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Aromaticity Rules

The concept of aromaticity is very elusive and is never directly observed. The existence of simple mathematical rules can take account for the aromaticity of a large number of organic and inorganic molecules. The field of aromaticity is in constant evolution and the variety of molecules that present properties related to aromaticity is growing bigger and bigger. 

Over the last two decades there has been a remarkable expansion in the number of different types of aromatic systems and also for our understanding of aromaticity. The aromaticity concept can be applied to the entire periodic table and is now widely accepted that there is not a unique type of aromaticity but chemical compounds can also have δ-, σ-, and even φ- aromaticity which combine together with combinations of these different types.

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Aromaticity Rules Definition

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The word aromaticity usually implies that a given molecule is stable, compared to the corresponding open chain hydrocarbon. The aromaticity rules are based on the Huckel Mobius concept. A cyclic polyene is called a Huckel system if its constituent p orbitals overlap everywhere in phase, or the p orbitals all have same sign above and below the nodal plane.

According to Huckel rule if such a system has 4n + 2 electrons, the molecule will be aromatic and stable, while on other hand a Huckel ring 4n electrons will be anti-aromatic. The classification of molecules into groups based on similar molecular properties structure or reactivity has been one of the goals of chemistry from the very beginning. Benzene and related compounds attracted a lot of attention because of their peculiar stability and rectivity and were gathered to form the group of aromatic molecules. 

Understanding the peculiarities of aromatic molecules and the features that a molecule should display to join the group of aromatic compounds became a goal in organic chemistry.  According to Huckel molecular orbital theory the topology of the molecule and the number of pie electrons determine the stability of the molecule. An outcome Huckel molecular orbital theory applied to cyclic conjugated hydrocarbons was well known 4n + 2 stability rule. Its connection to the concepts of aromaticity and anti- aromaticity has played a major role in organic chemistry. 

The Huckels rule of aromaticity states that monocyclic conjugated hydrocarbons with 4n + 2 electrons are aromatic whereas systems with 4n π electrons are antiaromatic. The number of π electrons is crucial to determine stability structure and reactivity of aromatic and antiaromatic systems.

Symmetry is one of the usual features of aromatic compounds and although not all aromatic species are symmetric the most archeptyle aromatic compounds are highly symetric and possess degenrate highest occupied molecular orbitals.

These orbitals can be fully occupied resulting in a closed shell structure or can be same spin half filled. The closed shell or same spin half filled electronic structure is the prigin of several rules of aromaticity like Huckel’s 4n + 2, Baired 4n, Wade Mingos 2n + 2 and Hirsch $2(n+1)^{2}$.

Structure Aromaticity Rules

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The concepts of aromaticity and anti-aromaticity can be a valuable tool for explaining structure, stability, chemical bonding and other molecular properties of inorganic compounds and their building blocks.

Extending the concepts of aromaticity and anti-aromaticity to include metal systems provides a new characteristics feature of chemical bonding. The particular the building blocks based on metal clusters frequently possess multiple aromaticity, multiple anti-aromaticity and conflicting aromaticity.

The multiple aromaticity and anti-aromaticity in inorganic systems lead to a new complication for electron counting rules. 
  • The electron counting rules for s – AO based σ-aromaticity are the same as Huckel 4n + 2/4n rules for aromaticity / anti-aromaticity for all cyclic structures.
  • The rules for counting the p – AO based σ-aromaticity are 4n + 4 aromaticity and 4n + 6 anti-aromaticity for cyclic structures with even numbers of atoms and 4n + 2 aromaticity.
  • For cyclic structures with odd numbers of atoms with 4n anti-aromaticity as only two types of σ- orbitals are found. They are p (σ – r) and p (σ – t) MOs which are considered together.
  • In the simplest case of occupation of just one p (σ – r) or one p (σ – t) MO, the system is also aromatic.
  • For p – AO based π aromaticity, the counting rules are 4n + 2 / 4n for aromaticity / anti-aromaticity for all cyclic structures.
  • For d – AO based σ- and π- aromaticity, the counting rules are 4n + 4 and 4n + 6 for cyclic structures with even numbers of atoms and 4n + 2 and 4n (anti-aromaticity) for cyclic structures with odd numbers of atoms as there are two types of σ orbitals.
  • For d-AO based δ aromaticity the electron counting rule is 4n + 2 / 4n for aromaticity / anti-aromaticity respectively.

