Rings of atoms are also common in organic structures. You may have heard the famous story of Auguste Kekulé first realizing that benzene has a ring structure when he dreamed of snakes biting their own tails. You have met benzene rings in phenylalanine and aspirin. Paracetamol also has a structure based on a benzene ring.

When a benzene ring is attached to a molecule by only one of its carbon atoms (as in phenylalanine, but not paracetamol or aspirin), we can call it a ‘phenyl’ group and give it the organic element symbol Ph.

Any compound containing a benzene ring or a related (Chapter 7) ring system is known as ‘aromatic’, and another useful organic element symbol related to Ph is Ar (for ‘aryl’). While Ph always means C6H5, Ar can mean any substituted phenyl ring, in other words phenyl with any number of the hydrogen atoms replaced by other groups. Of course, Ar = argon too but there is no confusion as there are no organic compounds of argon.

For example, while PhOH always means phenol, ArOH could mean phenol, 2,4,6-trichloro phenol (the antiseptic TCP), paracetamol, or aspirin (among many other substituted phenols). Like R, the ‘wild card’ alkyl group, Ar is a ‘wild card’ aryl group. The compound known as muscone has only relatively recently been made in the laboratory. It is the pungent aroma that makes up the base-note of musk fragrances. Before chemists had determined its structure and devised a laboratory synthesis the only source of musk was the musk deer, now rare for this very reason. Muscone’s skeleton is a 13-membered ring of carbon atoms. The steroid hormones have several (usually four) rings fused together. These hormones are testosterone and oestradiol, the important human male and female sex hormones.

Some ring structures are much more complicated. The potent poison strychnine is a tangle of interconnecting rings.

One of the most elegant ring structures is shown above and is known as Buckminster fullerene. It consists solely of 60 carbon atoms in rings that curve back on themselves to form a football-shaped cage. Count the number of bonds at any junction and you will see they add up to four so no hydrogens need be added. This compound is C60. Note that you can’t see all the atoms as some are behind the sphere. Rings of carbon atoms are given names starting with ‘cyclo’, followed by the name for the carbon chain with the same number of carbon atoms. Structure 1 shows chrysanthemic acid, part of the naturally occurring pesticides called pyrethrins (an example appears in Chapter 1), which contains a cyclopropane ring. Propane has three carbon atoms. Cyclopropane is a three-membered ring. Grandisol (structure 2), an insect pheromone used by male boll weevils to attract females, has a structure based on a cyclobutane ring. Butane has four carbon atoms. Cyclobutane is a four-membered ring. Cyclamate (structure 3), formerly used as an artificial sweetener, contains a cyclohexane ring. Hexane has six carbon atoms. Cyclohexane is a six membered ring.