Aromaticity antiaromatic hydrocarbons

As we mentioned earlier, at first we plan to discuss inorganic molecules, which can be obtained from aromatic/antiaromatic hydrocarbons by isoelectronic substitution. Two molecules borazine (BjNjH, ) and 1,3,2,4-diazadiboretiidine (B2N2H4) are thought to be prototypical aromatic and antiaromatic inorganic molecules, as they are isoelectronic to benzene and cyclobutadiene, respectively. 

Depending on the type of the heteroatom (X, Y, or Z) that replaces the —CH= group in the original hydrocarbon, heterocyclic structures may be designed with either the same number of 7r-electrons as in the parent molecule, or with a lesser or greater number of these. In the latter case, a change in the number of the 7r-electrons must reverse the aromatic (antiaromatic) character relative to the hydrocarbon analog. 

Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

Division of the Hess-Schaad resonance energy by the number of tt electrons gives the resonance energy per tt electron (REPE) of the compound. A compound with a substantially positive REPE value (greater than, say, 0.01 jS ) is predicted to be aromatic. A compound with a near-zero REPE is nonaromatic. A compound with a substantially negative REPE is predicted to be antiaromatic, being less stable than if its double bonds were isolated from one another. Some antiaromatic hydrocarbons are cyclobutadiene and fulvalene. 

Fig. 13 Alternative resonance forms for benzene (aromatic), cyclobutadiene and cyclo-octatetraene (antiaromatic). Other aromatic cyclic hydrocarbons and ferrocene are illustrated at the bottom...

Some of the essential properties of aromatic, non-aromatic, and antiaromatic hydrocarbons have been considered earlier (see p. 41) basically these differences may be attributed to the assignment of positive, zero, and 134... 

Huckel proposed his theory before ideas of antiaromaticity emerged We can amplify his generalization by noting that among the hydrocarbons covered by Huckel s rule those with An) tt electrons not only are not aromatic they are antiaromatic... 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

C. Aromaticity and Antiaromaticity of Heterocyclic Compounds in Which the Number of 77-Electrons Is Different from That in the Parent Hydrocarbon... 

Cyclooctatetraene is [8]annulene, with eight pi electrons (four double bonds) in the classical structure. It is a (41V) system, with N = 2. If Htickel s rule were applied to cyclooctatetraene, it would predict antiaromaticity. However, cyclooctatetraene is a stable hydrocarbon with a boiling point of 153 °C. It does not show the high reactivity associated with antiaromaticity, yet it is not aromatic either. Its reactions are typical of alkenes. 

The word aromaticity usually implies that a given molecule is stable, compared to the corresponding open chain hydrocarbon. For a detailed account on aromaticity, see, e.g., Reference [95], The aromaticity rules are based on the Hiickel-Mobius concept. A cyclic polyene is called a Hiickel system if its constituent p orbitals overlap everywhere in phase, i.e., the p orbitals all have the same sign above and below the nodal plane (Figure 7-23). According to HiickeTs rule [96], if such a system has 4n + 2 electrons, the molecule will be aromatic and stable. On the other hand, a Hiickel ring with 4n electrons will be antiaromatic. 

Jenks et al. studied the effects of conjugation and aromaticity on the sulfoxide bond by means of ab initio computation <1996JOC1275>. They calculated S-O bond dissociation energies (BDEs) and found that, in a formally aromatic system such as thiophene sulfoxide, the SO BDE is decreased by as much as 25kcalmoP relative to the BDE of DMSO. Although the BDE of the formally antiaromatic thiirene sulfoxide increased by about 15 kcal moP the authors concluded, on the basis of calculated geometries and isodesmic reactions with pure hydrocarbons, that cyclic unsaturated sulfoxides are neither significantly aromatic nor antiaromatic. 

The aromatic rings present the highest and the antiaromatic systems the lowest bifurcation values of ELF. The bifurcation of the ELF occurs close to 0.75, except in system where sigma delocalization exists. In this way, the aromaticity of polycyclic aromatic hydrocarbons was well predicted, and also the aromaticity of new molecules was corroborated. In the all-metal aromatic compound Al - important contributions to stability from the two tt aromatic electrons and the cr system in the plane of the molecule were observed. The isosurfaces of the total ELF, ELFOT and ELF functions are showed in the Figure 6. 

From the Table it can be seen that aromatic hydrocarbons tend to be more easily reduced as the molecular size increases. However, even for simple aromatic molecules both molecular size and molecular symmetry must be analysed for a quantitative understanding 37). Detailed 3H and 13C NMR studies of reduced hydrocarbons enabled to correlate between the topology of each system, its magnetic properties, and antiaromaticity. These studies will be discussed in the following sections. 

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