Huckels Rule Annulenes

PROBLEM 11.12 One of the chemical properties that make cyclobutadiene difficult to isolate Is that it reacts readily with itself to give a dimer  

What reaction of dienes does this resemble  

Structural studies of cyclobutadiene and some of its derivatives reveal a pattern of alternating single and double bonds and a rectangular, rather than a square, shape. Bond distances in a stable, highly substituted derivative of cyclobutadiene illustrate this pattern of alternating short and long ring bonds. 

Thus cyclobutadiene, like cyclooctatetraene, is not aromatic. Cyclic conjugation, although necessary for aromaticity, is not sufficient for it. Some other factor or factors must contribute to the special stability of benzene and its derivatives. To understand these factors, let s return to the molecular orbital description of benzene. 

Huckel was a German physical chemist. Before his theoretical studies of aromaticity, Huckel collaborated with Peter Debye in developing what remains the most widely accepted theory of electrolyte solutions. 

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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. 

This includes heteroannulenes, which comply with the HUCKEL rule, i.e. which possess An + 2) -electrons delocalized over the ring. The most important group of these compounds derives from [6]annulene (benzene). They are known as heteroarenes, e.g. furan, thiophene, pyrrole, pyridine, and the pyrylium and thiinium ions. As regards stability and reactivity, they can be compared to the corresponding benzenoid compounds [1]. 

The compound [18]-annulene is large enough to be planar and obeys the HUckel rule (4n + 2 = 18, with n=4). It is a brownish red fairly stable reactive solid. NMR evidence shows that it has aromatic character. The an-nulene with n= 7, [30]-annulene, can also exist in a planar form but is highly unstable. See also pseudoaromatic. 

The next higher annulene, cyclooctatetraene, is readily determined to be nonaromatic. The bond lengths around the ring alternate as expected for a polyene, and thermochemical data provide no evidence of special thermodynamic stability. Neither is the molecule antiaromatic. It is readily isolated and has the chemical behavior of a polyene. The molecule has a tub shape,and therefore is not a planar system of the type to which the Huckel rule applies. 

Larger annulenes permit the incorporation of irons double bonds into the rings hence, isomeric annulenes warrant consideration beginning with the cyclodecapen-taenes. [10]Annulene should, by the Huckel rule, possess aromatic stabilization if it were planar. All the isomeric cyclodecapentaenes suffer serious strain that prevents the planar geometry from being adopted. The trans,cis,trans,cis,cis-isomer, which has minimal angle strain, suffers a severe nonbonded repulsion between the two internal hydrogens ... 

The Huckel rule would predict nonaromaticity for [16]annulene. The compound has been synthesized and thoroughly characterized. The bond lengths show significant alteration (C=C, 1.34 A C-C, 1.46 A), and the molecule is significantly less planar than [14]annulene. These structural data are consistent with regarding [16]annulene as being nonaromatic. 

As with cyclooctatetraene, then, [10]annulene does not exist in a planar form and the Huckel rule cannot be applied. 

The term annulene was coined to refer to the completely conjugated monocyclic polyenes. The synthesis of annulenes has been extended well beyond the first two members of the series [4]aimulene (cyclobutadiene) and [6]annulene (benzene). The generality of the Huckel rule can be tested by considering the properties of members of the annulene series. 

Annulene offers a particularly significant test of the Huckel rule. The internal cavity in [18]annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. Most MO calculations find the delocalized structure to be more stable than the polyene. ... 

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... 

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. 

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