Huckel Rule Aromatic vs Anti-Aromatic

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Aromaticity is a special stabilization is some specific arrays of π system that leads to reactivity that is much different from that of normal alkenes.
A classic example is benzene and the attempts to hydrogenate its double bond as compared to other systems is approximately 151 KJ per mol. 
Under normal circumstance and system. 
Reduction reaction
 
Where, ∆H = -28.6 K Cal per mol

The expected value ∆H = 2(-28.6) K Cal = -57.2 Kcal per mol
 
∆H = 2(-28.6) K Cal = -57.2 Kcal per mol which is the expected value. The actual value comes to 3 (-28.6 Kcal per mol) = ∆H 85.8 Kcal per mol. 

Reduction
∆H = -49.3 Kcal per mol. And hence the difference of 36.5 Kcal difference is energy for aromaticity.

Huckel’s rule
  • Count all the number of pie Ï€ electrons
  • If there are 4n + 2 Ï€ electrons, then the molecule is aromatic
  • If there are 4n Ï€ electrons, then the molecule is an anti-aromatic (if there are 6 or fewer carbon atoms in the ring)
  • If anything else, the molecule is non-aromatic
As the anti-aromaticity is destabilising a molecule with more carbon atoms like cyclo octatetraene will adopt a non-planar shape to avoid it. 
Understanding aromaticity allows a comprehension of reactivity, particularly for aromatic rings possessing hetero atoms. 

Hetero Atoms
 
Aromaticity broken ------------- aromaticity retained
Huckel 4n + 2 rule and aromaticity
  • Benzene is cyclic and conjugated
  • Benzene is unusually stable and has heat of hydrogenation at 150 KJ per mol less negative than expected for conjugated cyclictriene
  • Benzene is planar and has a shape of regular hexagon with all bond angles of 120 and with sp2 hybridisation of carbon atoms
  • Benzene undergoes substitution reactions which retain the cyclic conjugation rather than electrophilic addition reactions that would destroy conjugation
  • Benzene is resonance hybrid whose structure is intermediate between two line bond structures.
Aromaticity provides the benefits of high stability and in order to get these facts true the above rules are followed to contain 4n + 2 electrons, where n = 0,1,2,3, 4 ,5.. if the conjugated cycle is found to have only 4n electrons then it is considered as antiaromatic and will be either highly reactive or will distort in order to violate one of the rules.

Aromaticity Rules Organic Chemistry

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According to Huckel criteria for aromaticity a molecule must be cyclic conjugated (nearly planar with a p orbital on each atom) and have 4n + 2 Ï€ electrons. It is never defined that the number of Ï€ electrons must be same as the number of atoms in the ring or that the substance must be neutral. The numbers can be different and substance can be an ion. Hence, both cyclo pentadienyl anion and cyclo pentatrienyl cation are aromatic even though both are ions and neither contains six membered ring. 

Cyclo Pentadienyl and Cyclo Pentatrienyl

Cyclo pentadienyl      and      cyclo pentatrienyl 
  • Hydrogen can be removed with both electrons (H from the C-H bond leaving a carbocation as product
  • Hydrogen can be removed with one electron H* from the C – H bond leaving a carbon radical as product.
  • Hydrogen can be removed with no electrons H+ from the C – H bond leaving a carbanion as product.
All the potential products formed by removing a hydrogen with numerous resonance structures but Huckel’s rule predicts that only six pie electrons should be aromatic while the other products are predicted by the 4n + 2 rule to be unstable and antiaromatic.
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