Acetylene (and Hydrocarbon Suffixes)

In a previous post on this blog, I discussed the possibility for confusion among modern students of chemistry due to older, generic names of organic molecules being incongruous with the systematic naming conventions we use today. The example I used to make this point was acetone, which unlike acetic acid, acetylene, and acetaldehyde, contains three carbons rather than two.

To illustrate this example, I put together a simple graphic summary, which was the image I used when I tweeted about the post (Figure 1).

Figure 1. The offending image (in its original form)

Ethene blunder

After a few very kind responses, a couple of eagle-eyed twitterers pointed out that acetylene, which I had equated with ethene, was actually the common name for ethyne. Frankly, I was mortified, and riding the wave of embarrassment I changed the image within the article at the first opportunity (although the original still remains on my twitter feed). But once my shame had abated, I realised that this mistake was another perfect example of the exact point I was trying to make about acetone. It only took a small lapse in attention for me to apply the familiar systematic naming framework to this older term, and assume that because acetylene ends in -ene it must be an alkene. So, of course, I had to look into why this wasn’t the case.

The Simplest of the Alkynes

The name acetylene was first coined in 1860, by the French chemist Berthelot.1 The gas had been isolated in an impure form by Edmund Davy in 1836, but it doesn’t seem that Berthelot was aware of this when he presented his work to the French Academy of Sciences.2 Berthelot expressed his belief that he had discovered the simplest example of a new chemical series of hydrocarbons, to go alongside the series for which methane and ethene (referred to as marsh gas and olefiant gas, respectively) represented the form.

While the chemical formulae he proposes are not quite what we use today, largely due to the common belief at the time that carbon had an atomic mass of 6 rather than 12, he is clearly demonstrating the basic concepts of alkanes, alkenes and alkynes (Table 1). He even goes on to explain how the addition of hydrogen to his new gas would product olefiant gas.

Table 1. The names and formulae of methane, ethene and ethyne, as they appear in The Proceedings of the French Academy of Sciences, 18601

Berthelots names

He calls this new gas ‘acétylène’, without any explanation whatsoever…not even a courteous footnote to allude to it being a novel name. It is entirely possible this was thought to be the obvious choice at the time and that no comment was needed. But from a modern point of view it is less clear, and a little historical context is needed to elucidate a possible rationale.

Radical theory

While the name of acetone was derived from its method of production (i.e. from acetic acid), this is not the case for acetylene. The more likely explanation centres on the acetyl radical. Around the 1830s, many chemists had started to think of organic molecules as being made of building blocks: groups of atoms that would stay together during reactions, and could be replaced in compounds by a single element: radicals.3 This theory has a massive impact on the development of systems of nomenclature in organic chemistry, allowing for binomial names such as ‘chloride of benzoyl’ and ‘oxide of ethyl’. It was even pointed out a few decades later that the advantages this theory gave to a simple nomenclature may have contributed to its success.3

The name ‘acetyl’ was assigned to the radical C4H6 by Justus Liebig in 1832.4 At the same time it was given the abbreviation ‘Ac’ which we still use today. As with all acet- names, the name traces back to the Latin word for vinegar, acetum, from which acetic acid was named. Ending the names of radicals with -yl had been suggested by Liebig, alongside his colleague Wöhler, in their publication regarding the benzoyl radical that same year. The suffix derives from the Ancient Greek word hyle, meaning matter or material, possibly to highlight that radicals are a more fundamental form of matter.5 However, Liebig clearly wasn’t fully committed to this convention as in the same work where he describes the acetyl radical he also discusses the N2H4 radical, which he refers to as “amid”.4

A couple of years later, in 1834, the French chemists Dumas and Peligot were working with compounds derived from methanol, or as they knew it “l’esprit de bois” (the spirit of wood). They describe a radical which they supposed must exist as the basis for the compounds they were describing and named it “méthylène”. In a footnote they explain that they derived this from the Ancient Greek words methü, meaning wine (close enough to spirit), and hyle, which can also mean wood. There is, however, no explanation for the -ène suffix, which has left the matter open to speculation.

A prevailing theory I’ve read online is that the chemical suffixes -ene, -ine and -one derive from suffixes used in Ancient Greek to mean “daughter of” (e.g. Hermione is the daughter of Hermes).6 Indeed, I have said this myself in previous blog posts. However, as I’ve been reading more of the primary literature from the time, I’m starting to think that this theory is going a bit far; I’ve never seen anyone describe a compound as the ‘daughter of’ another one. I can believe that these suffixes are taken from Greek, but I am inclined to veer more towards the explanation in the 1963 book “The Origins of Chemical Names”, that -ene is simply a name forming suffix, with no real meaning in itself.2 While researching the etymology of acetone I began to have doubts about the validity of this “daughter of” theory: the name margerone, from which the suffix was subsequently used for acetone, was coined as a feminine noun version of l’acide margarique, its synthetic precursor. But once again there was no claim of parenthood in the explanation, or even a mention that this was taken from Greek.

