The Benzene Molecule
Benzene is colorless, flammable, toxic liquid with a pleasant aromatic odor. It boils at 176°F and solidifies at 42°F. Benzene is a hydrocarbon, with formula C6
H6
, and is obtained for industrial purposes from the distillation of coal tar, a by-product in the manufacture of coke, and from petroleum by special reforming methods. It is used in the manufacture of plastics, synthetic rubber, dyes, and drugs. Benzene is a known carcinogen.
Once benzene was identified as a distinct molecule, it was observed almost immediately that all 6 carbon atoms are chemically equivalent; the only way this is possible is if the 6 carbon atoms are arranged in a ring. Regardless, this presented difficulties when trying to use Covalent Bond Theory and the Octet Rule to explain the structure of the benzene molecule.
A single covalent bond is created when two atoms share a pair of electrons. There is no net charge on either atom; the attractive force is produced by interaction of the electron pair with the nuclei of both atoms. If the atoms share more than two electrons, double and triple bonds are formed, because each shared pair produces its own bond. By sharing their electrons, both atoms are able to achieve a highly stable electron configuration.
For example, in methane
(CH4), carbon shares an electron pair with each hydrogen atom; the total number of electrons shared by carbon is eight; each hydrogen shares two electrons.
Covalent bonds are of particular importance in organic chemistry because of the ability of the carbon atom to form four covalent bonds. These bonds are oriented in definite directions in space, giving rise to the complex geometry of organic molecules. If all four bonds are single, as in methane, the shape of the molecule is that of a tetrahedron (triangular based pyramid); the reason is that the tetrahedron structure allows the maximum physical separation of the 4 bonds. If a carbon atom has 3 bonds (that is, two single and one double), then the bonds lie in the same plane, since that allows the maximum separation of the 3 bonds.
The first attempt to represent the benzene molecule was with single bonds between all of the carbon atoms. For greater clarity through less clutter in the diagram, the carbon atoms at each corner of the hexagon are not depicted.
The problem with this configuration is that each carbon atom has only 3 bonds, in violation of the octet rule. If one tries to fix this problem by putting an extra unshared pair of electron on each carbon atom, then the octet rule is satisfied, but then the geometry in the vicinity of each carbon atom will be tetrahedral. However, this would imply that the carbon atoms don’t lie in the same plane, which is known to be false since the benzene molecule is planar.
The German chemist Friedrich Kekulé (1865) proposed a model for benzene as a hexagon of six carbon atoms joined by alternating single and double bonds and each bearing one hydrogen atom.
This model accurately predicted the flatness of the benzene molecule. For greater clarity and less clutter in the diagram, and the appropriate number of hydrogen atoms attached to each carbon atom is understood (in accordance with the octet rule for carbon).
However, there was a problem with this depiction as well, the details of which i will skip here for reasons of brevity. Suffice it to say that in a model of the benzene molecule which has alternating single and double bonds, each carbon atom is not equivalent.
What to do? The octet rule had worked so well incredibly well in predicting bonding and geometry in molecules, and scientists often have a great deal of difficulty abandoning such useful models.
It was eventually shown that in carbon to carbon bonds, double bonds are actually shorter than single bonds. Thus, if the benzene molecule actually had alternating single and double bonds, it would not be a regular hexagon.
Modern studies have shown that the six carbon-carbon bonds are all of equal strength and length; in fact, the length is intermediate between the lengths of the single and the double bond. The modern interpretation of the benzene molecule is that the electrons are delocalized around the ring, and are not shared by any carbon-hydrogen pair.
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