Huckel annulenes

One of the most interesting and important of these theorems is known as the Pairing Theorem. In his study of the polyenes, and annulenes, Huckel demonstrated [149] that thdr energy levels... 

Use Frosts circle to construct orbital energy diagrams for (a) [lOjannulene and (b) [12]annulene Is either aromatic according to Huckel s rule ... 

Most of the synthetic work directed toward the higher annulenes was earned out by Franz Sondheimer and his students first at Israel s Weizmann Institute and later at the University of London Sondheimer s research systematically explored the chemistry of these hydrocarbons and provided experimental venfication of Huckel s rule... 

Annulene satisfies the Huckel (4n+2) tt electron rule for aromaticity and many of its proper ties indicate aromaticity (Section 11 20) As shown in Figure 13 10a [18]annulene contains two different kinds of protons 12 he on the ring s periphery ( out side ) and 6 reside near the middle of the molecule ( inside ) The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the NMR spectrum The outside protons have a chemical shift 8 of 9 3 ppm which makes them even less shielded than those of benzene The six inside protons on the... 

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. 

An extensive homologous series is also known for unsaturated organic ring systems of the type (-HC=) , which are known as annulenes. The most important example is benzene or [6]annulene. The stability of annulenes is governed by Huckel s rule, which states that planar ring systems of this type... 

Chlorophyll molecules are flat, completely conjugated ring compounds that obviously have a resonance form. Chlorophylls have a ring containing 97r bonds or 187r electrons, which fits the n = 4 situation in the Huckel (An + 2) rule. The (cis, trans, trans)3 double bond pattern of the chlorophylls also fits that of [18]annulene, which is aromatic. 

Aromaticity was found to be a general property of many (but not all) cyclic, conjugated jt systems. Moreover, it was found diat aromaticity in molecules can be predicted by Huckel s rule. The structural requirements implicit in Huckel s rule are that there be An + 2 (n is an integer) n electrons in a cyclic, conjugated n system. Obviously benzene, which has six jt electrons (An + 2, n = 1) in a conjugated it system, is aromatic. However, Huckel s rule predicts that molecules such as cyclodecapentaene An + 2 = 10 (n = 2) and [18]-annulene An + 2 = 18 (n = A) should be aromatic, have equal bond lengths, and be planar—and they are. 

Like benzene, cyclobutadiene ([4]annulene) has a continuous ring of overlapping p orbitals. But it has four pi electrons (two double bonds in the classical structure), which is a (41V) system with N = 1. Huckel s rule predicts cyclobutadiene to be antiaromatic. 

Use Huckel s rule (and the criteria for its application), rather than resonance, to determine which annulenes and ions are aromatic, antiaromatic, and nonaromatic. 

The antiaromatic geometry found along the concerted path of ground-state-forbidden pericyclic reactions, which is topologically equivalent to an antiaromatic HUckel [4/t]annulene or Mdbius [An 4- 2]annulene, is a particularly interesting type of biradicaloid geometry. (Cf. Section 4.4.) Other biradicaloid geometries and combinations of those mentioned are equally possible. 

Section 11.19 An additional requirement for aromaticity is that the number of tt electrons in conjugated, planar, monocyclic species must be equal to An + 2, where n is an integer. This is called Hiickel s rule. Benzene, with six TT electrons, satisfies Huckel s rule for n = 1. Cyclobutadiene (four tt electrons) and cyclooctatetraene (eight tt electrons) do not. Planar, mono-cyclic, completely conjugated polyenes are called annulenes. 

Fig. 1. Huckel 7c-elcctron molecular orbitals for some lower annulenes...

In the next section we take up a discussion of the magnetic properties of the annulenes. In striking analogy to Huckel s rule, an opposite directionality of these properties is found for the [4w]- and [4n + 2]-annulenes. For this reason magnetic properties have played the major role in the investigation of aromatic, non-aromatic and anti-aromatic character in the annulenes. By default, n.m.r. has been the standard experimental probe of annulene magnetism. 

This non-mobile spectrum shows [18]annulene to be aromatic, a fact which follows from Huckel s theory, together with the finding 118> that this molecule is almost planar, displacement of the carbon atoms from the mean plane being less than 0.1 A. Further X-ray crystallographic analysis 114> shows that there is no bond alternation in this molecule, (but not all bonds are exactly the same length). 

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