AROMATICITY.

AROMATICITY
AROMATICITY – SYLLABUS:
1. Aromaticity:
1.1. Aromaticity – Introduction.
1.2. Differenttheories of Aromaticity.
1.2.1. Aromatic sextet Theory.
1.2.2. Valence Bond Theory of Aromaticity.
1.2.3. Molecular orbital theory of Aromaticity.
1.3. Huckle’s rule and the concept of aromaticity.
1.4. Types of Aromaticity.
1.4.1. Anti – Aromaticity.
1.4.2. Homo – Aromaticity
1.4.3. Pseudo – Aromaticity
1.5. Aromaticity in benzenoid and non-benzenoid compounds.
1.6. Aromaticity in alternant and non-alternant hydrocarbons.
1.7. Aromaticity in metallocenes- Ferrocene.
1.8. Aromaticity in Azulenes.
1.9. Aromaticity in Fulvenes.
1.10. N.M.R spectroscopy of aromatic compounds.

1.1. Aromaticity – Introduction Class Video:

1.1. Aromaticity – Introduction:
In previous we learn about Resonance which arises due to conjugation i.e. delocalization of  – electrons and tells about the continuous delocalization of π electrons. But we don’t know, what happens when the resonance is continuous in a circular manner. That discussed by the concept of Aromaticity. Hence, Aromaticity is a property which tells about the continuous delocalization of π – electrons i.e resonance in a cyclic planar ring.
We are already aware that the aromatic compounds apparently contain alternate double and single bonds in a cyclic structure and resemble benzene in chemical behaviour. Hence, Originally the term aromaticity was used to describe all compounds that has the properties of benzene or condensed system of benzene rings and was confined. The reason for this description was that compounds which has aromatic character exhibited chemical reactivity and physical properties of benzene like the unusual stability of benzene, substitution reactions of benzene (although the benzene ring was unsaturated), the weaker basic properties of aromatic amines and acidic properties of phenols as compared with alcohols which were very much different from those of the analogues aliphatic and alicyclic i.e. aliphatic cyclic compounds.
Now let us discussed about the reason for armaticity of Benzene. In the kekule representation of Benzene, it can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies of the C = C, C – C and C – H, that lowers the molecular energy and causes stability and Aromaticity of the Benzene. Thuis Aromaticity is now generally associated with this property of lowered molecular energy. A major contribution to the stability of the aromatic systems is considered to be due to the delocalization of electrons in these molecules.
Hence, the concept Aromaticity gives rise to stability of the molecule in aromatic compound like benzene. But in some times, it can also lead to destability of the molecules in systems like cyclobutadiene. So, it is necessary to understand what conditions are required to classify the molecules into Aromatic, Anti – Aromatic, Homo Aromatic and Non – Aromatic. These differences in chemical properties led chemists to seek an explanation and in consequence to define aromaticity.
Let us try to know about the concept of aromaticity and the conditions required to classify any given compound into aliphatic, aromatic and anti-aromatic.

