Chemistry of Arenes

Topics: Benzene, Aromatic hydrocarbon, Hydrogen Pages: 6 (1460 words) Published: November 1, 2011
The aromatic hydrocarbons also have the name arenes. They contain in their molecule one or more cycles made up from 6 carbon atoms. When the molecule is formed out of a single cycle, the hydrocarbons are mono nucleuses; when the molecule contains more than one cycle, the hydrocarbons are poly nucleuses. The simplest aromatic hydrocarbon, benzene, is compound out of just one such cycle; its formula is C6H6. The representation of benzene through a cycle of 6 carbon atoms with 3 double bounds was proposed by Kekule in 1865:

In some special conditions, benzene can be hydrogenated, the result being a cyclic hexane: | +3H2 | |

Under the influence of light, chlorine or bromine addition at the benzene’s molecule giving hexachlorocyclohexane: C6H6 + 3Cl2 C6H6Cl6. With ozone, the benzene gives a trizonide, which by decomposing (with water), passes in glycoxal. These addition reactions prove the following: benzene has a cycle of 6 carbon atoms; there are three double bounds in the cycle. Yet, the addition reactions at benzene take place only in special conditions, benzene usually giving substitution reactions, like:

With halogens: C6H6 + Cl2 C6H5Cl + HCl
With sulfuric acid: C6H6 + HO-SO3H C6H5-SO3H + H2O
With nitric acid: C6H6 + HO-NO2 C6H5-NO2 + H2O
The easy formation of the substitution products is a proof that benzene’s character is less unsaturated than the hydrocarbons with conjugated double bounds. Furthermore, benzene has a pronounced saturated character. Yet, this behavior does not correspond to Kekule’s structural formula, which says that three conjugated double bound should exist. Another critic brought to this formula is that she predicts the existence of for isomers than in reality. If two hydrogen atoms in the benzene’s molecule are substituted with two bromine atoms, then, according tot Kekule’s rule, there should be two isomers containing two bromine atoms connected to two nearby carbon atoms (positions 1,2 and 1,6) at one the two carbon atoms being separated through a double bound, and at the other, through a simple bound. In reality, such isomers due exclusively to the position of the double bound are unknown. The symmetry of the benzenic cycle illustrates the existence of only one substituted product in the position 1,2 contrary to the representation in which benzene would have the structure 1,3,5-cyclohexatryene. There are three isomers of the dibrominobenzene due to the substitution of the bromine at different positions of the cycle. They have very different boiling points. At one of this isomers (with boiling point = 1.8 Celsius degrees), the two bromine atoms are connected to the nearby carbon atoms; at the second isomer (with boiling point = -7 Celsius degrees), the bromine atoms are connected to two carbon atoms which are separated by a CH group (in which the hydrogen atom is not substituted); at the third isomer (with boiling point = 87 Celsius degrees), the bromine atoms are connected to two carbon atoms separated by two CH groups (in which the hydrogen atoms are not substituted). Therefore the positions 1,2 and 1,6 are equivalent, same as the positions 1,3 and 1,5: | | |

Kekule tried to explain the unfit between the number of derivate substitute isomers of benzene in his formula and the existent ones, by evolving the hypothesis that in the molecules, the double bounds do not occupy steady position, but that they change their location with the simple bound, meaning they move, they “oscillate”. Since the formula of Kekule does not express the equivalence between the C-C bounds in the benzene’s molecule and neither does it express the important property of benzene of giving privileged substitution reactions (and not addition reactions, as the double bounds in the formula indicate), some researchers started looking for explanations for the benzene’s structure which will also reverberate the characteristic properties of benzene. Classification

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