Annoying Heterocycles

Nomenclature is one of the most annoying roadblocks encountered while reading the chemical literature, and in no structural class is this more apparent than the heterocycles. Indeed, the problem is only getting worse as molecular structures grow more and more complex and unique. I’ve been known to pass over a paper or two at the sight of some obnoxious heterocycle in the title.

The Hantzsch-Widman system is currently the accepted method for the systematic naming of heterocyclic compounds. Ring size and saturation are specified by a suffix:

Size: (unsaturated), (saturated)
3-membered: -irine, -irane
4-membered: -ete, -etane
5-membered: -ole, -olane
6-membered: -ine, -inane
7-membered: -epine, -epane
8-membered: -ocine, -ocane

Small nitrogen-containing rings are named slightly differently; -iridine, -etidine, and -olidine are used for 3-, 4-, and 5-membered rings respectively. The location of heteroatoms on the ring is specified by prefixes. Oxa-, thia-, selena-, aza-, phospha-, sila-, and bora- are examples, and elements from earlier groups are placed first. So, for example, a four-membered ring with an oxygen atom next to a nitrogen is a 1,2-oxazetidine. Rings that are not fully conjugated are named as hydrogenated derivatives of the fully unsaturated heterocycles.

We can thank history for a plethora of exceptions to the H-W system. The small nitrogen cycles of course come to mind, with names starting in pyrr- or pip-. 1,2-diazole is commonly known as pyrazole, for example. Fused heterocycles are an absolute mess; a picture’s worth a thousand words.

Put your thinking caps on boys and girls!

Note from personal experience: committing the H-W system to mind may help improve your love life. In my case, it was “oxazaborolidine”…but that’s a story for another time.



  1. Oxazaborolidine? That is one funny pet name for a significant other.

    This is one of those areas of chemistry that I’m glad that someone knows how it works, but I’m even more glad that person isn’t me.


  2. She asked me what my favorite “sciency” word was…at the time, under the influence of copious amounts of alcohol, that was all I could come up with. But now she thinks I’m a genius… ;-D


  3. that’s awesome! note to self: develop a favourite sciency word that isn’t oxazaborolidine, or, barring that, acutally figure out how to pronouce it under the influence of copious amounts of alcohol.


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