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Aromatic Ring

 The classification of Organic chemistry is based on the structure of the molecules. The aliphatic compounds have open chain structures such as hexane and can have C-C single bond, C=C double bond or even triple bond in between the carbons atoms. In aromatic compounds the cyclic molecules are unsaturated possessing additional stability with unsaturation of ring system.
These aromatic compounds have a different name and are known as arenes and they can be carbo cyclic which indicates that the ring skeleton contains only carbon atoms or heterocyclic with at least one atom other than carbon in the ring. These heteroatoms can be N, O or S atom.

Based on the elemental composition and relative molecular mass determination the formula of benzene is found to be C6H6. The saturated hydrocarbon hexane has the molecular formula $C_6H_{14}$ and hence it was concluded that benzene in which the carbon atoms were joined by alternate single and double bonds.

It was Kekule in 1865, who suggested the cyclic structure of benzene or better known as the aromatic ring.


Aromatic Ring Definition

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Based on elemental composition and by determining the relative mass the formula of benzene was arrived to as C6H6 and the entire structure has alterative double bonds. Kekule proposed structure keeping with the current knowledge of benzene but it does not explain how the double bonds differ from the aliphatic type.

The aromatic structure showed alternative position of the double bonds in between the carbon atoms but never explains why one particular form is more stable than the other or will be acceptable by the scientific community.

Finally Kekule proposed that the equivalent structures with oscillating double bonds which averaged out the single and double bonds so that the compounds were indistinguishable.

All the carbon atoms in an aromatic ring are $sp^2$ hybridized which gives an idea that each of the carbon atoms can form three sigma bonds ($\sigma $) and one pie ($\pi $) bond. The single bonds that are present are sigma bonds ($\sigma $) while each of the double bond consist of one sigma bond ($\sigma $) and one pie bond ($\pi $). 

Sigma Bond

The double bonds are shorter than single bonds and the aromatic structure would be deformed with longer single bonds as compared to shorter double bonds. 

Pi Bond

A benzene ring consist of 6 carbon atoms bonded to each other to form a regular hexagon with a carbon atom at each vertex of the hexagon. The carbon – carbon – carbon bond angle is 120o. The hydrogen atom attached to each carbon atom giving benzene the molecular formula of C6H6. There is a carbon and hydrogen bonding at each vertex in the aromatic ring structure.

The circle or lines inside the benzene or aromatic ring represent some of the electrons involved in carbon – carbon bonding.

These electrons are in orbitals that extend above and below the plane of the aromatic ring. These electrons circulate around inside the aromatic ring and not part of any specific carbon – carbon bond.

These electrons are delocalized and the C-C bond order in aromatic ring is somewhere between the single bond of an alkane and the double bond of an alkene.

The carbon – carbon bonds in aromatic ring are also termed as bonds and half and this unique bonding gives aromatic ring a unique structure, chemistry and infrared spectra.

The infrared spectrum of aromatic ring gives an idea of what it could be a structure with alternate single and double bond with 12 atoms in all having 30 normal mode of vibration.

Out of these 12 atoms normal mode of vibration, only a few are infrared active because of the high symmetry of the molecule.

All the substituted benzenes have lower symmetry and can exhibit quite a number of infrared bands. As substituents are arranged around a aromatic ring in different ways the symmetry changes results in some bands to appear while the rest just disappear. The intensities vary depending on the dipole moment change for a particular vibration.

The aromatic ring has narrow and sharp infrared spectrum bands which is a characteristic of aromatic ring spectra and the sharpness of these are used to distinguish aromatic bands due to other functional groups.

The C-H stretches of aromatic ring are found between 3100 and 3000 / cm. So this ring has a trio of bands above 3000 / cm and another trio of just below 3100 / cm.

These aromatic spectra bands are called ring modes due to stretching and contracting of the carbon-carbon bonds of the aromatic ring.

The carbon-carbon bonds in benzene are all the same length and as all the carbons are sp2 hybridized, there is a 2py orbital left over on each carbon which can overlap with a 2py orbital on either side.

This leads to a molecular orbital which involves all the 2py orbitals where the upper and lower lobes merge to give two doughnut like lobes above and below the aromatic ring plane.

Substituents on an Aromatic Ring

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A substituent in a benzene ring influences the course of electrophilic substitution in two ways:
  • Effects the reactivity of the molecule
  • Controls the orientation of attack, and hence isomers are formed
It is important to understand why this actually happens and it is these properties which influences the course of the reactions of aromatic compounds with electrophiles.

An electron releasing group increases the electron density of the benzene or aromatic ring promoting electrophilic attack. Such substituents are known as activating groups. An electron withdrawing group is deactivating and reduces the electron density of the aromatic ring. This makes the attack by the electron by the electron deficient reagent more difficult.