1.2. Aromaticity – Concept & Characterstics:
Aromaticity is a concept which tells about the continuous delocalization of π electrons i.e. resonance in a cyclic planar ring. The concept of aromaticity can explains about the following charecterstics of Aromatic and Anti Aromatic compounds.
1. Compounds chemical behavior: The concept of aromaticity can explains the chemical behavior of compounds weather they are Aromatic or Anti Aromatic, by observing their electrophilic aromatic substitution reactions. If the compound will undergoes electrophilic aromatic substitution reaction then the compound is said to be aromatic. While if the compound wiil not undergoes electrophilic aromatic substitution reaction then the compound is anti Aromatic. Hence, Aromatic compounds undergoes electrophilic aromatic substitution reactions rather than addition reactions.
2. Compounds structural features: The concept of aromaticity can explains the structural features of compounds weather they are Aromatic or Anti Aromatic by observing Bond length equalization. If the Bondlengths are equalization, then the compound is Aromatic. While if the Bondlengths are not equalization, then the compound is Anti aromatic. Hence, In aromatic compounds all the bondlenths are in equal i.e. Bond length equalization. But in other than aromatic compounds the bondlenths are not in un equal i.e. Not Bond length equalization.
3. Compounds Energetic Behaviour:The concept of aromaticity can explains the Energetic Behaviour of compounds weather they are Aromatic or Anti Aromatic by observing the Resonance Energy. The energetic stabilization can be explained on the basis of chemical and physical properties of compounds. If the compound posses Low Resonance Energy then the compound is said to be Aromatic in nature. While if the compound posses High Resonance Energy then the compound is said to be anti aromatic in nature.
4. Compounds Magnetic Behaviour: The concept of aromaticity can explains the Magnetic Behaviour of Compounds weather they are Aromatic or Anti Aromatic by observing their Magnetic susceptibility If the compound posses high diamagnetic susceptibility, then the compound is said to be Aromatic in nature. While if the compound posses high paramagnetic susceptibility then the compound is said to be anti aromatic in nature. It can also explains “Ring current” effects such as abnormal or anomalous chemical shifts in NMR and large Magnetic anisotropies and High diamagnetic susceptibility.
a) Anomalous chemical shifts in NMR.
b) Large Magnetic anisotropies.
c) High diamagnetic susceptibility.

Huckle’s rule (4n+2) - electron rule for Aromaticity:
1. Huckel in 1937 carried out Molecular Orbital calculations on mono cyclic system CnHn which containing, n  – electrons.
2. According to this Huckle’s rule i.e. (4n+2) - electron rule, in mono cyclic system which containing (4n+2)  – electrons in a closed shell, each carbon atom providing one electron and as a result connected aromatic stability through it’s high delocalisation energy or high resonance energy.
3. According to Huckle’s rule i.e. (4n+2)  electron rule for Aromaticity, the delocalised  electron cloud must contain a total of (4n+2) - electrons, where n is a whole number i.e., n = 0,1, 2, 3 and so on. Putting n = 0 in (4n+2)we get 2- electrons, similarly putting n = 1, we get 6- electrons; n= 2 gives 10- electrons; n= 3 gives 14electrons and so on.
4. According to Huckle’s rule i.e. (4n+2)  electron rule for Aromaticity, In the description of aromaticity, there is no need to mention how many no of carbon atoms are involved in the ring. But the essential requirements are the presence of (4n+2)  electrons and planarity of the ring. However, if the ring is not planar, overlap of the p – orbital is diminished or absent.
5. Thus, if a molecule is a monocyclic, planar system and contains (4n+2) electrons, that molecule will exhibit aromatic character, ie. Will have unusual stability.
6. For benzene, n = 1 and the molecule has a closed shell of six electrons, i.e all occupied bonding molecular orbitals are double filled.

1.4. Types of Aromaticity:
Homo – Aromaticity & Homo Aromatic compounds:
Homo Aromaticity: The phenomenon in which the contentious delocalization of  electrons or overlapping of p atomic orbitals in a cyclic arry i.e. Aromaticity through a saturated sp3 hybridized carbonatom, causes decreases in energy is called as Homo Armaticity and the compounds which exhibit Homo Aromaticity are called as Homo Aromatic compounds.
Examples for Homo Aromaticity:
Homotropylium ion – cyclooctatrienyl cation: Cyclooctatetraene is when subjected to dissolved in con.H2SO4, a proton from con.H2SO4 adds to one of it’s double bond and produces the Homotropylium ion cyclooctatrienyl cation.

The Homotropylium ion or cyclooctatrienyl cation is Homo aromatic, because in this species an aromatic sextet is spread over seven carbon atoms as in the tropylium ion. The eighth carbon is an sp3 carbon which is in out of plane, so cannot takepart in the aromaticity. In order to overlap the orbitals most effectively as to close loop the sp3 atoms are forced to lie almost vertically above the plane of the aromatic atom. In homotropylium ion, Hb is directly above the aromatic sextet and so is shifted far up filed in the NMR.
NMR spectral data of Homotropylium ion: In NMR spectroscopy the Homotropylium ion or cyclooctatrienyl cation shows a diatropic ring current. Hence different protons of Homotropylium ion shows different chemical shift values as follows, the proton Hb is found at = – 0.3, since, in homotropylium ion, Hb is directly above the aromatic sextet and so is shifted far up filed in the NMR, H1 and H7 at 6.4, H2 – H6 at 8.5. this ion is an example of homoaromatic compound, which may be defined as a compound that containes one or more sp3 hybridised carbon atoms otherwise conjugated cycle.

