Aromatic Hydrocarbons

Submitted by jwmoore on Sat, 01/15/2011 - 11:23

AromaticReferring to a hydrocarbon containing benzene-like rings and exhibiting characteristic stability and patterns of reactivity. hydrocarbons are one of the three classes of compounds found in petroleum. They are less abundant than the alkanes and cycloalkanes, amounting to only a few percent of the total, but they are quite important commercially. All aromatic hydrocarbons contain a benzene ring. You will recall from the discussion on resonance that benzene, C6H6 contains a flat ring of six carbon atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. joined by bonds which are intermediateIn chemical kinetics, a species that is formed in an early step in a reaction mechanism and then consumed in a later step; evidence of existence of an intermediate may be important for the interpretation of a rate law. in character between single and double bonds. The benzene ring is usually indicated by

Image:benzene.jpg

In the latter structure the lines represent C—C bonds, but carbon and hydrogen atoms, as well as C—H bonds, have been omitted. The benzene ring is very stable, surviving unchanged in most chemical reactions. It is very different in reactivity and shape from the puckered six-membered rings found in cycloalkanes. Below are 3D Jmol models of both cyclohexane and benzene.


Examples of aromatic hydrocarbons found in crude oil are

Image:Aromatic Hydrocarbons in Crude Oil.jpg

Note that the three xylenes are also isomers. Compounds containing two benzene rings joined together, such as naphthalene, are also found in crude oil, though they are much rarer than benzene-related compounds.

Image:Napthalene.jpg

Aromatic hydrocarbons are much more common in coal than in petroleum, though in the United States they are mostly manufactured from the latter. In addition to their use in motor fuel, they may be made into dyes, plastics, explosives, detergents, insecticides, medicines, and many other products. In 2000, a total of 6.74 × 1012 liters of benzene were produced in the US, after compensation for exportation and importation[1].

Some aromatic compounds, benzene among them, are toxic. The compound 1,2-benzopyrene was the cause of the first demonstrated case of occupational disease.

Image:1,2-Benzopyrene.jpg

During the eighteenth century chimney sweeps in London were found to have extremely high rates of skin cancer relative to the average person. This was eventually traced to the carcinogenic (cancer-causing) properties of 1,2-benzopyrene in the soot which coated the insides of the chimneys they cleaned. Small quantities of the compound were produced by inefficient combustionVigorous combination of a material with oxygen gas, usually resulting in a flame. of coal in the fireplaces used to heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. London houses.

References
  1. Price, C. "Summary of available production volumes in millions of gallons per year for selected volatile organic compounds(VOCs) studied by the National Water Quality Assessment(NAWQA) Program." USGS. March 07, 2005. water.usgs.gov/nawqa/vocs/national_assessment/aboutvocs/prod_data.xls