Saturated Fatty Acids

Before chemists had a detailed understanding of molecular structure, newly discovered chemicals were named on the whims of the discoverers. By the end of the 19th century the number of organic molecules known to science had started to increase dramatically, and the list of unconnected names that had to be remembered was getting longer and longer. It became apparent that this mess had to be sorted out, and the process of developing the systematic naming conventions that we have today began. However, despite being less descriptive, many of the old names are retained in the language today and referred to as trivial or common names.

Saturated fatty acids are a good example of a class of compounds where the systematic names are quite simple and easy to remember, being generally based on the Greek for the number of carbons, but common/trivial names are often used preferentially. This balance of use does vary significantly, illustrated very well by which one is used as the title for their Wikipedia article. At one end of the scale we have compounds such as palmitic acid where the name is widely used, and at the other end we have compounds like enanthic acid for which the systematic name heptanoic acid is much more commonly used in modern times. I would also include formic and acetic acids in the former group (momentarily ignoring that they aren’t “fatty”) and point out that they are almost never referred to as methanoic and ethanoic acid outside A-Level exams!

There are also several compounds in this class which simply don’t have trivial names, suggesting that they weren’t discovered until a time when the number of carbons could be identified, and the compound could be named systematically based on this knowledge. In the graphic above I have only included compounds that have a trivial name. It is interesting to note that after the 10-carbon point, the majority of the odd numbered possibilities don’t have trivial names. This highlights the fact that odd numbered fatty acids are much less common in nature. This is due to the primary method by which fatty acids are biosynthesised; multiple units of acetic acid, with its pair of carbons, are sequentially strung together, producing chains of even length. Some organisms have evolved processes to produce odd chain lengths, often by cleaving a carbon off a longer chain, but this is very rare.

The etymologies of the common names for fatty acids are in the graphic above, but I’ve picked out a few interesting ones here to discuss in more detail.

Enanthic Acid

Enanthic acid was the first compound for which I found conflicting information available. More commonly referred to nowadays as heptanoic acid, the trivial name seems to survive mainly for its esters. The hormone derivative testosterone enanthate is an example, and is a medication used mainly in the treatment of low testosterone levels in men, sold under the brand name Delatestryl. Some modern sources suggested (and probably guessed) that there was a relationship with the plant Oenanthe crocata, otherwise known as hemlock water dropwort. As many of the fatty acids are named after the plants they were isolated from, this is a reasonable presumption. The Latin name for the genus Oenanthe is said to derive from the Greek oinos, meaning wine, and anthos, meaning flower, a reference to the wine-like odour of the flowers. However, as I was unable to find any reputable reference to this acid ever having been prepared from this plant, this didn’t seem to ring true and so I kept searching for more information.

Further digging led me to a textbook written by Thomas Graham in 1842, and then to a dictionary of English published in 1878, which describe œnanthic acid (the first letter of which has clearly morphed over time) as being a derivative of œnanthic ether. This is an oil left over after the distillation of large quantities of wine, and Graham claimed it to be responsible for “the characteristic odour of wine by which it is distinguished from dilute alcohol”, i.e. it makes wine smell like wine. The words used in the derivation of the name œnanthic ether are exactly the same as those used to derive the name of the previously discussed plant, namely the Greek for ‘wine’ and ‘flower’. Graham talks about the ‘flower’ of wine as being synonymous with its ‘aroma’ or ‘bouquet’, although the word doesn’t seem to be used in this way by wine tasters today. But it was in this sense that the compound appears to have been named, not because it smells like flowers.

This is a very interesting example of convergent etymology, where both a flower and a chemical have been named seemingly independently based on their association with the smell of wine and two different uses of the word flower.

Behenic Acid

Behenic acid (the fatty acid with a 22-carbon long chain) provided another case of conflicting information. Ben oil, also known as behen or moringa oil, is produced from the seeds of moringa oleifera, also known as the horseradish tree, the drumstick tree, or the ben oil tree. Ben oil is sold as a cosmetic product with various dubious medicinal claims, and often marketed with lots of pseudo-scientific nonsense and references to how it’s been used for thousands of years to keep skin looking young and livers toxin free. Many sites selling this product, as well as its Wikipedia page, claim that ben oil is named thus due to its high concentration of behenic acid. I was immediately suspicious of this explanation, as it seemed odd for a natural oil to be named after a constituent rather than the other way around, and it still doesn’t explain where the name ‘behenic’ originates from (being unconnected to the tree’s Latin name). It seems probable that someone trying to write sciency-sounding spiel made a false assumption, and this misinformation has been copied, without confirmation, across the internet.

Luckily, other sources discussing the acid, including an etymological dictionary from 1929, claim the more logical order of events, with behenic acid being named after ben oil. Looking more into the colloquial name for the tree revealed that it is believed to have been derived from the Persian bahman, the eleventh month of the Persian, which straddles January and February; traditionally the time when the roots of the tree, which taste similar to horseradish, were harvested. This provides a much more convincing and logical origin story for the naming of behenic acid.

Margaric Acid

Finally, the derivation of margaric acid (the C17 fatty acid) provided at unexpected story. I had initially surmised there to be a link with margarine, but was surprised to learn that margaric acid is not cited as being a component of this foodstuff. There is, however, a very odd connection.

When ‘margaric acid’ was first isolated in 1813, from pig fat, it was named using the Greek word for a pearl, margaron, due to the pearly lustre of its crystals. We now know that margaric acid is very rare in nature and is not found at high concentrations in any animal and vegetable fats, and so it is very unlikely that the 17-carbon fatty acid which is now known as margaric acid was actually the chemical that was isolated. It is believed that what had actually been isolated was probably a mixture of stearic and palmitic acids, which would have been effectively impossible to identify with the techniques available at the time.

However, when this substance was discovered, it was believed to be one of the three fatty acids which formed animal fats, alongside oleic and stearic acids. Due to this misunderstanding, when a butter alternative was patented in 1869 it was marketed under the trade name margarine, as it was made from beef fat and, therefore, believed to contain large quantities of margaric acid. True margaric acid was later produced synthetically, and while its crystals may not have had the same pearly lustre, the name has continued to be used ever since (despite having no real link with margarine!).

References

  1. Bevan, Gregg, Rosseinsky, Gregg, S. J., & Rosseinsky, Angela. (1976). Concise etymological dictionary of chemistry. London: Applied Science.
  2. Markley, K. (1960). Fatty acids: Their chemistry, properties, production, and uses.(2nd ed). New York: Interscience.
  3. Bailey, D., & Bailey, Kenneth C. (1929). An etymological dictionary of chemistry and mineralogy. London: Arnold & co.
  4. Worcester, J. (1878). A Dictionary of the English Language. Philadelphia: J. B. Lippincott & co.
  5. Graham, T. (1842). Elements of chemistry, including the applications of the science in the arts. London: Hippolyte Baillière.

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