Benzene

Benzene

Introduction

Benzene is one of the most important organic compounds with the chemical formula C6H6. It is the simplest organic, aromatic hydrocarbon. Benzene is parent compound of the various aromatic compound. Benzene is one of the elementary petrochemicals and a natural constituent of crude oil. It has a gasoline-like odor and is a colorless liquid. Benzene is highly toxic and carcinogenic in nature. It is primarily used in the production of polystyrene.

Structure of Benzene

The structure of benzene has been of interest since its discovery. Benzene is a cyclic hydrocarbon (chemical formula: C6H6), i.e., each carbon atom in benzene is arranged in a six-membered ring and is bonded to only one hydrogen atom. According to molecular orbital theory, benzene ring involves the formation of three delocalized π – orbitals spanning all six carbon atoms, while the valence bond theory describes two stable resonance structures for the ring.

Discovery of Benzene

The word benzene derives historically from gum benzoin, sometimes called ‘benjamin’. Gum benzoin was known as an aromatic resin. Michael Faraday, an English scientist first discovered Benzene in illuminating gas.  

Properties of Benzene

The various properties of benzene are mentioned below:

  • Benzene is immiscible in water but soluble in organic solvents.
  • It is a colorless liquid and has an aromatic odor.
  • It has a density of 0.87g cm-3. It is lighter than water.
  • Benzene has a moderate boiling point and a high melting point. (Boiling point: 80.5°C, Melting point: 5.5°C)
  • Benzene shows resonance.
  • It is highly inflammable and burns with a sooty flame.

Resonance of Benzene

The oscillating double bonds in the benzene ring are explained with the help of resonance structures as per valence bond theory. All the carbon atoms in the benzene ring are sp2 hybridized. One of the two sp2 hybridized orbital of one atom overlaps with the sp2 orbital of adjacent carbon atom forming six C-C sigma bonds. Other left sp2 hybridized orbitals combine with s orbital of hydrogen to form six C-H sigma bonds. Remaining unhybridized p orbitals of carbon atoms form π bonds with adjacent carbon atoms by lateral overlap.

This explains an equal possibility for the formation of C1 –C2, C3 – C4, C5 – C6 π bonds or C2 – C3, C4 – C5, C6-C1 π bonds. The hybrid structure is represented by inserting a circle in the ring as shown below in the figure. Hence, it explains the formation of two resonance structures proposed by Kekule.

Benzene

Aromaticity of benzene

Benzene is an aromatic compound, as the C-C bonds formed in the ring are not exactly single or double, rather they are of intermediate length. Aromatic compounds are divided into two categories: benzenoids (one containing benzene ring) and non-benzenoids (those not containing benzene ring), provided they follow Huckel rule. According to Huckel rule, for a ring to be aromatic it should have following property:

  • Planarity
  • Complete delocalization of the π electrons in the ring
  • Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .)

Uses of Benzene

Benzene is used in various industrial processes such as in the manufacture of lubricants, plastics, rubbers, dyes, synthetic fibers, etc. However, it has non-industrial uses too which are limited due to the reason benzene is toxic and carcinogenic. The different uses of Benzene are mentioned below.

  • Benzene is used in the preparation of phenol. It is also used to prepare aniline used in dyes and in dodecylbenzene used for the detergents.
  • In early times, benzene was used in degreasing of metal.
  • It is used for manufacturing of nylon fibers.
  • The main use of benzene is that it is used in the manufacture of other chemicals such as ethylbenzene, cyclohexane, cumene, nitrobenzene, alkylbenzene, etc.

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Aqueous solution of HCl has the pH = 4. Its molarity would be