Suffixes — or combining forms?

In a number of IUPAC publications, the entities that are referred to as “suffixes” include

• Suffix for the principal characteristic group, such as ‘-amine’, ‘-one’ or ‘-oic acid’ [1];
• Suffixes indicating charge [2, p. 5];
• Suffixes indicating loss or addition of one or more hydrogens from/to parent hydrides, e.g. ‘-ium’, ‘-ylium’, ‘-ide’ or ‘-uide’ [2, p. 105] ;
• Suffixes ‘yl’, ‘ylidene’ or ‘ylidyne’ in the names of radicals and substituent groups [2, p. 108];
• Subtractive suffixes ‘ene’ and ‘yne’ [3];
• Composite suffixes [4, p. 82 ] aka combined suffixes [2, p. 251] that contain multiplicative prefixes, as in ‘diyl’ or ‘triylium’;
• Suffixes ‘quinone’, ‘quinol’, ‘chromenol’ and ‘chromanol’ in names of quinones [5].

You may recall that in substitutive names the (name of) parent structure is a root and the (names of) substituents are either “prefixes” or “suffixes”. However, as we just discussed, the substitutive “prefixes” such as ‘chloro’ are not prefixes but combining forms. Following this logic, we have to conclude that the substitutive “suffixes” are in fact also combining forms. Often the “prefix” and “suffix” that mean the same substituent have the common root. For example, the substitutive “prefix” ‘amino-’ and substitutive “suffix” ‘-amine’ share the root ‘amin’^*. In some other cases, however, the “prefix” and “suffix” for the same substituent have nothing in common: cf. ‘hydroxy-’ and ‘-ol’, or ‘formyl-’ and ‘-carbaldehyde’.

One does not have to be a linguist to see that ‘-carboxylic acid’ or ‘-oic acid’ simply cannot be suffixes, the obvious reason being that ‘acid’ is a standalone word^†. It is clear that ‘-oic’ is a derivational morpheme because it converts a noun, e.g. hexane, to an adjective, e.g. hexanoic. Since ‘oic’ on its own does not mean much, we can say that it is, indeed, a suffix^‡. The same cannot be said about ‘carboxylic’.

It’s also quite obvious that ‘quinone’, ‘quinol’, ‘chromenol’ and ‘chromanol’ are not suffixes. They are independent words, each of them consist of at least two content morphemes. That they could be further joined with other combining forms, as in 2-methyl-6-chromanol or 2,3-dimethoxy-5-methylbenzoquinone, does not make them suffixes.

What about “suffixes” indicating negative charge, viz. ‘-ate’, ‘-ide’ and ‘-ite’? The terminal ‘-e’s are endings but ‘-at-’, ‘-id-’ and ‘-it-’ look like genuine suffixes. Ditto suffixes indicating loss/addition of one or more hydrogens from/to parent hydrides. Unlike substitutive names, where only one “suffix” is allowed, there is no such restriction here.

(a)	(b)
(c)	(d)
(e)	(f)

borane (parent hydride) trihydridoboron (additive) boranylium (parent hydride-based) dihydridoboron(1+) (additive) boranuide (parent hydride-based) tetrahydridoborate(1−) (additive) boranium (parent hydride-based) tetrahydridoboron(1+) (additive) boranuidyl (parent hydride-based) trihydridoborate(•1−) (additive) boraniumyl (parent hydride-based) trihydridoboron(•1+) (additive)

For example, the name for (e) combines the negative charge suffix ‘uid’ with radical suffix ‘yl’, thus boranuidyl, while (f) combines the positive charge suffix ‘ium’ with radical suffix ‘yl’, thus boraniumyl. The suffix ‘ylium’, however, indicates a formal hydride loss from the parent hydride and is not a combination of ‘yl’ and ‘ium’, thus the cation (b) is called boranylium. Confusing? I bet it is. I prefer to stick to additive names.

Are the subtractive suffixes ‘ene’ and ‘yne’ real suffixes? They used to replace ‘ane’ in the names of hydrocarbons to indicate unsaturation, as in propane, propene and propyne. Consider “diene”, a generic name for any hydrocarbon containing two double bonds. Of course, ‘di’ is a multiplier, and I suggested earlier that multipliers are not prefixes but combining forms. Can an independent word such as “diene” comprise just a multiplied suffix? I doubt it. In my view, ‘-an-’, ‘-en-’ and ‘-yn-’ are also combining forms and terminal ‘-e’s are endings.

I just realised that the construction multiplier + “suffix” could be an ad hoc test to determine whether the “suffix” is in fact a combining form. For example, “diene” is a meaningful standalone word, so ‘en’ is not a suffix. On the contrary, words like “diide” or “triate” on their own make no sense whatsoever.

*	We cannot say that they are completely semantically equivalent though as choosing the latter implies that it is the principal group of the structure being named. On the contrary, choosing the former implies that the structure contains at least one more functional group which also is senior to the “prefix” form.
†	This is especially obvious in Romance languages where adjectives normally follow nouns. Cf. English “hexanoic acid” with French acide hexanoïque or Spanish ácido hexanoico.
‡	The semantic load of ‘-oic’ is quite low. Cf. English “hexanoic acid” with German Hexansäure which shows that it is possible to construct an unambiguous systematic name without ‘-oic’.

References

1. Hellwich, K.-H., Hartshorn, R.M., Yerin, A., Damhus, T. and Hutton, A.T. (2020) Brief guide to the nomenclature of organic chemistry (IUPAC Technical Report). Pure and Applied Chemistry 92, 527—539.
2. 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.
3. Powell, W.H. (1993) Revised nomenclature for radicals, ions, radical ions and related species (IUPAC Recommendations 1993). Pure and Applied Chemistry 65, 1357—1455.
4. Leigh, G.J., Favre, H.A. and Metanomski, W.V. Principles of Chemical Nomenclature: A Guide to IUPAC Recommendations. Blackwell Science, 1998.
5. IUPAC-IUB Commission on Biochemical Nomenclature (CBN) (1974) Nomenclature of quinones with isoprenoid side-chains (IUPAC Recommendations 1993). Pure and Applied Chemistry 38, 439—447.