Radicofunctional names

I’m sure you came across names such as “ethyl alcohol” or “vinyl chloride”. Do they remind you of binary-type names so common in inorganic chemistry? This is because they also consist of two words. There is an important difference though. The inorganic binary-type names always comprise positive/negative pairs, as in “sodium chloride”. The names like “ethyl alcohol” consist of “specific” part followed by “generic” part^*. Thus ethyl alcohol is an alcohol. All alcohols have a generic formula R–OH; in our alcohol, R = ethyl group.

These names are easily interpretable in terms of linear formulae. So ethyl alcohol (substitutive name ethanol) has a formula C₂H₅–OH and vinyl chloride (substitutive name chloroethene) is H₂C=CH–Cl.

This naming method is known as radicofunctional nomenclature although IUPAC prefers calling it functional class nomenclature [1]. And for a good (enough) reason: the word radical, in modern usage, refers to a molecular entity that has an unpaired electron, while the meaning of radical in “radicofunctional” is that of substituent group. (A recent review by Constable and Housecroft [2] provides an illuminating insight into the history of the word “radical” in chemistry.) Yet, in my opinion, the adjective “radicofunctional” is a better way to express the binary nature of such names: radical (i.e. substituent, such as ethyl) followed by a functional class name (such as alcohol). Incidentally, “R” in generic formulae like R–OH or R–Cl is an abbreviation for “radical” (in an older sense of the word). On the other hand, “functional class name” (of a particular structure) could be easily misinterpreted as “name of a functional class” (which, being a class, contains many different structures).

Class	Example	Structure	Functional class name	Substitutive name
alcohols	(a)		isobutyl alcohol	2-methylpropan-1-ol
aldehydes	(b)		valeryl aldehyde	pentanal
cyanides	(c)		methyl cyanide	acetonitrile
ethers	(d)		diphenyl ether	1,1′-oxydibenzene; phenoxybenzene
glycols	(e)		ethylene glycol	ethane-1,2-diol
halohydrocarbons	(f)		benzyl chloride	(chloromethyl)benzene
ketones	(g)		diethyl ketone	pentan-3-one
sulfides (formerly thioethers)	(h)		diphenyl sulfide	1,1′-sulfanediyldibenzene
sulfones	(i)		diphenyl sulfone	1,1′-sulfonyldibenzene

It’s a matter of taste whether to use substitutive or radicofunctional names. In my book, shorter is often better, so I prefer ethanol to ethyl alcohol and acetonitrile (c) to methyl cyanide. At the same time, I like when the names tell that the structure is symmetrical; moreover, I try to avoid locants as much as possible, at least in spoken names. To me, diphenyl ether (d) sounds better than phenoxybenzene and diethyl ketone (g) better than pentan-3-one.

Then there are situations when you simply have no choice [1]:

Salts, esters, acyl halides and anhydrides cannot be named substitutively when the characteristic group is chosen as the principal group, and functional class names <...> are then used.

Class	Example	Structure	Functional class name	Substitutive name
acyl halides	(j)		benzoyl chloride	—
anhydrides	(k)		benzoic anhydride	—
esters	(l)		benzyl benzoate	—
salts	(m)		sodium benzoate	—

For those of us with background in inorganic chemistry, the names like vinyl chloride, methyl cyanide, diphenyl sulfide, or benzyl benzoate strike a somewhat dissonant note. Why? Because we expect the words ending with -ide or -ate to indicate anionic species, while those ending with -yl refer to either radicals or groups, not cations. So these names seemingly imply a negative charge imbalance. However, radicofunctional names are not about any balance: we have the name of the group on the left and the class name on the right. Salt names such as (m) are, indeed, charge-balanced but should not even be called radicofunctional: they are just extended binary-type names.

In the rather special case of glycosides, which are derivatives of cyclic forms of monosaccharides where the anomeric –OH group is replaced with a group –OR, the way of naming depends on the nature of R.

(n)	(o)	(p)

β-D-glucopyranose prunasin (trivial) (β-D-glucopyranosyloxy)(phenyl)acetonitrile (substitutive) O-β-D-glucopyranosylmandelonitrile (trivial + substitutive) mandelonitrile β-D-glucopyranoside (trivial + functional class) methyl β-D-glucopyranoside (functional class)

Consider the structures (o) and (p), both derivatives of β-D-glucopyranose (n). The glycoside (o), known as prunasin^†, could be given a completely systematic substitutive name (β-D-glucopyranosyloxy)(phenyl)acetonitrile. Or, using as a parent mandelonitrile^‡, a trivial name for hydroxy(phenyl)acetonitrile, it could be named semi-systematically as O-β-D-glucopyranosylmandelonitrile. Or it could be given a binary name: mandelonitrile β-D-glucopyranoside, which is the “classical” way of naming glycosides [3]. But when the group R is “relatively simple”, the “classical” way is the only way. How simple is simple? For me, the group R should be less complex than the glycosyl group. For example, the only reasonable name for (p) is methyl β-D-glucopyranoside: you cannot seriously call it β-D-glucopyranosyloxymethane!

*	It is interesting to see how radicofunctional names are formed in other languages. The phrase “ethyl alcohol” translates to Russian as этиловый спирт, where этиловый is an adjective and спирт is a noun. In Spanish, the adjective follows the noun: alcohol etílico. And in German, as you may have suspected, the “binary” name is actually one word: Äthylalkohol.
†	So called because it is found in plants of the genus Prunus which includes plums, cherries, peaches, apricots and almonds.
‡	Nitrile of mandelic acid, from German Mandelsäure, from Mandel, “almond”. Mandelic acid was discovered in 1831 by the German pharmacist Ferdinand Ludwig Winckler who prepared it from “bitter almond water” (Bittermandelwasser) and hydrochloric acid.

References

1. Leigh, G.J., Favre, H.A. and Metanomski, W.V. Principles of Chemical Nomenclature: A Guide to IUPAC Recommendations. Blackwell Science, 1998, p. 96.
2. Constable, E.C. and Housecroft, C.E. (2020) Before radicals were free — the radical particulier of de Morveau. Chemistry 2, 293—304.
3. McNaught, A.D. (1996) Nomenclature of carbohydrates (IUPAC Recommendations 1996). Pure Appl. Chem. 68, 1919—2008. 2-Carb-33. Glycosides and glycosyl compounds.