Antiaromaticity cyclobutadiene

Why does the number of n electrons determine whether a compound is aromatic Cyclobutadiene is cyclic, planar, and completely conjugated, just like benzene, but why is benzene aromatic and cyclobutadiene antiaromatic ... 

Stabilizing resonances also occur in other systems. Some well-known ones are the allyl radical and square cyclobutadiene. It has been shown that in these cases, the ground-state wave function is constructed from the out-of-phase combination of the two components [24,30]. In Section HI, it is shown that this is also a necessary result of Pauli s principle and the permutational symmetry of the polyelectronic wave function When the number of electron pairs exchanged in a two-state system is even, the ground state is the out-of-phase combination [28]. Three electrons may be considered as two electron pairs, one of which is half-populated. When both electron pahs are fully populated, an antiaromatic system arises ("Section HI). 

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. 

Thus cyclobutadiene like cyclooctatetraene is not aromatic More than this cyclo butadiene is even less stable than its Lewis structure would suggest It belongs to a class of compounds called antiaromatic An antiaromatic compound is one that is destabi lized by cyclic conjugation... 

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... 

Section 11 19 An additional requirement for aromaticity is that the number of rr elec Irons m conjugated planar monocyclic species must be equal to An + 2 where n is an integer This is called Huckel s rule Benzene with six TT electrons satisfies Huckel s rule for n = 1 Square cyclobutadiene (four TT electrons) and planar cyclooctatetraene (eight rr electrons) do not Both are examples of systems with An rr electrons and are antiaromatic... 

The rectangular structure is calculated to be strongly destabilized (antiaromatic) with respect to a polyene model. With 6-3IG calculations, for example, cyclobutadiene is found to have a negative resonance energy of—54.7 kcal/mol, relative to 1,3-butadiene. In addition, 30.7 kcal of strain is found, giving a total destabilization of 85.4 kcal/mol. G2 and MP4/G-31(d,p) calculations arrive at an antiaromatic destabilization energy of about 42kcal/mol. ... 

It is clear that simple cyclobutadienes, which could easily adopt a square planar shape if that would result in aromatic stabilization, do not in fact do so and are not aromatic. The high reactivity of these compounds is not caused merely by steric strain, since the strain should be no greater than that of simple cyclopropenes, which are known compounds. It is probably caused by antiaromaticity. ... 

It would be useful if triple bonds could be similarly epoxidized to give oxirenes. However, oxirenes are not stable compounds.Two of them have been trapped in solid argon matrices at very low temperatures, but they decayed on warming to 35 Oxirenes probably form in the reaction, but react further before they can be isolated. Note that oxirenes bear the same relationship to cyclobutadiene that furan does to benzene and may therefore be expected to be antiaromatic (see p. 58). 

Antiaromatic molecules are kinetically unstable, and aromatic molecules are kinetically stable (Scheme 13). In cyclobutadiene, the n orbitals can be combined out of phase and the n orbitals can be combined in phase. Cyclobutadiene is kinetically unstable toward electron donors and acceptors. In benzene, all neighboring pairs of n orbitals cannot be combined out of phase, and all neighboring pairs of 71 orbitals cannot be combined in phase. Benzene is kinetically stable toward donors and acceptors. 

The kinetic stabilities and the donor-acceptor properties of cyclic conjugated molecules [68] have been described (Scheme 12) in the theoretical subsection (Sect. 2.2.2) to be controlled by the phase property. There is a parallelism between the thermodynamic and kinetic stabilities. An aromatic molecule, benzene, is kinetically stable, and an antiaromatic molecule, cyclobutadiene, is kinetically unstable (Scheme 13). 

Fig. 9 illustrates that the two acetylenic systems become nearly parallel at C1-C6 distances close to 3 A where the constructive overlap of the re-orbital with one of the re -nodes is compensated by a destructive overlap with the other rc -node (Fig. 9, bottom). From a conceptual point of view, the properties of the in-plane re-system at the 3 A threshold bear a striking resemblance to the interaction of the two re-bonds in D2h cyclobutadiene where the re-re interaction is zero and the re-re repulsion is considerable, thus accounting for the extreme instability of this antiaromatic molecule.41 Even more relevant is a comparison with the TS of the symmetry forbidden thermal [2S + 2S] cycloaddition (Fig. 10) which prompted us to call this region antiaromatic .42... 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

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