Huckels rule

but we must realize that this attempts to place two hydrogens in the same space. This is clearly not feasible as the hydrogens are pushed away from each other, therefore, this must lead to a non-planar molecule. 

Structures that are also aromatic are the cyclopropenyl cation (2 jt electrons n = 0) and the cyclopentadienyl anion (6 n electrons n = 1). Although we do not wish to pursue these examples further, they are representative of systems where the number of jr electrons is not the same as the number of carbon atoms in the ring. 

The stabilization conferred by aromaticity results primarily from the much lower energy associated with a set of electrons in molecular orbitals compared 

Structure like benzene, but has two nonbonding electrons it does not have the special stability we see in benzene. As we have seen in Section 2.9.2, cyclooc-tatetraene also adopts a non-planar shape, lacks the stabilization associated with conjngation, and behaves like four separate normal alkenes. 

The criteria for a compound to be aromatic were developed by Erich Huckel. 

Cyclic, fully conjugated, planar molecules with 4n + 2 pi electrons (n = any integer including zero) are aromatic. 

Let s analyze each aspect of this rule. First, the molecule must have a ring with a series of conjugated p orbitals that extends completely around the cycle, like benzene. If the cycle of conjugated orbitals is interrupted, as in the case of cyclopentadiene, then the compound is neither aromatic nor antiaromatic. It is just an alkene. 

Second, the ring must be planar so that the p orbitals overlap in pi fashion completely around the cycle. If the ring is not planar, the p orbitals are twisted so that they are not parallel, resulting in a decrease in overlap. This decreases or even eliminates the aromatic or antiaromatic effect of the conjugation. 

Finally, the number of pi electrons must equal 2 plus a multiple of 4 (or An + 2 pi electrons). Some of the possible numbers are as follows  

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We saw in Chapter 12 that aromaticity reveals itself in various ways Qualitatively aro matic compounds are more stable and less reactive than alkenes Quantitatively their heats of hydrogenation are smaller than expected Theory especially Huckels rule furnishes a structural basis for aromaticity Now lets examine some novel fea tures of their NMR spectra... 

Huckels rule (Section 11 19) Completely conjugated planar monocyclic hydrocarbons possess special stability when... 

Annulene, [22]annulene, and [24]annulene have all been reported. The NMR spectrum of [22]annulene is consistent with regarding the molecule as aromatic, whereas those of the [20] and [24] analogs are not. In each case, there is some uncertainty as to the preferred conformation in solution, and the NMR spectra are temperature-dependent. Although the properties of these molecules have not been studied as completely as those of the smaller systems, they are consistent with the predictions of the Huckel rule. 

PATr+ of about 0-2. Several methods for generating the phenalenyl cation have been developed.Because the center carbon is part of the conjugated system, the Huckel rule, which applies only to monocyclic conjugated systems, cannot be applied to just the peripheral conjugation. The nature of the phenalenyl system is considered further in Problem 12 at the end of this chapter. 

The Huckel rule is strictly applicable only to monocyclic compounds, but the genera] concept of aromaticity can be extended beyond simple monocyclic compounds to include polycyclic aromatic compounds. Naphthalene, with two... 

Bond angle/bond length relationships do not readily account for the bond localization noted for starphenylene (116) and triphenylene (124). A cursory examination of the structures reveals that in these cases the annelated cycles can have an aromatic character of their own. In staiphenylene the cycle would contain four electrons and be antiaromatic, whereas in triphenylene the cycle would have six electrons and be aromatic. From the simple Huckel rule, the antiaromatic cycle should be disfavored. In such a case, structural stabilization can be accomplished by greater contribution from the resonance form that has single-bond character at the endo-honA. The reverse is expected for the aromatic cycle. This model is simple, predictive, and accurate ... 

Q Which of the following heterocycles conform to the Huckel rule (4h +2) for aromaticity (i) furan (ii) l//-azepine (iii) pyrylium perchlorate [chlorate(VIl)] ... 

Anions of small heterocyclics are little known. They seem to be involved in some elimination reactions of oxetan-2-ones (80JA3620). Anions of large heterocycles often resemble their acyclic counterparts. However, anion formation can adjust the number of electrons in suitable systems so as to make a system conform to the Huckel rule, and render it aromatic if flat geometry can be attained. Examples are found in Chapter 5.20. Anion formation in selected large heterocycles can also initiate transannular reactions (see also Section 5.02.7 below). 

Like porphyrins, heteroporphyrins are aromatic and confirm the [4n+2] it-elecfron Huckel rule (00MI3). Replacing N with O, S, Se, and Te perturbs jt-delocalization and electronic properties. The alteration in jt-delocalization is evidenced from downfield shifts in H NMR spectra. The extent of shift in monoheteroatom is more than diheteroatom-substituted porphyrins (1979JA7055, 19840MR561). The large heteroatoms... 

On basis of this type of argument we come to Huckel Rule [ 4n + 2) rule]. 

Suprafacial or Huckel transition state in a pericyclic reaction is associated with a plane of symmetry and is particularly favourable if the number of cyclically conjugated Tr-electrons in the transition state equals [4n - - 2] (the Huckel rule, where = 0,1, 2,...). 

Moreover, the high stabilization energy of cyclopentadienyl anion (comparable with that of benzene 31.19 kcal mol 1) demonstrates the aromatic character of this species, in agreement with the 4n + 2 Huckel rule. 

Systems such as cyclopentadienyl anion, benzene and tropylium cation with (4N + 2) 7t-electrons (N=Q, 1, 2,. ..) will thus have a closed shell, that is, four electrons in the degenerate pair of the highest occupied MOs in their ground configuration, and that is especially favourable energetically. This is the basis of the well-known Huckel rule of aromaticity. 

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