All aromatic compounds so far discovered are ion, and its is question as to whether homoaromatic character can exist in uncharged . homoaromatic ions of two and them electrons are also known.
The cyclobutenylcation is the homoaromaticanalog of the very stable cyclopropeniumcation. This ion can be prepared from 3-acetoxy cyclobutene ring super acid conditions.

1.5. Aromaticity in benzenoid and non-benzenoid compounds:
1.5.1. Aromaticity in Benzenoid compounds:
1.5.2. Aromaticity in Non-Benzenoid compounds:

Azulenes – C10H8 – Bicyclo [5.3.0] decapentaene – Class video:

Azulenes – C10H8 – Bicyclo [5.3.0] decapentaene:

• Azulene is a Blue coloured solid with the structure as shown and the name is derived from the Spanish word ‘Qzul’ meaning Blue.
• Azulene is a fused cyclo-heptatriene and cyclopentadiene system and has melting point as 990C.
• Azulenes is one of the few benzenoid structure which show significant aromatic stabilization. There are two kekule resonating structures containing 10 electrons. Azulenes has 49 K.cal/ mole resonance energy.
• The molecular formulae of Azulene is C10H8, and the Bicyclic nomenclature is Bicyclo[5.3.0] decapentaene.
• Azulene is a Bicyclic system which contains one five membered ring – cyclo pentadienyl anion i.e. cyclopentadienide and one seven membered ring – cyclo hepta trienyl cation i.e. tropylium ion. Hence Azulene is a combination of cyclopentadienide ion and tropylium ion.
• Azulenes is an isomer of Naphthalene: In Azulenes, the five membered ring has five  – electrons and seven – membered ring has seven  – electrons. In total it has 12  – electrons, out of these 12  – electrons, two  – electrons are common to both rings. In order to achieve the aromatic stabilization, one electron from the seven – membered ring is transferred to the five – membered ring, then each ring will have a closed shell of six  – electrons as in Napthalene. Hence Azulenes is an isomer of Naphthalene.
• Azulenes shows the dipole moment of 0.80 D: In Azulenes, In order to achieve the aromatic stabilization, one electron from the seven – membered ring is transferred to the five – membered ring, then the seven – membered ring posses positive charge and the five – membered ring posses negative charge results a dipolar structure which leads to the dipole moment of Azulenes as 0.80 D.
Azulenes has the dipole moment 0.80 D. But it’s isomer Napthalene has 0 dipoe moment indicating it’s polar form or charge stabilization.
• Azulene also behaves chemically as an aromatic compound: In contrast to the fact that cyclopentadiene – 5 – electrons and cycloheptatriene – 7 – electrons independently possess antiaromatic characteristics which are responsible for their instability. But, this fused compound is aromatic and is stable. In total it has 12  – electrons, out of these 12  – electrons, two  – electrons are common to both rings. In order to achieve the aromatic stabilization, one electron from the seven – membered ring is transferred to the five – membered ring, responsible for a charged resonating structures – Dipolar structures, which shows the –ve charge on the five membered ring, making it equivalent to 6e- cyclopentadienyl anion and seven membered ring bearing +ve charge is similar to 6e- tropylium cation, giving it an aromatic character.
• Azulenes undergoes Electrophilic substitution & Nucleophilic substitution Reactions: Azulenes because of it’s aromatic character, will undergoes in both electrophilic and nucleophilic substitution reactions. It will undergoes electrophilic substitution reactions like bromination, nitration and the friedel-crafts reaction and the nucleophilic substitution reactions like etc.,
• Further more, electrophilic substitution occurs preferentially in the five memberd ring , since this is more electron – rich than the seven – membered ring. Highly probable positions for electrophilic substitutions are 1 and 3. While, the Nucleophilic substitution Reactions occurs preferentially in the seven memberd ring , since this is more electron – poor than the five – membered ring.

