Conjunctive names

Here is the eternal problem of organic nomenclature: which part of the molecule is a skeleton and which is a substituent? Let’s have a look at the structure C₆H₅–CH₂–COOH (a).

(a)	(b)	(c)

phenylacetic acid (substitutive) carboxymethylbenzene (substitutive) benzeneacetic acid (conjunctive) benzene (trivial, parent hydride) acetic acid (trivial, functional parent)

We can name it substitutively phenylacetic acid; that would be its preferred IUPAC name (PIN). So its parent structure is acetic acid (c) where a hydrogen atom is substituted by phenyl group (–C₆H₅). Nothing prevents us, however, from naming it, also substitutively, carboxymethylbenzene. In this case, its parent structure is benzene (b) where a hydrogen is substituted by carboxymethyl group (–CH₂–COOH).

There is yet another way of naming (a), viz. taking the names of both parent structures and simply sticking them together without any modification: benzene + acetic acid = benzeneacetic acid^*. (The loss of one hydrogen atom from each parent structure is implied.) This method is widely used by Chemical Abstracts Service (CAS) and is known as conjunctive nomenclature [1].

(d)	(e)	(f)

histamine (trivial) 2-(1H-imidazol-4-yl)ethanamine (substitutive) 1H-imidazole-4-ethanamine (conjunctive) 1H-imidazole (trivial, parent hydride) ethanamine (substitutive)

In the same fashion, we can name the structure (d) by joining the names of its cyclic and acyclic parent structures together: 1H-imidazole (e) + ethanamine (f) = 1H-imidazole-4-ethanamine. Here, we have to indicate the attachment point in the imidazole ring by the locant ‘4’.

Unfortunately, this simple method — and I like simple methods — is restricted to relatively simple cases of a saturated hydrocarbon chain (or several identical hydrocarbon chains), containing just one principal group (each), attached by a single bond to a ring system (which also could have functional groups attached)^† [2]. This system uses Arabic numerals as locants in the ring and Greek letters (α, β, γ, etc.) as locants in the chain(s)^‡.

Why use conjunctive nomenclature? According to Hellwinkel [3], it

offers nothing that could not be settled equally well and, above all, more uniformly by substitutive nomenclature. Since, however, it makes allowance for the generation of new index names encompassing larger molecular units it has evolved into an irrevocable key element of Chem. Abstr. registry nomenclature.

So, after all, it does offer something that substitutive nomenclature does not: the conjunctive names can be used as new parent names and further modified (substitutively, additively, or subtractively).

(g)

3-hydroxymandelic acid (trivial + substitutive) hydroxy(3-hydroxyphenyl)acetic acid (substitutive) α,3-dihydroxybenzeneacetic acid (conjunctive + substitutive)

Thus, benzeneacetic acid (a) could be substitutively modified to yield α,3-dihydroxybenzeneacetic acid (g). The locants ‘α’ and ‘3’ indicate the attachment points for hydroxy groups in the chain and the ring, respectively.

And for structures with several identical chains conjunctive nomenclature “offers manifest advantages” [3].

(h)	(i)

tetraxetan (trivial, INN) 2,2′,2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (multiplicative + skeletal replacement) 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (conjunctive) cyclen (trivial) 1,4,7,10-tetraazacyclododecane (skeletal replacement)

For example, the conjunctive name for tetraxetan (h), a derivative of 1,4,7,10-tetraazacyclododecane (i), is much shorter than the corresponding multiplicative name, 2,2′,2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid, where ‘2,2′,2″,2‴’ and ‘1,4,7,10-tetrayl’ appear to be superfluous.

*	In linguistics, this type of word formation is called agglutination. English examples include artwork, desktop, foolproof, groundbreaking, lighthouse, troubleshooting, gentlemanlike, etc.
†	If the hydrocarbon chain is unsaturated or bound to the ring by a double bond, or contains heteroatoms or more than one principal group; if the principal group is oxo group or its chalcogen analog; or if the ring contains more principal groups than attached to hydrocarbon chain(s) — in all these cases conjunctive nomenclature is not supposed to be used [2]. Personally, I don’t see the reason behind most of these restrictions.
‡	As the Greek alphabet contains only 24 letters, assigning locants to longer hydrocarbon chains seems problematic.

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. Bünzli-Trepp, U. Systematic Nomenclature of Organic, Organometallic and Coordination Chemistry. EPFL Press, 2007, pp. 30—32.
3. Hellwinkel, D. Systematic Nomenclature of Organic Chemistry: A Directory to Comprehension and Application of its Basic Principles. Springer, 2001, pp. 91—92.