We’ve already examined the functional groups that can be present in organic molecules in a previous post; here, we take a look one step further, at heterocycles. Heterocycles are hugely important in organic chemistry – they make up more than half of all known organic compounds. Caffeine is a prime example of an everyday chemical that is composed of heterocycles, as is nicotine, and there are plenty of others in pharmaceuticals and natural products we use on a natural basis. Parts of our DNA are even made up of compounds which contain heterocycles.
Perhaps as a result of the wide range of possible heterocycles, their naming can sometimes seem a little more complicated. For many heterocycles, discovered several generations ago, archaic names are still preferred to the more systematic names that are used for more recent discoveries. All heterocycles can, however, be referred to by these systematic names, which are given in italics under the more commonly used names in the graphic above.
The Hantzsch-Widman naming system is used to name heterocycles. Firstly, a prefix is given for the element other than carbon which makes up the heterocycle:
Following this, the vowel at the end of the prefix is removed, and a suffix is added. This suffix is based on the number of atoms that make up the ring, and also depends on whether the heterocycle is saturated (contains only single bonds) or unsaturated (contains double bonds):
The main classes of heterocycles are those containing oxygen, nitrogen or sulfur. However, examples containing selenium, phosphorus, silicon, boron and arsenic are also possible, although rarer. The quicker on the uptake will have deduced that the compound ‘arsole’ is a possibility (an unsaturated, 5-membered ring containing an arsenic heteroatom). Although arsole itself has not been isolated in its pure form, derivatives of it have been, to the amusement of chemists everywhere. Whilst we’re on the subject, an alternative name for imidazole, shown in the graphic, is ‘miazole’, and there’s also a molecule named ‘urazole’. Who says chemists don’t have a (crude) sense of humour?
Moving swiftly on: as we already mentioned, heterocycles can be found everywhere. It’s estimated that 90% of new drugs contain heterocycles, and they’re also found in abundance in natural processes. Many of the biological molecules in your body contain heterocycles, from the neurotransmitter serotonin, to the enzymes that help you break down and metabolise food. Part of the reason for them being so widespread may be because of their usefulness in being able to undergo a wide range of reactions in differing conditions.
This is just a very brief overview of heterocycles, which has focused mainly on their naming and occurrences. If you’re interested in learning more about them, there are several links included in the further reading section at the bottom of the page, which you might find of interest.
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References & Further Reading
- Pharmacological significance of heterocycles – R Dua and others.
- Nomenclature of heterocyclic compounds
- Heterocyclic compounds – Virtual Textbook of Organic Chemistry
- Heterocyclic chemistry – Dr. P Baran, Scripps Research Institute