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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:
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.
The term "aromatic" was used by chemists to describe certain chemicals with peculiar odors long before anyone understood what atoms were or how they might be connected to form molecules. Many aromatic molecules such as benzene and those shown below have distinctive odors (aromas), but not all aromatic molecules have an odor. Can you apply Hückel's Rule to each one?
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.
See also: aliphatic, hydrocarbon, organic.
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