It appears that the naming of methylene was the first time that the -ene suffix was applied to organic chemistry, and my interpretation is that Dumas and Peligot just wanted a chemically sounding suffix to finish off their name! I also noticed in the same paragraph as the coining of the name the phrase “Ce radical est un hydrogène carboné” which Google-translates to “this radical is a carbonaceous hydrogen” (what we would now call a hydrocarbon). I can’t help but wonder if the ‑ène suffix was inspired by the end of the French spelling of hydrogen, either unconsciously, or as a way of emphasising that the new radical was a hydrocarbon.7

From methylene to acetylene

In the following year, the Swedish chemist Berzelius used the term methylene to coin the name “methyl” for the C2H6 radical, as well as “ethyl” for the C4H10 radical (noting the relationship from the volatile liquid ‘ether’ which was named roughly a century previously after Aether, one of the Greek primordial deities).8,9 By the time acetylene was named in 1860, it had become the practice to add the suffix ‑ene to hydrocarbon radical names to denote a hydrocarbon containing fewer equivalents of hydrogen (so not quite the same relationship as between methylene and methyl). By 1856 we see the names propylene, butylene, and amylene being used, and by 1859 we see ethylene (I haven’t been able to find exactly when these names were coined).10,11 Based on this, it would therefore be perfectly logical for Berthelot to name acetylene along similar lines, with this compound being related to Liebig’s formula for the acetyl radical (C2H6) by the loss of a few equivalents of hydrogen (C2H2).1,4

The slight issue that crops up with this is that in 1852 the French chemist Charles Gerhardt had made the decision to reassign the name “acetyl” to the radical C2H2O,12 which is much closer to the formula we give the radical today. But I think we can probably accept that Bertholet either didn’t accept this, or it had simply passed him by.

-ane, -ene, -ine, -one, -une

The systematic naming conventions we know today began their life a mere 6 years after the naming of acetylene, as a musing by Hofmann in 1866 which he published in an extended footnote in a paper about something else entirely.13 The footnote proposes the basic ideas that would later become the rules laid out by IUPAC in the 20th century (Figure 2). The most relevant suggestion to this discussion is that the suffixes on the end of hydrocarbons should be used to denote the degree of saturation (i.e. how much hydrogen is in the compound). Hofmann had clearly noticed the use of ­‑ene, ‑ine, and ‑one as suffixes, and proposed the use the entire vowel series. Thus, the most saturated hydrocarbons would end with -ane, then as pairs of hydrogen atoms were lost the compounds would go through -ene, -ine, -one, and -une.

Figure 2. Hofmann’s proposed scheme for naming hydrocarbons13

Hofmanns scheme

This has translated rather well into today’s systematic nomenclature. The last two in the list were never needed due to the non-existence of carbon-carbon quadruple and quintuple bonds, which is convenient as -one was used for ketones. We’ve kept -ane and -ene as they are, and at some point -ine morphed into -yne, either by an accident of transliteration or to distinguish it from all the other chemicals that end in -ine, which would definitely have been a good plan. But despite ethine being suggested as a replacement name for acetylene so soon afterward it’s original naming, this potentially confusing moniker still manages to persist into the 21st century.

Conclusion

The logic running from the naming of methylene to the naming of acetylene described here isn’t perfect; I spent far too much time agonising over how many fewer equivalents of hydrogen the -ene suffix was supposed to represent. But I eventually came to the conclusion that trying to find watertight logic in these old names is probably just another mistake of a mind so used to systematic nomenclature. In the 19th century there were a lot of opinions flying around, with the same compound being given different names by different people. In the case of radical theory, it also worked the other way round, with similar names having different compositions in the eyes of multiple chemists. One chemist would borrow terms or ideas from another when it was convenient, and not always abide by the naming rules of their source material.  As such, trying to trace any sensible thread through the history of hydrocarbon nomenclature is, to put it mildly, messy.

 

References

  1. Berthelot. Sur une nouvelle série de composés organiques, le quadricarbure d’hydrogène et ses dérivés. C R Hebd Seances Acad Sci. 1860:805-808. https://gallica.bnf.fr/ark:/12148/bpt6k3007r/f817.image.
  2. Flood WE. The Origin of Chemical Names. London: Osbourne Book Co. Ltd.; 1963.
  3. Crosland MP. Historical Studies in the Language of Chemistry. London: Heinemann Educational Books Ltd; 1962.
  4. Liebig J. Der pharmacpe. Justus Liebigs Ann Chem. 1839;30(2):139. doi:10.1002/jlac.18390300202
  5. Wöhler, Liebig. Untersuchungen über das Radikal der Benzoesäure. Ann der Pharm. 1832;3(3):249-282. doi:10.1002/jlac.18320030302
  6. Development of Systematic Names for the Simple Alkanes. http://chem125-oyc.webspace.yale.edu/125/history99/5Valence/Nomenclature/alkanenames.html.
  7. Dumas J, Peligot E. Mémoire sur l’Esprit-de-Bois et les Divers Composés Éthéres qui en Proviennent. Ann Chim Phys. 1834;58:5-74. https://gallica.bnf.fr/ark:/12148/bpt6k6569005x/f15.item.
  8. Frobenius JSA. An account of a spiritus vini æthereus, together with several experiments tried therewith. Philos Trans R Soc London. 1730;36(413):283-289. https://royalsocietypublishing.org/doi/abs/10.1098/rstl.1729.0045.
  9. Berzelius J. Årsberättelse Om Framstegen I Fysik Och Kemi. 1835:376. https://books.google.co.uk/books?id=1DM1AAAAcAAJ&pg=PA376&redir_esc=y#v=onepage&q&f=false.
  10. Berthelot. Synthese des carbures d’hydrogene. C R Hebd Seances Acad Sci. 1856;43:236-238. https://www.biodiversitylibrary.org/page/1211512#page/248/mode/1up.
  11. Wurtz. Sur l’oxide d’ethylene. C R Hebd Seances Acad Sci. 1859;48:101-105. https://www.biodiversitylibrary.org/item/16550#page/107/mode/1up.
  12. Gerhardt C. Ueber wasserfreie organische Säuren. Ann der Chemie und Pharm. 1852;83:112-116. https://babel.hathitrust.org/cgi/pt?id=uva.x002457938;view=1up;seq=128.
  13. Hofmann AW. On the Action of Trichloride of Phosphorus on the Salts of the Aromatic Monamines. Proc R Soc London. 1866;15:54-62.

 

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