Let us come back to inorganic oxoacids and their anions. Observe the structures (a) through (c):
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| (a) | (b) | (c) |
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You may remeber that (a) is carbonic acid and (c) is its fully deprotonated anion, carbonate. What about (b)? Well, it’s called ‘hydrogencarbonate’ in the Red Book [1, IR-8.4] and ‘hydrogen carbonate’ in the Blue Book [2, P-65.6.2.3.2]. What’s the difference?
‘Hydrogen carbonate’ (with a space) is a typical extended binary-type name like that of a salt. So we can write a salt-like formula for (b): H+ (CO3)2−. On the other hand, ‘hydrogencarbonate’ (without a space) is one word, which supposedly confers the idea that the anion (b) is a single entity: HCO3−.
Now let’s have a look at a real salt (d):
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| (d) |
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It is commonly known as baking soda or sodium bicarbonate. This latter term is explicitly discouraged by IUPAC yet is still widely popular*. The Red Book recommends an extended binary-type name ‘sodium hydrogencarbonate’ while the Blue Book gives it the PIN ‘sodium hydrogen carbonate’ [2, P-65.6.2.3.2] which could be viewed as “generalised salt” name. It’s really a matter of taste which one to use†. I prefer shorter names, so ‘sodium hydrogencarbonate’ it is for me.
According to the Red Book, ‘hydrogencarbonate’ is an “accepted simplified hydrogen name” [1, IR-8.5]. And what is a “non-simplified hydrogen name”? It is a variant of additive name where
- ‘hydrogen’ is attached to the rest of the name,
- the number of hydrogens is specified by a multiplicative prefix,
- the anionic part is placed in enclosing marks, and
- the charge of the total structure is specified.
For (b), it would be ‘hydrogen(trioxidocarbonate)(1−)’. Now if we look at the additive name for (c), ‘trioxidocarbonate(2−)’, we’ll see that both anions share the same part, ‘trioxidocarbonate’. Moreover, we also can name (a) ‘dihydrogen(trioxidocarbonate)’. So hydrogen names are useful “if one wishes to emphasize the conception of the structure as hydrons attached to the anion in question” [1, IR-8.4, p. 135], however they don’t tell us where those hydrons are attached. Note that the purely additive names, ‘dihydroxidooxidocarbon’ for (a) and ‘hydroxidodioxidocarbonate(1−)’ for (b), provide us with complete connectivity and are shorter than the corresponding “full” hydrogen names.
The Red Book contains a very short list of “accepted simplified hydrogen names” [1, IR-8.5]:
| Molecular formula | Simplified hydrogen name | Hydrogen name | Structural formula | Additive name |
|---|---|---|---|---|
| H2BO3− | dihydrogenborate | dihydrogen(trioxidoborate)(1−) | [BO(OH)2]− | dihydroxidooxidoborate(1−) |
| HBO32− | hydrogenborate | hydrogen(trioxidoborate)(2−) | [BO2(OH)]2− | hydroxidodioxidoborate(2−) |
| HCO3− | hydrogencarbonate | hydrogen(trioxidocarbonate)(1−) | [CO2(OH)]− | hydroxidodioxidocarbonate(1−) |
| H2PO4− | dihydrogenphosphate | dihydrogen(tetraoxidophosphate)(1−) | [PO2(OH)2]− | dihydroxidodioxidophosphate(1−) |
| HPO42− | hydrogenphosphate | hydrogen(tetraoxidophosphate)(2−) | [PO3(OH)]2− | hydroxidotrioxidophosphate(2−) |
| HPHO3− | hydrogenphosphonate | hydrogen(hydridotrioxidophosphate)(1−) | [PHO2(OH)]− | hydridohydroxidodioxidophosphate(1−) |
| H2PO3− | dihydrogenphosphite | dihydrogen(trioxidophosphate)(1−) | [PO(OH)2]− | dihydroxidooxidophosphate(1−) |
| HPO32− | hydrogenphosphite | hydrogen(trioxidophosphate)(2−) | [PO2(OH)]2− | hydroxidodioxidophosphate(2−) |
| HSO4− | hydrogensulfate | hydrogen(tetraoxidosulfate)(1−) | [SO3(OH)]− | hydroxidotrioxidosulfate(1−) |
| HSO3− | hydrogensulfite | hydrogen(trioxidosulfate)(1−) | [SO2(OH)]− | hydroxidodioxidosulfate(1−) |
I don’t see why this list should not be extended to include hydrogen names based on other acceptable common names of mononuclear oxoacid anions.
