Seniority criteria

Last time I took part in the Intra-Universal Panel of Astronomical Chemistry, I had a most edifying and enjoyable discussion with an alien (to me) colleague who, for reasons unknown, showed an interest in terrestrial chemical nomenclature. I can’t tell you her name, and not due to confidentiality considerations: I am simply unable either pronounce or write it^*.

We got along very well. Even though she spoke with a thick Arcturian accent, I understood most of her English. What she made of my English, I know not, but she assured me that the latest Google Translate app was doing a decent job despite ignoring important words like “not”.

I told her that chemical nomenclature, with all its shortcomings, is the best scientific nomenclature developed on Earth because, knowing the rules, one can reconstruct the structure from the name. Looking back, I wish I hadn’t said that. I guess now she doesn’t think much of our science as a whole.

Her interest in nomenclature was intriguing given that she regarded it, along with other prescriptive systems, a form of authoritarianism. Among Arcturians, she explained, it’s considered to be poor taste to talk about that, but she’s always had a rebellious streak.

One thing we agreed on was that any nomenclature system requires some seniority criteria. These criteria better be both objective and consistent, and the fewer of them the better.

Atomic number

And if there is one truly universal criterion of seniority for elements — universal in the sense that it will be accepted anywhere in our Universe — it must be atomic number. Consider the atoms i and j with corresponding atomic numbers Z_i and Z_j. Saying that if Z_i > Z_j then atom i is senior to atom j should stir no controversy whatsoever. So you would expect the atomic number criterion to be the main ordering principle in chemical nomenclature, or at least to be used a lot. You’d be wrong.

Nevertheless, atomic number is used consistently in the Cahn–Ingold–Prelog (CIP) sequence rules. Now there are many rules, but the first among them is this one [1, P-92.1.3.1a]:

higher atomic number precedes lower.

How curious, says the Arcturian. She has nothing against atomic numbers, it is just that in her view “senior” means what it means: elder. Hydrogen, helium, lithium and beryllium have been around the longest, practically since the Big Bang, so they must be senior to the rest. And the superheavies are the newest, the youngest. In other words, junior. Also, they die young, so they’ll never grow to be seniors. For me, she says, if Z_j > Z_i then atom i is senior to atom j. But please, go on.

Let’s see how it works, I continue. If all atoms directly attached to the chiral centre are different, applying the CIP rules is straightforward. This is the case of the enantiomers of halothane, (a) and (b).

(a)	(b)

(S)-halothane (trivial) (2S)-2-bromo-2-chloro-1,1,1-trifluoroethane (substitutive) (R)-halothane (trivial) (2R)-2-bromo-2-chloro-1,1,1-trifluoroethane (substitutive)

The chiral centre (C-2) is linked to four different atoms: Br, C, Cl and H. On the diagram (a), the chiral centre is positioned in such a way that the least-preferred ligand — in this case, hydrogen — points away from the viewer. The rest of the ligand sequence, Br > Cl > C, go anticlockwise, therefore, the configuration of the chiral carbon is ‘S’. The complete name of (a) is (2S)-2-bromo-2-chloro-1,1,1-trifluoroethane and of its mirror image (b) is (2R)-2-bromo-2-chloro-1,1,1-trifluoroethane.

If at the chiral centre there are two or more atoms of the same element, we can assign priorities according to the atoms directly attached to them [1, P-92.2.1.1.2]. Consider two common amino acids, L-serine (c) and L-cysteine (d) [1, P-103.1.1.1]:

(c)	(d)

L-serine (retained) (2S)-2-amino-3-hydroxypropanoic acid (substitutive) L-cysteine (retained) (2R)-2-amino-3-sulfanylpropanoic acid (substitutive)

In both amino acids, the order of preference at the chiral centre (C-2) is N > C = C > H. But no, not all carbons are equal. In L-serine (c), the carbon (C-1) of the carboxy group, –COOH, is linked to two oxygen atoms, and therefore is senior to the other carbon (C-3) that is linked to only one oxygen, that of a hydroxy group, –OH. The hydrogen at C-2 points away from the viewer and the rest of the ligand sequence, N > C-1 > C-3, go anticlockwise, so the configuration of the chiral carbon is ‘S’. On the other hand, in L-cysteine (d), the carbon (C-3) bears a sulfanyl group, –SH. Since sulfur is senior to oxygen, the ligand sequence N > C-3 > C-1 go clockwise and the configuration of C-2 is ‘R’.

