Benzene is a colourless liquid hydrocarbon, C6H6, b.p. 80 °C. It is now made from petroleum by catalytic reforming (formerly obtained from coal tar). Benzene is the archetypal aromatic compound. It has an unsaturated molecule, yet will not readily undergo addition reactions. On the other hand, it does undergo substitution reactions in which hydrogen atoms are replaced by other atoms or groups.
In 1865, Friedrich August Kekulé purposed the benzene molecule structure as a hexagonal ring which consists of six carbon atoms with alternate carbon-carbon single and carbon-carbon double bond. But such a structure should be highly reactive, and so didn't account for the unreactive nature of benzene. We now know that the best representation for the structure of benzene is indeed, hexagonal, with each C-C bond distance being identical and intermediate between those for a single and double bond. The π-orbitals from each neighbouring carbon atom overlap to form a delocalised molecular orbital which extends around the ring, giving added stability and with it, decreased reactivity. That is the reason the structural formula of benzene represents as a hexagon with a circle in the center which represents the delocalized electrons.
Aromatic compounds are a major group of unsaturated cyclic hydrocarbons containing one or more rings, typified by benzene, which has a 6-carbon ring containing three double bonds. All the bonds in benzene (C6H6) are the same length intermediate between double and single C-C bonds. The properties arise because the electrons in the p-orbitals are delocalised over the ring, giving extra stabilization energy of 150 kJ/mol over the energy of Kekulé structure. Aromatic compounds are unsaturated compounds, yet they do not easily partake in addition reactions.
Historical use of the term implies a ring containing only carbon (e.g., benzene, naphthalene), but it is often generalized to include heterocyclic structures such as pyridine and thiophene.
Azo compounds are organic compounds containing the group -N=N- linking two other groups. They can be formed by reaction of a diazonium ion with a benzene ring.
Coal tar is a material obtained from the destructive distillation of coal in the production of coal gas. The crude tar contains a large number of organic compounds (e.g. benzene, naphthalene, methylbenzene, etc.), which can be separated by fractional distillation.
Cyclic describing a compound that has a ring of atoms in its molecules. In homocyclic compounds all the atoms in the ring are of the same type, e.g. benzene (C6H6) and cyclohexane (C6H12). These two examples are also examples of carbocyclic compounds; i.e. the rings are made of carbon atoms. If different atoms occur in the ring, as in pyridine (C5H5N), the compound is said to be heterocyclic.
Dielectric constant or permittivity (ε) is an index of the ability of a substance to attenuate the transmission of an electrostatic force from one charged body to another. The lower the value, the greater the attenuation. The standard measurement apparatus utilises a vacuum whose dielectric constant is 1. In reference to this, various materials interposed between the charged terminal have the following value at 20 °C:
vacuum | 1 |
air | 1.00058 |
glass | 3 |
benzene | 2.3 |
acetic acid | 6.2 |
ammonia | 15.5 |
ethanol | 25 |
glycerol | 56 |
water | 81 |
The exceptionally high value for water accounts for its unique behaviour as a solvent and in electrolytic solutions. Dielectric constant values decrease as the temperature rises.
Meta position in organic chemistry is the one in which there are two same functional groups tied to a ring of benzene in position 1 and 3. The abbreviation m- is used, for example, m-Hydroquinone is 1,3-dihydroxybenzene.
Ortho position in organic chemistry is the one in which there are two same functional groups, tied to a ring of benzene in the positions 1 and 2. The abbreviation o- is used, for example, o-Hydroquinone is 1,2-dihydroxybenzene.
Generalic, Eni. "Benzen." Croatian-English Chemistry Dictionary & Glossary. 29 June 2022. KTF-Split. {Date of access}. <https://glossary.periodni.com>.
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