1.10. N.M.R spectroscopy of aromatic compounds:
• From the adventage of magnetic techniques, such as Nuclear Magnetic Resonance – NMR etc., it has been possible to determine experimentally whether a compound has a closed ring of electrons or not.
• Aromaticity can now be defined as the ability to sustain an induced ring current, a compound with this ability is called diatropic.
• When an aromatic compound is placed in an external magnetic field, the delocalized  – electrons in the ring start to circulate in one direction i.e. perpendicular to the applied magnetic field and there by producing a ring current which induces a magnetic field perpendicular to the molecular plane called as secondary magnetic field or local magnetic field or induced magnetic field. The distribution of Induced magnetic field is non – uniform said to by Anisotropy i.e. volumes above and below boundary by the area of the ring.
• The Induced magnetic field, either oppose or reinforce the applied magnetic at the proton, depending upon its location in the induced magnetic field.
• If the induced magnetic field opposes the applied magnetic field, the proton experiences a weaker magnetic field than the applied magnetic field and the proton is said to be shielded. The shielded protons experiences smaller  – chemical shift values. This type of shielding is called as diamagnetic shielding.
• One the other hand if the induced magnetic field reinforces the applied magnetic field, the proton experiences a stronger magnetic field than the applied magnetic field and the proton is said to be deshielded. The deshielded protons experiences larger  – chemical shift values. This type of shielding is called as paramagnetic shielding. Thus, there are volumes in which deshielding – negative and shielding – positive occur.
• In the case of aromatic compounds, the Induced magnetic field opposes the applied magnetic field inside the ring and assists the applied magnetic field out side the ring. Hence Inner side protons are shielded, experiences smaller  – chemical shift values and outer side protons are deshielded, experiences larger  – chemical shift values.
• It follows that aromaticity can be determined from an NMR spectrum. If the protons attached to the ring are shifted downfield (higher values) from the normal olefinic region, we can conclude that the molecule is diatropic and hence aromatic. In addition, if the compound, has protons above or within the ring, then if the compound is diatropic, these will be shifted upfield (to lower)
• From the above all discussion, it follows that when we look at a compound for aromaticity, we look for:
1. Chemical stability
2. The ability to undergo aromatic substitution
3. Equal or approximately equal bond distances, except when the symmetry of the system is disturbed by a heteroatom or in some other way.
4. Planarity of the ring.
• The presence of a diamagnetic ring current.

1.10.1. N.M.R spectroscopy of aromatic compound – Benzene:
When an aromatic compound like Benzene is placed in an external magnetic field, it takes perpendicular orientation with respect to the direction of applied magnetic field H0. The induced circulation of delocalized  – electrons above and below plane of the ring generates Ring current which interms produces a secondary magnetic field or local magnetic field or induced magnetic field. The distribution of Induced magnetic field is non – uniform said to by Anisotropy i.e. above and below plane of Benzene ring. Since all the six protons are located in paramagnetic region  – chemical shift values are large and lies between 7 and 8 those of olefinic protons are 5 to 6. The Anisotropic effect of aromatic compounds is stronger because of it’s participation of more  cloud compared to an Anisotropic effect ofolefins. It is therefore assumed that a compound is aromatic if the NMR absorption peaks of hydrogen atoms attached to the carbon atoms of the ring are at a lower field higher values than that expected for olefininc hydrogen atoms.

1.10.2. N.M.R spectroscopy of aromatic compound – 2a1-methyl-2a1H-benzo[cd]azulene: In the case of 2a1-methyl-2a1H-benzo[cd]azulene, the methyl protons are out side the ring and ethylenic protons are inside the ring. Hence the methyl protons are deshielded, experiences larger  – chemical shift values. ethylenic protons are shielded, experiences smaller  – chemical shift values.