Benzene

Topics: Benzene, Atom, Chemical bond Pages: 5 (1433 words) Published: October 9, 2013

Benzene is a special case of conjugation that leads to an especially stable bonding arrangement. The benzene ring consists of six sp2 hybridized carbon atoms in a regular hexagon. Each carbon forms two σ-bonds with adjacent carbons, and a third σ-bond with a hydrogen atom. The 2p orbital on each atom is available for π-bonding.

Interaction between the p orbitals on the six carbon atoms forms a conjugated system of π-electrons. Molecules with this bonding arrangement are called aromatic molecules.

For convenience, we usually draw benzene as one of its resonance forms.
The shape of benzene
Benzene is a planar regular hexagon, with bond angles of 120°. This is easily explained. It is a regular hexagon because all the bonds are identical. The energetic stability of benzene
This is accounted for by the delocalisation. As a general principle, the more you can spread electrons around - in other words, the more they are delocalised - the more stable the molecule becomes. The extra stability of benzene is often referred to as "delocalisation energy". The reluctance of benzene to undergo addition reactions

With the delocalised electrons in place, benzene is about 150 kJ mol-1 more stable than it would otherwise be. If you added other atoms to a benzene ring you would have to use some of the delocalised electrons to join the new atoms to the ring. That would disrupt the delocalisation and the system would become less stable. Since about 150 kJ per mole of benzene would have to be supplied to break up the delocalisation, this isn't going to be an easy thing to do. The symbol for benzene

Although you will still come across the Kekulé structure for benzene, for most purposes we use the structure on the right. The hexagon shows the ring of six carbon atoms, each of which has one hydrogen attached. (You have to know that - counting bonds to find out how many hydrogens to add doesn't work in this particular case.) The circle represents the delocalised electrons. It is essential that you include the circle. If you miss it out, you are drawing cyclohexane and not 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...
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