Ants, apples, amber

Let’s turn our attention now to other kind of acids. You know what I’m talking about: carboxylic acids. Here’s the simplest one (a):

(a)

HCOOH formic acid (common, PIN) methanoic acid (substitutive) hydridohydroxidooxidocarbon (additive)

If we compare the structure (a) with that of our old friend, carbonic acid (b), we’ll notice that the only difference between them amounts to one oxygen atom. More specifically, the hydrido group –H in (a) is replaced by hydroxido group –OH in (b):

(b)

[CO(OH)2] carbonic acid (common, PIN) dihydroxidooxidocarbon (additive)

Yet this difference marks a fine line between inorganic and organic worlds — as far as nomenclature is concerned, that is. The common name ‘carbonic acid’ is constructed exactly like that of the other mononuclear inorganic oxoacids: the name of an element, in this case carbon, plus a suffix. On the other hand, in organic nomenclature carbon is not even mentioned: it is a default element.

There are three ways of naming carboxylic acids systematically [1, P-65.1.2]. The first method is adding the ‘-oic acid’ bit to the name of the parent hydride. We can think of it as “oxidising” the terminal methyl group, –CH₃, of the parent hydride to the carboxy group, –COOH:

R–CH₃ → R–COOH

Deriving the substituve name for (a) from its parent hydride, methane, we arrive to ‘methanoic acid’ (H–CH₃ → H–COOH).

The second method is to add the ‘-carboxylic acid’ to the name of the parent hydride. We can visualise it as if we were substituting the hydrido group, –H, of the parent hydride by the carboxy group, –COOH:

R–H → R–COOH

It is weird to think of (a) as derived from hydrogen (H–H → H–COOH), but the name ‘hydrogencarboxylic acid’ also exists.

The third alternative is to stick the ‘carboxy-’ in front of the name; this method is used when we cannot use the ‘-oic acid’ or ‘-carboxylic acid’, for example if there is a higher priority group in the structure [1, P-65.1.2.2.3]. This is clearly not applicable to (a) but we’ll come back to the ‘carboxy-’ method later.

All these methods notwithstanding, the name the molecule (a) is universally known by, as well as its preferred IUPAC name (PIN), is ‘formic acid’ [1, P-65.1.8].

As it is the case with inorganic oxoacids, ‘formic acid’ is a noun phrase consisting of the noun ‘acid’ and the adjective ‘formic’. Once again, we owe this name — as a number of others — to Méthode de nomenclature chimique [2, p. 108]:

Noms anciens.	Noms nouveaux.
Acide des fourmis.	Acide formique.
Acide des pommes.	Acide malique.
Acide du succin.	Acide succinique.
Acide du sucre.	Acide oxalique.
Acide du vinaigre.	Acide acéteux.

Here, the somewhat awkward “old” denomination acide des fourmis, i.e. “acid from ants”, is replaced by a more elegant binary name, acide formique. Likewise, acide des pommes (acid from apples), acide du succin (acid from amber), acide du sucre (acid from sugar)^*, and acide du vinaigre (acid from vinegar) become acide malique, acide succinique, acide oxalique and acide acéteux. This all happens on the same page where Lavoisier & Co. introduce names such as acide sulfurique instead of old acide du soufre. See the similarity? Just like ‘sulfuric acid’ means an acid derived from sulfur, ‘formic acid’ means an acid derived from ants.

It’s not that the 18^th-century French academics were not aware of the difference between pure elements (e.g. sulfur) and vegetable, animal, or mineral matter (e.g. apples, ants and amber). They were. Still, for the naming purposes, the indication of the source in a consistent manner made perfect sense.

The common names for carboxylic acids as proposed in Méthode got adopted by the rest of Romance languages, including Neo-Latin, with minimal changes, as well as by English (with an obvious noun-adjective order swap). In many other languages, those names got translated more thoroughly: cf. Danish myresyre, Finnish muurahaishappo, German Ameisensäure, Greek μυρμηκικό οξύ, Polish kwas mrówkowy and Russian муравьиная кислота. Common names and sources of some carboxylic acids (in English, Latin, German and Russian) are summarised here.

Let’s look at a few more carboxylic acids. Replacing –H in (a) with methyl, ethyl and propyl, we’ll get the structures (c), (d) and (e), respectively:

(c)	(d)	(e)

CH3–COOH acetic acid (common, PIN) ethanoic acid (substitutive) CH3–CH2–COOH propanoic acid (substitutive, PIN) propionic acid (common) CH3–[CH2]2–COOH butanoic acid (substitutive, PIN) butyric acid (common)

Once more, IUPAC chose the common name, ‘acetic acid’, as PIN for (c), and not the systematic ‘ethanoic acid’ [1, P-65.1.1.1], let alone ‘methanecarboxylic acid’. The PINs for (d) and (e) sound quite systematic: ‘propanoic acid’ and ‘butanoic acid’, as you would expect from the substitutive names based on the corresponding parent hydrides ‘propane’ and ‘butane’. You might recall though that the root ‘prop’ of propane was derived from propionic acid and the root ‘but’ of butane from butyric acid. The pairs ‘propionic acid/propanoic acid’ and ‘butyric acid/butanoic acid’ are what is known in linguistics as doublets, with a peculiar twist that the former term in each pair is also a grandparent of the latter one.

