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If a molecule contains an aromatic sub-unit, this is often called an aryl group.
A prototypical aromatic compound is benzene, so a layperson might prefer to think of an aromatic compound as something that has a ring structure like that of benzene, C6H6. Shown here are 4 equivalent ways of representing the structural formula of benzene where C = a carbon atom, H = a hydrogen atom and a line is a chemical bond:
An introductory organic chemistry definition of an aromatic compound is one that has a planar ring with 4n + 2 pi-electrons where n is a non-negative integer (Hückel's Rule). For those with limited chemistry experience, in the drawings of benzene above there are six pi-electrons. If you look at the first or third drawing, the double lines (bonds) between carbon atoms each have 2 pi-electrons. There are 3 double bonds in these drawings, and therefore six pi-electrons. Systems with six pi-electrons in a planar ring are aromatic according to Hückel's Rule where n = 1, i.e. 4(1) + 2 = 6.|
Aspiring chemists will note that our current discussion is omitting a feature of aromatic molecules called resonance that explains their chemical properties. For detailed information about the special chemical properties and chemistry of aromatic molecules, see the Further Reading links below.
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For comparison, here are some molecules with ring structures that are not aromatic because they do not meet Hückel's Rule which requires that they have 2 (n=0), 6 (n=1), 10 (n=2) or 14 (n=3) etc. pi-electrons in a ring:
|Aromatic molecules containing several fused (joined) rings are called polycylic aromatics or sometimes simply "polycyclics" for short. Those polycyclic aromatics made up only of carbon and hydrogen are called polycyclic aromatic hydrocarbons or PAH's.|
Many PAH's are extremely potent carcinogens or mutagens. For example, the molecule shown on the right, benzo[a]pyrene is an exceedingly potent carcinogen found commonly in coal tar and soot, including tobacco smoke and diesel exhaust!
Side note: PAH's don't always appear to obey Hückel's Rule, but individual rings or groups of rings in the molecule do.
Treat all aromatic compounds with respect as many of them pose serious physical and health hazards such as carcinogenicity and flammability. Use proper personal protective equipment (PPE) and minimize your exposure to aromatic compounds by using proper engineering and administrative controls.
Disclaimer: The information contained herein is believed to be true and accurate, however ILPI makes no guarantees concerning the veracity of any statement. Use of any information on this page is at the reader's own risk. ILPI strongly encourages the reader to consult the appropriate local, state and federal agencies concerning the matters discussed herein.