Rings and Aromatics

In many organic molecules chains of carbon atoms link together to form rings.  The most common rings are those that contain five, six or seven carbon atoms.  To name a ringed compound, the prefix cyclo- is added to the root name.

Perhaps the most important of the ringed compounds goes by its common name, benzene.  It is formed as a planar hexagon of six carbon atoms.  If the benzene ring exits without substitution or attachement to other structures, it also contains six hydrogen atoms.  Each carbon atom in C₆H₆ is bonded to two carbons and one hydrogen and is doubly bonded to one of its neighbors.  The two ways of showing this are called kekule structures but the molecule actually forms a consistent bond between each of the carbons that is an average of a single and a double bond for most purposes.  As a result, the benzene ring is usually shown with a continuous circle inside the hexagon to represent the double bonds. The CHIME image of a benzene ring is shown to the right of the line representations of benzene.  Place your mouse anywhere on the molecule in the CHIME image and hold the button down to view a menu of representations of the model.  You can rotate the image by placing your mouse anywhere on the background and holding down the mouse button while moving the mouse around. 

As a result of this bonding structure, benzene is very stable and survives intact in environments that would destroy other carbon-carbon double bonds.  Organic molecules that contain one or more of these stablilized structures are called aromatic compounds, since many are responsible for the distinctive aromas of plants and spices.

Aromatic compounds can be destablized when free radicals such as a chlorine atoms are present.  As you may recall, free radicals are atoms, or collections of atoms, with unpaired electrons.  Molecular chlorine Cl₂ cannot react with the double bonds in benzene. A single chlorine atom, a free radical, does.  Michael Faraday first noticed this reaction as far back as 1825.  When benzene was exposed to chlorine gas in strong light the chlorine was found to react with the benzene.  Strong light splits the Cl₂ molecules into free radicals.  Once such a reaction starts, it continues unitl all the carbon atoms have added one chlorine atom.  The result is a 1,2,3,4,5,6,-hexachlorocyclohexane molecule, also known as hexachlorobenzene (HCB). Benzene also undergoes substitution reactions durng which the hydrogens can be replaced by other functional groups as well as chlorine.