Both the types of substitution in aromatic ring, affects the electron density at all positions of the ring but exert their greatest effects at the ortho and para positions.

These sites become very electron rich in case of donor group is present and electron deficient in case electron withdrawing groups are present.
Donor groups of the electrophile therefore attack directly the ortho and para positions and are known as ortho / para directors.

While aromatic ring containing electron acceptor groups are attacked at meta position since this is least electron deficient site and are aptly called as meta directors.

Not all substituents fit exactly into this situation as halogens are deactivating but attack directly to the ortho / para positions.
  • Electron donating substituents activate the aromatic ring to electrophilic attack which results in the formation of ortho / para di-substituted benzene derivatives
  • Electron withdrawing substituents deactivate the aromatic ring to attack by electrophiles which occurs at meta position
  • Substituents exert their influence on a molecule through either the sigma bonds or the pie bonding system or by inductive and mesomeric (resonance) effects.
These interactions influence both electron density at various aromatic ring positions and the stability of the intermediate carbocation.

In a sigma bond between two atoms of differing electro negativities there is an unequal sharing of the electron pair with the electrons being attracted towards the more electronegative atom. This results in a permanent polarization of the molecule. This influencing of an atom or group on the distribution of the electron pair is called the inductive effect. The inductive effect rapidly goes away along a saturated carbon chain.

Substituents in an aromatic ring that withdraws electrons in this manner exerts a - (I) inductive effect. These include not only halogens and hydroxyl or nitro groups where an electronegative atom is attached to aromatic ring, but also groups such as carbonyl and nitrile in which an electron deficient carbon atom is bonded to the ring.

Substituents which falls into this category are $NO_{2}$, $CO_{2}R$, COR, CN and $SO_{3}R$. All of these are characterized by the atom attached to the ring being linked to a more electronegative atom by a multiple bond and could be represented by X = Y, where Y is more electronegative than X.

Electrons are therefore attracted towards Y, which makes X more electron deficient and hence strongly electron withdrawing. A positive charge is placed on ortho and para positons. 
Resonance Effect

Nitration of an Aromatic Ring

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The nitration of aromatic ring occurs when the nitronium ion (NO2+) acts as an attacking electrophile.

It is an important means of introducing functionality into the aromatic ring because the nitro group can readily be reduced to the amino group and hence give access to many other functional groups.

Direct nitration:

The most common method of introducing a nitro group into an aromatic compound is by direct nitration and variety of reagents have been used to get this. The choice is based on nature of the substituent and hence compounds containing an electron withdrawing group generally require more force conditions and give the 3 nitro derivative whereas those substituted with an electron donor are more easily nitrated than benzene produce a mixture of 2 and 4 isomers.

The standard method of nitration uses a mixture of concentrated nitric acid and concentrated sulfuric acid, but when stronger conditions are required fuming nitric acid replaces the rest of reagents. 

Nitration of Benzene
The presence of an electron withdrawing group in an aromatic ring makes attack by electrophiles more difficult, while electron donor groups makes this attack easier.

Alternative method:

The nitration of aromatic ring can proceed through nitrosation by the nitrosonium ion NO+ and subsequently oxidation of the nitroso compound by nitric acid. There is only a small concentration of NO+ in dilute nitric acid and so catalytic amounts of sodium nitrite are occasionally added to increase the quantity. This is better for smooth effective low temperature nitrations. 

Nitration of Phenol

5 Membered Aromatic Ring

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These are compounds with one heteroatom. The presence of two iodine atoms on one of the aromatic rings on venerable antiarrhythmic agent amiodarone may give an idea that it was originally conceived as an agent for treating various thyroid anomalies.

The synthesis begins with an unusual scheme for building the furan ring. The reaction of benzyl bromide
  • With triphenylphosphine gives phosphonium salt
  • The salt is then treated with valeryl chloride in the presence of pyridine results acylation
  • The benzylic carbon goes through cyclization to give benzofuran
  • Friedel crafts acylation in presence of stannic chloride proceeds to give electron rich furan ring to give the five member aromatic ring

6 Membered Heterocyclic Aromatic Ring

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  • These are compounds with one heteroatom. The synthesis of a 6 membered heterocyclic aromatic ring begins with the di-halogenated pyridine.
  • The di-halogenated pyridine is obtained from nicotine.
  • The coupling of this with monosilyl derivative from acetylene in presence of palladium helps in replacing the iodine on the aromatic ring by acetylene moiety.
  • The chlorine on the aromatic ring is then removed by reduction with zinc in acetic acid.
  • The silyl protecting group is then cleaved with the help of fluoride ion to finally give antinicline.
  • The moiety is finally replaced by a 6 membered pyridine ring.
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