Home > General chemistry > The history of benzene

The history of benzene

Benzene (C6H6) is known as the most famous aromatic (cyclic) hydrocarbons. It could be famous because of its structure which is cyclic or due to its stability compared to others hydrocarbons (benzene is more stable than other hydrocarbons). As it is categorized as organic compaund, benzene can be treated by common organic reaction, e.g. nucleofilic attact, substitution reaction, dehidrogenation, and so on. It is also hydrophobic (Greek: water afraid) molecule but less hydrophobic than other organics.

Currently, chemists know very well about this molecule but benzene itself has a nice history how people try to explain this molecule. Since it was found for the first time, benzene was known has un-ussual structure. This molecule was discoverd by Michael Faraday (1791-1867) in 1825. Farady is more famous for his insights into electricity and magnetism but he was the first person who identify benzene and determine its composition as containing six carbon atoms and six hydrogen atoms. But he confused, how it connected each others. At that time, benzene was known to be unsaturated hydrocarbon which has doube or triple bond but unfortunately, it was not reactive to hydrogen. Hydrogenation reaction is common way to check the un-saturity degree of organic compound to distinguish the bond types that the molecule has.

It needs time to obtain a correct structure of benzene. The first people who proposed the benzene structure were Scottish chemist Archibald Scott Couper (1831-1892) and the German chemist Josef Loschmidt (1821-1895) but without any empirical (scientifical) evidence.

Until 1865 when a young German, Friedrich August Kekule (1829-1896) proposed the hexagonal structure which we all know and use it until now. However, he has an interesting chronology before finding that cyclic structure. At that time, he stayed at Ghent and that idea came into his dream during his afternoon nap. This story was known as Kekule snake dream as the snake was came out and inspired him to propose the new idea of benzene molecule.

Another interesting point of benzene is its non-polarity. Usually, non polar compounds resulted from well distributed electron within the atom or molecule. When we measure the polarity of benzene, we find benzene as an iso-electric molecule. However, inside the molecule there is a polarization charge between C and H. C atom will attract electron closer to its nucleus (core) and it makes C atom more negative (negative dipole). On the other hand, H atom will have a positive dipole. It is shown by electrostatic potential analysis which has ‘blue’ colour (means negative) to the center and read colour (means positive) near H atom.

If we notice more, we will find that this un-even electron distribution was also located above and below the hexagonal plane. This is known as famous pi bond interaction which create a conjugation in the benzene molecule. So, now we realize the although benzene does not have ‘net’ dipole moment, it has un-even distribution of electrone within the molecule or called as quadrupole phenomena.

X-ray data results the distance between the carbon atoms of a normal double bond is 1.34 angstroms and 1.54 for single bond. However, benzene bond has 1.397 and 1.084 for C-H bond [1,2].

Reference:

1. Herzberg, G. H. Molecular Spectra and Molecular Structure: Electronic Spectra and Electronic Structure of Polyatomic Molecules; D. Van Nostrand: Princeton, NJ, 1967; Vol. III.

2. Tamagawa, K.; Iijima, T.; Kimura, M. J. Mol. Struct. 1976, 30, 243

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Categories: General chemistry
  1. Edward Copcutt
    June 29, 2010 at 8:03 am

    Thank you for this it was very helpful for a school essay I need to write!

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