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Aromatic Hydrocarbons

An aromatic hydrocarbon is any hydrocarbon that contains one or more benzene rings in its molecule. The name "aromatic" is historical in origin, and does not at all imply that these compounds have pleasant aromas. Aromatic hydrocarbons are the basis of many aromatic compounds containing other atoms such as oxygen and nitrogen in addition to the carbon and hydrogen that are of extreme biological and industrial importance.

The simplest aromatic hydrocarbon is benzene itself, C6H6, whose molecule is a hexagonal ring of six CH groups. Various carbon-and-hydrogen groups can be substituted for any or all of the hydrogen atoms in benzene to form substituted benzenes. Benzene's own phenyl groups, C6H5, can bond to each other end to end, to form polycyclic (multiple-ring) hydrocarbons, or they can fuse together along the hexagons' sides to form condensed ring or fused ring hydrocarbons.

In this figure, the bonds leading to all the hydrogen atoms are omitted for simplicity as is the usual practice among chemists. Also, the benzene rings are drawn with alternating double and single bonds between the carbon atoms. In reality, however, resonance makes all the carbon-carbon bonds equal at an intermediate value between single and double. Chemists therefore usually draw the benzene ring simply as a hexagon with a circle inside:

Boiling-temperature rangea Name of fraction Number of carbon atoms in moleculea Uses
a The exact temperature ranges and numbers of carbon atoms differ in different refineries, and according to various legal definitions in various states and countries.
Below 36˚C Natural gas 1-5 Fuel; starting material for making plastics
40-60˚C Petroleum ether 5-6 Solvent
70-90˚C Naphtha 6-7 Solvent; lighter fuel
69-174˚C Gasoline 6-10 Fuel for engines, industrial solvent
174-288˚C Kerosene (coal oil) 10-16 Fuel for lamps, heaters, tractors, jet airplanes
250-310˚C Fuel oil (gas oil) 15-18 Heating oil; diesel fuel
300-370˚C Lubricating oils 16-20 Lubrication
Melts at 40-55˚C Petrolatum (petroleum jelly) 17-30 Lubrication; ointments
Melts at 50-60˚C Paraffin wax 23-29 Candles; waterproof coatings
Above 515˚C Pitch, tar Over 39 Paving, roofing

The hexagon represents the six carbon atoms and their attached hydrogen atoms, while the circle represents all the bonding electrons as if they were everywhere in the molecule at once. In chemists' shorthand, then, naphthalene would be depicted as shown in the above figure.

Among the important substituted benzenes are methyl benzene, commonly known as toluene, and dimethyl benzene, commonly known as xylene. They are both powerful solvents for organic compounds and are used as starting materials for the synthesis of drugs, dyes, plastics, and explosives. Treatment of toluene with nitric and sulfuric acids produces the explosive trinitro-toluene, or TNT.

Among the important condensed ring aromatic hydrocarbons are naphthalene and anthracene whose molecules consist of two and three hexagonal benzene rings, respectively, fused together along one side.

Both are derived from coal tar and are used as starting materials for the synthesis of many useful compounds. Naphthalene is a crystalline solid with a strong, pungent odor; it is used as a moth repellant and a deodorant-disinfectant.

Additional topics

Science EncyclopediaScience & Philosophy: Hydrazones to IncompatibilityHydrocarbon - Carbon's Chemical Bonding, Aliphatic Hydrocarbons, Alkanes, Alkenes, Alkynes, Aromatic Hydrocarbons