwithstanding the fact that the SHM, when applied without regard to considerations like the Jahn-Teller effect (see above) incorrectly predicts 4n species like cyclobutadiene to be triplet diradicals. The Hiickel rule also applies to ions for example, the cyclopropenyl with system two n electrons, the cyclopropenyl cation, corresponds to n = 0, and is strongly aromatic. 

The pattern of experimental results on charged species with cyclic conjugated systems is summarized in Table 8.1. It is consistent with the applicability of Huckefs rule to charged, as well as neutral, conjugated planar cyclic structures. 

HuckeFs rule leads one to expect resonance stabilization in the cyclopentadienate and cycloheptatrienyl (tropyllum) ions. With a view to determining the resonance energy of the latter, Turner has measured the heat of hydrogenation of tropylium chloride to cycloheptaiie and hj drogen chloride, in acetic acid solution, ag — 86 23 rt 0 08 kcal/mole. An energetic comparison of the relative stabilities of tropylium chloride and the isomeric benzyl chloride... 

HuckeFs rule (Sect. 18.4) The requirement for an odd number of it electron pairs in order for a compound to be aromatic. 

HuckeFs rule can be extended to heterocyclic aromatic compounds. A heteroatom such as oxygen or nitrogen can contribute either zero or two of its unshared electrons as needed to the -IT system so as to satisfy the (4n + 2) -ir electron requirement. 

In Table 1 we present schematic molecular orbital diagrams, possible electronic configurations and HuckeFs rules for s-, p-, d-, and f-AO based o-aromaticity/antiaromaticity, p-, d-, and f-AO based K-aromaticity/antiaromaticity, d-, and f-AO based 8-aromaticity/antiaromaticity, and f-AO based (()-aromaticity/antiaromaticity of a singlet/triplet model triatomic system as a general example of odd-number cyclic systems. The same information for a tetratomic system as a general example of even-number cyclic systems is listed in Table 2. In Figs. 1-20 we give a... 

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