We all make mistakes. 'To err is human; to forgive, divine', as a very sagacious person¹ once said. Even supposedly very intelligent people, such as research chemists, make mistakes. They made one particular howler that still has the capacity, many years after it was made, to reduce whole lecture theatres full of undergraduates to paroxysms of laughter. It is a mistake that, with 20-20 hindsight, should not have been made but organic chemistry would be a much more dreary discipline without it.

It's a classic story of people painting themselves into a corner; although, this time, they did it not with paint and brushes but chemical structures and rules for naming them. The story illustrates that it is possible to make a horrendous mistake in chemistry using just paper and ink, without even the merest drop of nitroglycerine in sight.

The Recipe

First, you need a science in its infancy: organic chemistry. Organic chemistry studies the behaviour of compounds made up mostly of carbon and hydrogen, and is so-called because these compounds are the stuff of which living organisms are built.

Organic chemistry was not even into short trousers in the 19th Century; for a large part of this century, the idea of 'synthesising' organic compounds by starting off with basic chemical building blocks, such as pure carbon and hydrogen, was regarded as being preposterous. Organic chemicals were supposed to have a 'vital force' within them that could only originate from living organisms.

All the same, the stuff of living organisms can be found in many places other than the organism itself. When plants die they can get compressed into coal and oil, and this was recognised as a viable source of organic substances. Friedlieb Ferdinand Runge, a German pioneer of organic chemistry, liked to experiment with coal-tar, and he isolated several important chemicals from it. Aniline and carbolic acid (phenol) both came to be extremely important raw materials - one for dyes and the other for disinfectants and resins.

He also managed to isolate a smelly oil he called pyrrole. He called it this because it was produced by heating coal-tar² and because it was oily³. Pyrrole is very important in its own right as a chemical building block, but it is also structurally interesting. The exact chemical term for this class of compound is heterocycle; it contains a ring made up of five atoms, one of them a nitrogen atom and the remainder carbons. Five hydrogen atoms festoon the ring, like baubles on a crown, so its formula is C₄H₅N. Pyrrole has some very interesting chemistry, called aromaticity, that originates from its electronic configuration, and has been extensively investigated.

Now, the element nitrogen is a member of Group V of the Periodic Table. One of the features of the Periodic Table is that elements with similar properties all appear in columns. Nitrogen is at the top of its column, and bismuth at the bottom. In between are various well-known elements, one of these being phosphorus. Chemists like to develop rules and general principles, and so once the structure of pyrrole had been unambiguously determined, they wanted to see if the aromatic behaviour could be extended to rings containing other kinds of element, such as other Group V elements.

Over the cliff, lemming!

The logical next step was to replace nitrogen with phosphorus. The molecule phosphole resulted, being so named because it was a phosphorus-containing analogue of pyrrole. If you are French, then you will realise that it has a rather silly homonym, apparently meaning 'crazy woman' or 'ladyboy'.

The last - and loudest - laugh is reserved for the English-speaking chemist. Right below phosphorus in the Periodic Table is the notorious element arsenic, beloved of poisoners and penny-dreadful writers. Now if one were to make an arsenic analogue of pyrrole it would of course have to be called ... arsole.

You can bet your life it was. Having realised the terrible mistake they had made in setting this naming precedent, chemists decided to make light of it. Hence the hilariously-titled research papers that have been published about this esoteric (and fittingly noxious) molecule: although rarely found as the simplest form C₄H₅As⁴, arsole rings are sometimes found in other organic compounds, hence the paper entitled Studies on the Chemistry of the Arsoles⁵. And, if you want to go just one little stage further, you can do interesting chemical reactions with them, such as in the unforgettably titled paper Unusual Substitution in an Arsole Ring, by the same authors.

Who says that chemists don't have a sense of humour?

Further Reading

This website provides plenty of light relief for jaded organic chemistry students, or even laypeople just wanting to learn more about stupidly-named chemicals.