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Aromaticity

Aromatic compounds are those, which resemble benzene in chemical behavior. These compounds contain alternate double and single bonds in a cyclic structure. They undergo substitution reactions rather than addition reactions. This characteristic behavior is called aromatic character or aromaticity the criteria for which are as follows.

  • Contains a cyclic cloud of delocalized p electrons above and below the plane of the molecule.
  • Electrons cloud must contain a total of (4n+2) a electrons, where p is an integer equal to 0,1,2,3 ...........
This is known as 'Huckel rule' according to which the aromatic compounds have delocalized electron cloud of p electrons of 2 or 6 or 10 or 14 electrons.

The term 'aromatic compounds' was first used by Kekule to classify benzene and its derivatives, many of which possessed fragrant odor or aroma. This he did, to distinguish them from the already known aliphatic compounds which lacked such odor. As new facts came to light, the term 'aromatic' came to be associated with 'chemical stability', rather than aroma.

 

Aromatic Definition

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With the development of structural theory, the stable character of aromatic compounds was attributed to the presence of a ring structure specific of benzene. So that, now the term aromatic compounds stands for "The entire series of compounds which contains one or more benzene rings in their molecule."

The introduction of the new name 'arenes' for all aromatic hydrocarbons like benzene, naphthalene, anthracene, etc, a very particular definition for aromatic compounds was given and that is arenes and their derivatives.

Like the aliphatic compounds, aromatic compounds include the hydrocarbons, hydroxy derivatives (phenols), ethers, aldehydes, ketones, carboxylic acids, etc.

Aromatic Compounds

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Compounds with benzene are called as aromatic compounds. There are many sources from which aromatic compounds are derived from.

1. Coal as a source of Aromatic compounds


Earlier than 1950, coal was the chief source of aromatic compounds. With the development of the petroleum industry, this source is no longer that important. Coal is formed by the thermal decomposition of trees buried under earth's crust long time back. It has a sieve carbon structure in which complex aromatic molecules are entrapped. The coal tar obtained from the carbonization of coal contains high proportion of aromatic compounds, or aromatics.

2. Petroleum as a source of aromatic compounds


Petroleum has now become a major source of aromatic compounds, due to its increasing demands. Most of the benzene and almost all the toluene and xylenes, are derived from petroleum. These in turn are basic raw materials for most other aromatic compounds.

Aromatic Hydrocarbons

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Benzene and its homologue constitute the most important class of Arenes or aromatic hydrocarbons. Benzene and its homologues are frequently called by common names which are accepted by the IUPAC system. The homologues of benzene having a single alkyl group are named as Alkylbenzenes. These are made of a benzene ring to which is attached the alkyl group. The alkyl group bonded to a ring carbon is designated as the side-chain.

Aromatic hydrocarbons consist of benzene and compounds containing benzene like rings. As with aliphatic compounds, aromatic hydrocarbons also has a blend of common and IUPAC names. The use of common name is more prevalent in the case of aromatic hydrocarbons.

1. Naming of Mono substituted Benzene


Benzene is represented by regular hexagon with a circle inscribed in it. The mono-substituted derivatives of benzene are those in which a hydrogen of the ring is replaced by another substituent. These are named by prefixing the name of the substituent to the word 'Benzene'.

For example - Benzene in which one of the hydrogens is replaced by bromine (Br) is named as Bromobenzene. Here, the bromine atom may occupy any of the six positions on the benzene ring.

2. Naming substituted benzenes


When there are two substituents on the ring, three positional isomers are possible. Their relative positions are indicated by prefixes Ortho- (o), Meta (m) and Para - (p) or by numbers indicating their positions.

3. Naming of poly substituted benzenes


When three or more substituents are placed on the benzene ring, numbers must be used to designate their positions. One of the substituents is put on the op of the hexagon and numbered '1'. The numbering of the remaining positions is done clockwise or anticlockwise, whichever gives lowest possible group of numbers to the other substituents.

4. Fused Polycyclic Arenes


There are numerous polycyclic aromatic hydrocarbons or arenes, having one or more benzene rings fused together in ortho positions. From each of these an IUPAC numbering system is used to indicate positions of substituents. For example- Naphthalene (two fused benzene rings), anthracene (three fused benzene rings), etc.

Aromaticity Rules

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Rules of aromaticity or the Huckel rule states the rules for aromatic compounds.

The aromatic compounds apparently contain alternative double bond and single bonds in a cyclic structure and resemble benzene in chemical behavior. They undergo substitution rather than the addition reaction. This characteristic behavior is called aromatic character or aromaticity.

Aromaticity is, in fact, a property of the sp2 hybridized planar rings in which the p orbitals (one on each atom) allow cyclic delocalization of π electrons.

Criteria for Aromaticity


On the basis of the above consideration, certain criteria or rules are laid out to identify if a compound is aromatic or not.
  1. An aromatic compound is cyclic and planar.
  2. Each atom in an aromatic ring has a p orbital. These p orbitals must be parallel so that a continuous overlap is possible around the ring.
  3. The cyclic π molecular orbital (electron cloud) formed by overlap of p orbitals must contain (4n + 2) pi electrons, where n = integer 0, 1, 2, 3, etc. This is known as Huckel rule.

When we apply this rule to Benzene

1. Benzene


It is cyclic and planar compound. It has a p orbital on each carbon of the ring involved in a double bond. It has three double bonds and six pi electrons, which is in accordance to the Huckel rule.

4n + 2 = 6 or 4n = 6- 2 = 4
n = 1

2. Cyclo-heptatriene


This is a cyclic and planar compound. It has three double bonds and six π electrons. But, one of the carbons is saturated and does not possess a p orbital. Hence a continuous overlap around the ring is not possible. This compound is, therefore, non-aromatic.

3. Cyclo octatetraene


It is cyclic and has p orbital on each atom of the ring. The Huckel rule is not satisfied, since there are 8 pi electrons.

Applying the Huckel rule

4n + 2 = 8
4n = 6
n = 1.5.

This compound is non-aromatic.

More topics in Aromaticity
Aromatic Ring Aromatic Hydrocarbons
Huckel's Rule of Aromaticity Aromatic Compounds
Aroma Chemicals Polynuclear Aromatic Hydrocarbons
Aromaticity Rules Non Aromatic Compounds
Antiaromatic
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