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Antiaromatic

The difference between the electron arrangement in benzene and that in a cyclo butadiene is the key to idea whether a cyclic, conjugated compound will be especially stable, like benzene or especially unstable quite like cyclo butadiene. Benzene has its highest occupied molecular orbitals completely filled with electrons and hence cyclic compounds completely conjugated around the ring with filled highest occupied molecular orbitals are especially stable and are aromatic. 

In comparison the cyclo butadiene has only enough electrons that its highest occupied orbitals are half filled. Since the highest occupied orbitals are half filled resulting in unstable configuration, so is termed as antiaromatic.

Compounds like cyclo butadiene is highly reactive in nature and because of the shorter double bonds alternating with longer single bonds helps in relieving some of the antiaromatic destabilisation by decreasing the overlap between the ‘p’ orbitals where longer bonds occur and is considered to be a characteristic of compound which are antiaromatic.

 

Antiaromatic Definition

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Benzene with six pi electrons or sometimes called an aromatic sextet is considered as the prototypical aromatic compound.

The criteria for an antiaromatic compound can be generalised in a similar manner and the requisite number of electrons to have the highest occupied molecular orbital half-filled is a multiple of four pairs. Hence, the cyclic, the fully conjugated planar molecules with 4n pi electrons are antiaromatic. 

 Number ‘n’   Pi electrons 
 Number of pairs 
 1  4 2
 2  8  4
 3  12  6
 4  6  8

As mentioned earlier, molcules like cyclo butadiene has four double bonds and it has eight pi electrons. And as this is a simple multiple of four, the cyclo butadiene is considered as antiaromatic if its in a planar form. Now, the planar cyclo octatetraene is also considered as antiaromatic if it is in planar form. But at the same time the cyclo octatetraene would face severe angle strain as its bond angle would be somewhere 135 degres rather than the trigonal planar bond angle of 120 degrees.

In order to remove both the antiaromatic destabilisation and angle strain, the molecule of cyclo octatetraene adapts to a non-planar geometery with a tub shape. Each of these existing duble bonds are twisted in comparison to adjacent double bonds which makes the cyclo octatetraene behaving normal aand as nonconjugated alkene.

As this new geometry does not show any specific instability associated with antiaromatic compounds, this can be readily made but at the same time does not show any features of the special stability associated with aromatic compounds. This molecule shows bond alternation, with shorter double bond alternating with longer single bonds and provide addition products to give nonaromatic.

Antiaromatic and Nonaromatic Compounds

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The aromatic and antiaromatic energies of pericyclic transition states are likely to have smaller than those of the analogous pi electron systems. All the overlapping pairs of 2p atomic orbitals are close together because the atoms are linked by sigma bonds. This is not found in pericyclic transition state. The orbital overlap is correspondingly reduced and so is the effect of the cyclic conjugated system. It is now possible to isolate the minute amounts of by product formed in the opening of cyclo butenes through antiaromatic disrotatory route.

The corresponding difference in activation energy between the con-rotatory and disrotatory pathways is around 14 Kcal per mole as compared to aromatic and antiaromatic energies of 7 Kcal per mole. There is another difference existing between the aromatic and antiaromatic which revolves around the topology of overlapping of atomic orbitals in pericyclic transition state and not on symmetries of molecular orbitals.

If the symmetries are involved the distinction between the accepted structure and unacceptable structure and reactions and would be attenuated as lost symmetry. The aromaticity is best described in terms of stability which is derived from the delocalisation of the participating bonding electrons. An aromatic molecule is characterised by appreciable stabilisation compared to noncyclic polyene while an antiaromatic molecule is one that is destabilised and the calculated energy is comparable to the polyene.

Antiaromatic molecules mostly have planar forms. These are cyclic with conjugated systems along with an even electron pair in the overall structure. So any compound which satisfy the stability part, the planar part and having conjugated systems are considered for such category. 
These also have even number of pairs of pi electrons. Compared to this an aromatic compound is considered to be more stable with localised electrons. With delocalisation of electrons, the stability decrease.

Antiaromatic Examples

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The best example to understand the antiaromaticity is cyclo butadiene.
Cyclo propylene anion
Cyclo penta dienyl
Cyclo octatetraene  
Annulene 
Porphyrins
N – N dihydro diazatetracene 

Why are Antiaromatic Compounds Unstable?

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Antiaromatic compounds are especially found to be unstable as compared to their acyclic analogs. Huckel formulated a set of rules which were used to predict compounds stability, especially benzene. A compound that exhibits this special stability is considered as aromatic compound. 
  • The aromatic part of the compound need to have a ring
  • Each of these ring atoms need to have a ‘p’ or ‘d’ orbital
  • The ring of p or d orbitals need to be a planar structure
  • The electron system of p and d orbitals need to have 4n + 2 settings where n = 0, 1, 2, ….

Any of the compounds that obey the first three rules but differ on the last one are considered as antiaromatic in nature. For a compound to be considered as antiaromatic form the last rule is applicable for 4n electrons in the system. Since the compounds do not follow the last of the rules laid out by Huckel, these are considered as unstable. Compounds which fail to obey these specific rules are categorized as non-aromatic.

As compared to this, aromaticity is the bonding phenomenon associated with increased symmetry, stability and having specific reactivity. It was developed for organic molecules and has been indispensable for changing or rationalizing their symmetrical shape and their reactivity. The discovery of aromaticity in all metal clusters is one of the most remarkable developments in cluster chemistry.

As much of the bonding in bulk metals is delocalized, the bonding in all metal clusters exhibit similar delocalized behavior. Delocalized bonding is considered traditionally by chemists as aromatic or antiaromatic. The stability part of antiaromatic compounds is basically the degenerate sets of molecular orbital in electron counting and thus is specifically agreed upon as the reason.

Antiaromatic vs Aromatic

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 Antiaromatic   Aromatic 
 Unstable compared to acyclic analogs   Relatively stable 
 The p orbitals do not overlap continuously   Occasional overlapping of p orbitals is seen
 Follow the 4n rule of electron distribution  Follow the Huckel’s rule of 4n + 2 electron distribution 
 Both p and d orbitals present
 P and d orbitals are present
 Ring structure present
 Structure is usually in ring form
 The p and d orbitals need to be in planar form 
 The structure is in planar form

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