Continuing with saturated aliphatic chains: from pentanoic acid up, there are no surprises. PINs are regular and dead boring. I like the common names valeric acid, myristic acid and palmitic acid better than their systematic counterparts (pentanoic, tetradecanoic and hexadecanoic acids): they are shorter and tell us something about their origin (Valeriana officinalis, nutmeg and palm oil, respectively). Then again, the common names caproic acid (C6), caprylic acid (C8) and capric acid (C10) could be easily confused: these acids are all named after she-goat (Latin capra) because they are found in goat milk fat. I think I prefer systematic names for them.

What if we replace –H in (a) with another carboxy group, –COOH? The resulting compound will be the simplest dicarboxylic acid (f):

(f)

HOOC–COOH oxalic acid (common, PIN) ethanedioic acid (substitutive)

IUPAC recommends the “Lavoisier” name ‘oxalic acid’ as PIN for (f), not the systematic ‘ethanedioic acid’ [1, P-65.1.1.1]. The PINs for higher dicarboxylic acids are completely regular: propanedioic acid (g), butanedioic acid (h), pentanedioic acid (i) and so on. Nevertheless, the Blue Book lists the corresponding common names ‘malonic acid’, ‘succinic acid’ and ‘glutaric acid’ as “retained for general nomenclature” [1, P-65.1.1.2.2].

(g)	(h)	(i)

HOOC–CH2–COOH malonic acid (common) propanedioic acid (substitutive, PIN) HOOC–[CH2]2–COOH succinic acid (common) butanedioic acid (substitutive, PIN) HOOC–[CH2]3–COOH glutaric acid (common) pentanedioic acid (substitutive, PIN)

Now, can we have three carboxy groups? Of course we can. Check out the molecule (j):

(j)

HOOC–CH2–C(COOH)(OH)–CH2–COOH citric acid (common) 2-hydroxypropane-1,2,3-tricarboxylic acid (substitutive, PIN) 3-carboxy-3-hydroxypentanedioic acid (substitutive)

The PIN for (j) is ‘2-hydroxypropane-1,2,3-tricarboxylic acid’. Wait. Why isn’t it a ‘trioic acid’? According to the Blue Book’s rule P-65.1.2.2.1,

If an unbranched chain is linked to more than two carboxy groups, all carboxy groups are named from the parent hydride by substitutive use of the suffix ‘carboxylic acid’, preceded by the appropriate numerical prefix ‘tri’, ‘tetra’ etc. and appropriate locants.

Since the ‘-carboxylic acid’ method implies adding carbons to the structure, we have to start not with pentane — as is the case with pentanedioic acid (i) — but with propane. Planting three –COOH groups at positions 1, 2 and 3 of propane, we get propane-1,2,3-tricarboxylic acid, and attaching a hydroxy group at the position 2 gives us the complete name, 2-hydroxypropane-1,2,3-tricarboxylic acid.

Can’t we start with pentanedioic acid (i) and just add one more carboxy group and one hydroxy group? The result, 3-carboxy-3-hydroxypentanedioic acid, is slightly shorter than the PIN. The drawback here is mixing two different methods, viz. that of ‘-oic acid’ with ‘carboxy-’, in the same name.

The only way to arrive at ‘trioic acid’ seems to “oxidise” all three terminal methyls of 3-methylpentane (k) to 3-hydroxy-3-methylpentanetrioic acid. Alas, 3-methylpentane does not qualify to be a parent hydride because it is branched.

(k)

CH3–CH2–CH(CH3)–CH2–CH3 3-methylpentane (substitutive, PIN)

The good news is that the common name of (j), ‘citric acid’, is also retained: “no substitution is recommended, but the formation of salts and esters is allowed” [1, P-65.1.1.2.3].

Speaking of which: anions, salts and esters of carboxylic acids are named in a regular fashion by replacing ‘-ic acid’ with ‘-ate’, no matter whether we are dealing with common or systematic names: formic acid → formate, acetic acid → acetate, butanoic acid → butanoate, hexanedioic acid → hexanedioate, 2-hydroxypropane-1,2,3-tricarboxylic acid → 2-hydroxypropane-1,2,3-tricarboxylate, etc. One exception is tartaric acid → tartrate^†. In languages where the common names of carboxylic acids are properly translated, the names of the corresponding anions (salts, esters) are still of Latin (Latinate?) persuasion: cf. German Bernsteinsäure → Succinat, Russian янтарная кислота → сукцинат, etc.