Hydrogen names can be really helpful in situations where we don’t know (or don’t care about) proton connectivity. Consider triphosphoric acid (e):
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| (e) | (f) |
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Since (e) is a pentaprotic acid, it can go on shedding protons all way up to the fully deprotonated triphosphate pentaanion (f):
| H5P3O10 | ⇌ | H4P3O10− + H+ | ⇌ | H3P3O102− + 2H+ | ⇌ | H2P3O103− + 3H+ | ⇌ | HP3O104− + 4H+ | ⇌ | P3O105− + 5H+ |
Naming the partially protonated anions systematically could be tricky because of tautomerism. For example, trianion H2P3O103− can theoretically exist as either PO(OH)(O−)–O–PO(O−)–O–PO(OH)(O−), PO(OH)2–O–PO(O−)–O–PO(O−)2 or PO(OH)(O−)–O–PO(OH)–O–PO(OH)(O−)2. Instead of struggling to name three different tautomers, we can furnish H2P3O103− with the hydrogen name ‘dihydrogen[bis(tetraoxidophosphato)dioxidophosphate](3−)’, or, even better, the simplified hydrogen name ‘dihydrogentriphosphate’.
In organic chemistry, esters derived from phosphoric acid (diphosphoric acid, triphosphoric acid, etc.) are also named ‘phosphates’ (diphosphates, triphosphates, etc.), even if the acid in question is completely protonated. As is the case with the neutral form of one of the most important molecules of life (g):
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| (g) |
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Here a hydrogen name is truly called for. Even though biochemists refer to (g) and any of its anions as ‘adenosine 5'-triphosphate’ (ATP), the proper name for this structure would be ‘adenosine 5′-(tetrahydrogen triphosphate)’, with space [2, P-106.2].
Similarly, we can give hydrogen (space) names to the partially protonated anions of polycarboxylic acids, such as our friend citric acid (h):
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| (h) | (i) |
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The trianion (i) is known as citrate. What if we need to name a dianion? Two tautomers are possible, (j) and (k):
-2-hydroxysuccinate.png) |
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| (j) | (k) |
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Simply moving a proton from one carboxy group to another results in two wildly different systematic names, 2-(carboxymethyl)-2-hydroxybutanedioate for (j) and 3-carboxy-3-hydroxypentanedioate for (k). But if we are unconcerned about proton connectivity, we can employ ‘hydrogen citrate’ for both structures.
| * | The name ‘bi-carbonate of potash’ for NaHCO3 was originally coined in 1814 by William Hyde Wollaston “in order to refer at once to the double dose of carbonic acid contained in it” [3] — that is, “double dose” per equivalent of sodium, compared with “single dose” in Na2CO3. Wollaston called this latter salt ‘subcarbonate of potash’. |
| † | I wonder what German speakers make of this conundrum: in German, Natriumhydrogencarbonat is one word. |
References
- Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005.
- Favre, H.A. and Powell, W.H. Nomenclature of Organic Chemistry: IUPAC Recommendations 2013 and Preferred IUPAC Names. Royal Society of Chemistry, Cambridge, 2014.
- Wollaston, W.H. (1814) I. A Synoptic scale of chemical equivalents. Philosophical Transactions of the Royal Society of London 104, 1—22.
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