Heterocycles in Organic Chemistry
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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:

Heterocycle Prefixes

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):

heterocycle naming table


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

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  • Jayarava
    Posted August 3, 2014 at 8:08 am 0Likes

    Ah. The arsole joke. 🙂 First heard in 1985 in Org Chem Lecture in NZ where it was introduced as an analogue of phosphole – with -R instead of -H. Apparently one of the main reaction these 5 rings undergo is “an insertion into the ring” to make a 6 ring.

    • Compound Interest
      Posted August 4, 2014 at 7:13 am 0Likes

      I remember one of my organic chemistry lecturers introduced it in a similar way – he took great enjoyment in using the phrase ‘insertions into arsoles’…

  • Trackback: A Guide to Simple Heterocycles in Organic Chemistry | Compound … | Organic Chem Tutor
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  • Daniel
    Posted August 6, 2014 at 3:03 pm 0Likes

    The last heterocycle (Morpholine) has the wrong formula (C2H4O, which is Ethylene Oxide). Wikipedia says it should be O(CH₂CH₂)₂NH , which looks right following the drawing of the molecule.
    Thanks for your efforts in this blog, I discovered it recently and I’m really enjoying it (studied chemistry many years ago, but haven’t practiced in a very long time).

    • Compound Interest
      Posted August 6, 2014 at 3:12 pm 0Likes

      Thanks for the heads up! Just a simple error on my part, I copy-pasted the text boxes in the graphic then changed them, so they all started off as ethylene oxide. Clearly missed changing that last one! I’m pushed for time at the minute but I’ll correct it this evening.

      • Daniel
        Posted August 6, 2014 at 3:26 pm 0Likes

        No rush, of course! I imagine you have “real work” to do…
        While you’re at it, Pyrazine also slipped through the cracks 😉
        And thanks again for making the Intertubes interesting.

        • Compound Interest
          Posted August 7, 2014 at 6:31 pm 0Likes

          Thanks – should be all updated and correct now 🙂

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