At this point I think it is appropriate to ask what the term ‘succinate’ really means^‡. Why, it’s an anion of succinic acid (h), right? Right. But which anion, (l) or (m)?

(l)	(m)

−OOC‒[CH2]2‒COO− succinate (common) butanedioate (substitutive, PIN) HOOC‒[CH2]2‒COO− hydrogen succinate (common) hydrogen butanedioate (substitutive) 3-carboxypropanoate (substitutive, PIN)

Since (h) is a diprotic acid, two deprotonation reactions occur:

HOOC‒[CH2]2‒COOH	⇌	HOOC‒[CH2]2‒COO− + H+	⇌	−OOC‒[CH2]2‒COO− + 2H+
(h)		(m)		(l)

It is reasonable to suggest that ‘succinate’ corresponds to the fully deprotonated structure (l), which is also referred to as succinate²⁻ or succinate(2−). Its substitutive name will be butanedioate (to be expected for the dianion of butanedioic acid). However, at pH 7 the dianion (l) exists in equilibrium with the monoanion (m), sometimes called succinate⁻ or succinate(1−). Yes, (m) is both an anion and a carboxylic acid. Anions are senior to acids, therefore we will name (m) as an anion modified with a carboxy group: finally, a legitimate use of ‘carboxy-’ method! As carboxy group brings a carbon atom into the structure, we have to add it to the three-carbon anion, viz. propanoate. The full systematic name of (m) will be 3-carboxypropanoate — very different to that of its conjugate base (l), butanedioate. Alternatively, we can give (m) a hydrogen name ‘hydrogen butanedioate’ or ‘hydrogen succinate’.

To sum up:

• Common names of several natural carboxylic acids appeared in Méthode de nomenclature chimique [2] more than 200 years ago. The Blue Book lists many of them as retained names while ‘formic acid’, ‘acetic acid’ and ‘oxalic acid’ are PINs.
• These names are noun phrases consisting of an adjective derived from the name of a natural source and the noun ‘acid’.
• The adjectives are constructed from the Latin roots of the natural source names followed by the suffix ‘-ic’.
• Systematic names of carboxylic acids are created using substitutive nomenclature. There are three methods:
  • (substituted) parent hydride + ‘-oic acid’ (R–CH₃ → R–COOH, no carbons added). Example: pentanedioic acid
  • (substituted) parent hydride + ‘-carboxylic acid’ (R–H → R–COOH, carbon added). Example: 2-hydroxypropane-1,2,3-tricarboxylic acid
  • ‘carboxy-’ + substituted parent hydride (R–H → R–COOH, carbon added). Example: 3-carboxypropanoate
• In the names of anions, salts and esters of carboxylic acids, ‘-ic acid’ is replaced by ‘-ate’.

*	The preparation of oxalic acid by reacting sugar with concentrated nitric acid was reported in 1776 by Torbern Olof Bergman [3], hence the name (acidum sacchari in Latin).
†	Yet again, we can trace this terminology back to the Méthode. For example, acétite de soude (sodium acetate) [2, p. 107] and tartrite de potasse (potassium tartrate) [2, p. 140]. The then “new” terms like acétite and tartrite became obsolete and have been replaced by acétate and tartrate.
‡	There is an annoying custom in biochemical literature and discourse to use the names of acids and their corresponding anions interchangeably. Carboxylic acids are the prime example [4]. Not only are carboxylic acids and carboxylates mixed up in the spirit of “don’t know, don’t care”: I was witness to many a discussion where the very attempts to differentiate between them were dismissed as irrelevant or futile. How did it come to that? Distinguishing between the chemical species is not a terminological question. No inorganic chemist will ever confuse sulfate with sulfuric acid, whether on paper or in the lab. Why is it acceptable to confuse succinate and succinic acid then? Making distinction could be of a fundamental and practical value. For instance, a patent was recently granted for a method of production of succinic acid — not succinate! — by yeast at low pH [5].

References

1. Favre, H.A. and Powell, W.H. Nomenclature of Organic Chemistry: IUPAC Recommendations 2013 and Preferred IUPAC Names. Royal Society of Chemistry, Cambridge, 2014.
2. Guyton de Morveau, L.-B., Lavoisier, A., Berthollet, C.-L., de Fourcroy, A.-F., Hassenfratz, J.H. and Adet, P.A. Méthode de nomenclature chimique. Chardon, Paris, 1787.
3. Bergman, T. Dissertatio chemica de acido sacchari. Edman, Uppsala, 1776.
4. Roosterman, D. and Cottrell, G.S. (2021) Rethinking the citric acid cycle: Connecting pyruvate carboxylase and citrate synthase to the flow of energy and material. International Journal of Molecular Sciences 22, 604.
5. Jansen, M. and Verwaal, R. (2009) Succinic acid production by fermentation at low pH. EP 2297297B1 / US 9012187B2.

See also:

• Common names and sources of some carboxylic acids in English, Latin, German and Russian.