Aromatic Structure: Detailed Explanations

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In this article, “aromatic structure”, what is aromatic structure, properties and relevant topics are discussed briefly.

 Aromatic compounds have (4n+2) number of electrons with a planar structure with alternating single and double covalent bonds. Aromatic compound must be cyclic and sp2 hybridized. One of the most important fact about aromatic structure is they have extra stability with comparing to similar aliphatic or non-aromatic compounds.

Main outlook of this article is to specify the definition of aromatic structure and its characteristics.

Definition of Aromatic Structure

Compounds having aromatic structure is known as aromatic compounds. Aromatic structure is defined as those structure observed in organic compounds known as aromatic compound. Aromatic compounds have extra stability with comparing to other nonaromatic or aliphatic compound and the aromatic compounds obey Huckel’s rule. According to this rule aromatic compounds must satisfy the following points-

  •  Aromatic compounds must have planar structure.
  • The must have (4n+2) number of pi electrons in a conjugated system of p orbitals. [n is a non-negative integer].
  • Aromatic compounds must be cyclic.
  • They must have resonance energy.

Benzene Model Structure

Benzene (C6H6) is a well known aromatic compound having 6 pi electrons (4n+2 rule, n=1). Benzene has total six carbon and six hydrogen atoms with alternating single and double bonds.

aromatic structure
Benzene Resonance Structure.
Image Credit: Wikimedia Commons.

Structure of benzene can be shown by the above two models. Both the structures are equivalent and contribute in equal amount to the hybrid structure. In benzene all the C-C bond lengths are equal (1.39 angstorm) due to rapid flipping from one structure to another whereas C-C single bond length is 1.5 angstorm and C-C double bond length in benzene is 1.3 angstorm. They are actually hybrid of carbon-carbon single bond and carbon-carbon double bond.

All of the carbon atoms present in benzene are sp2 hybridized with planar structure. The unhybridized p-orbitals overlap in side by side and from the pi bonds. Sigma bonds between carbon atoms are formed due to head on overlap of sp2 hybrid orbitals.

Some examples of benzene like aromatic structures are phenol (one hydrogen atom of benzene is substituted by hydroxyl group), toluene (substitution by CH3 group). Some bi substituted benzene like structures are p-xylene, diethyl benzene, salicylic acid etc.

Resonance Energy in Aromatic Compounds

 Resonance energy is defined as the extra stability gained by conjugated system with respect to those compounds having no conjugation. Resonance energy of any aromatic compounds is greater and thus they have extra stability.

A compound is aromatic or not can be decided through the amount of resonance energy of that compound. Resonance energy = Energy of most stable contributing structure – Energy of resonance hybrid. The calculated resonance energy of benzene is 150 KJ/mol.  This high amount of resonance energy is due to the resonance present in benzene molecule.

Why aromatic compounds are stable?

It is proved that aromatic compounds have some extra stability. One of the reason behind this extra stability is the carbon hydrogen ratio. This ratio is greater in alkane is greater than 2, where as this ratio is relatively smaller in aromatic compounds due to presence of double bonds participate in conjugation.

The electrons are delocalized to all over the aromatic structure due to conjugation. More delocalization of electrons indicates lesser interelectronic repulsion. And we all know if any type of repulsion is present in any compound, that will not gain stability. This repulsion is lesser in aromatic compounds due to resonance present in aromatic structure.

Nonaromatic and Antiaromatic Compounds

Antiaromatic compounds have the following characteristics-

  • They have 4n number of pi electrons. [n is non-negative integer].
  • They have also cyclic and planar structure.
  • Conjugation is also present in antiaromatic compounds.
  • They are not stable like aromatic compounds.

For example, biphenylene, cyclopentadienyl cation (B), cycle butadiene (A).

Antiaromats2 jpeg
Some antiaromatic compounds.
Image Credit: Wikimedia Commons.

Nonaromatic compounds are also one type of classification which satisfies the following properties-

  • Non aromatic compounds are non-cyclic non planar.
  • Conjugation is totally absent in nonaromatic compounds

Aliphatic compounds are generally the examples of nonaromatic compounds. For example- 1-hexyne, 4-vinyl cyclo hexane.

There is another category known as homoaromatic compounds.  It is a special case of aromaticity in which electrons cannot be delocalized through all over the molecular structure due to presence of one sp3 hybridized atom. If this sp3 atom can be avoided then rest of the molecule will be aromatic. For example, homotropylium cation (C8H9+).

Homotropylium Cation and Bishomotropylium Cation side by side jpeg
Homotropylium Cation.
Image Credit: Wikimedia Commons.

Frequently Asked Questions (FAQ)

What are heterocyclic aromatic compounds?

Answer: Heterocyclic aromatic compounds are one type of organic aromatic compounds in which one or more than one carbon atom is replaced another atom like oxygen, nitrogen, sulfur etc. For example, pyrrole, furan, thiophene.

What is the significance of aromatic compounds in organic chemistry?

Answer: Aromatic hydrocarbons are basically used as non-polar solvent. Non polar molecules will be soluble in non-polar solvent.