Element sequence

Could it be that the atomic number criterion is so obvious that Earth chemists felt embarrassed to use it for nomenclature? Anyhow, they came up with a different seniority sequence for elements, the one based on electronegativity [2, IR-2.15.3.1, IR-4.4.2.1]. The element sequence is the reason why we (are supposed to) say hydrogen chloride and write HCl (rather than chlorine hydride and ClH). The main purpose of the element sequence is to order the atoms in binary compounds, but it is “also adhered to when ordering central atoms in polynuclear compounds for the purpose of constructing additive names” [2, IR-1.6.3].

Here is an updated version of the element sequence [3]:

	(1)

In general, electronegativity decreases when we go from top to bottom in a group and from right to left in a period. This is how the element sequence (1) is organised: zigzagging, starting from the group 17 (F > Cl > Br, etc.), then groups 16, 15 and so on until the group 1 (Li > Na > K etc.) and then to the group 18 (He > Ne > Ar, etc.). Interestingly, hydrogen is placed not in the group 1 but on a “hedge” between groups 16 (chalcogens) and 15 (pnictogens)^†.

Wait, my Arcturian colleague interrupts. This couldn’t be right. No matter whether you choose Pauling or Allen scale, caesium is not more electronegative than krypton, and hydrogen is not more electronegative than nitrogen, or even carbon. So you should write H₃N instead of NH₃. On the other hand, tellurium is less electronegative than hydrogen. So you should say tellurium dihydride, not dihydrogen telluride.

I don’t really know how to counter it. Perhaps, I wager, this is not true electronegativity for each element but average electronegativity in a group?

The Arcturian smiles, a bit condescendingly. How can you deal with the fact that electronegativity of Group 11 (individual or average) is higher than that of Group 12? And you still didn’t explain what the noble gases are doing in the very end of the element sequence. Why on Earth you use a criterion that does not even have an accepted definition?

Indeed. To make matters worse, chemists came up with a number of other element sequences. The Red Book gives the following sequence for ordering mononuclear parent hydrides [2, IR-2.15.3.2]:

N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl > O > S > Se > Te > C > F > Cl > Br > I	(2)

Here, it goes from the group 15, then group 14 starting from silicon, then group 13, then group 16, then carbon, then group 17. Confusing? You bet. To select senior atoms in parent structures and to choose between rings and chains, the Blue Book recommends the variant of (2) without halogens [1, P-44.1.2, P-68.1.5]:

N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl > O > S > Se > Te > C	(3)

While for skeletal replacement (‘a’) and in Hantzsch-Widman (H-W) names, the seniority order of heteroatoms is as follows [1, P-22.2.3.1]:

F > Cl > Br > I > O > S > Se > Te > N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl	(4)

Naturally, not being a heteroatom, carbon is not even here. And sometimes, a shorter (halogen-less) variant of sequence (4) is used [1, P-23.3.2.2, P-25.4.2.3.1, P-26.5.4.2, P-28.4.2]:

O > S > Se > Te > N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl	(5)

It’s a mess, the Arcturian shrugs. I concur.

Chains and rings: size matters

You may recall that the names of branched hydrocarbons are constructed using the longest-chain method. For example, (e) is named 3-methylpentane and not 2-ethylbutane because its principal chain is the one that contains the greater number of skeletal atoms, viz. pentane.

(e)

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

Likewise, in cyclic hydrocarbons larger rings (rings with the greater number of skeletal atoms) are senior to smaller ones [1, P-61.2.2]. Thus, (f) is named cyclopropylcyclohexane and not cyclohexylcyclopropane. Two rings are senior to one ring, so (g) is named 1-phenylnaphthalene, not (naphthalen-1-yl)benzene (or 1-naphthylbenzene). It’s only logical. My alien friend nods in agreement.

(f)	(g)

cyclopropylcyclohexane (substitutive, PIN) 1-phenylnaphthalene (substitutive, PIN)

Other seniority criteria applied are rather arbitrary, I admit. For instance, the structure (h) is called cyclohexylbenzene because “benzene has more multiple bonds than cyclohexane” [1, P-61.2.2]. This is because, caeteris paribus, priority is given to the system with greater number of multiple bonds [1, P-44.4.1]. The Arcturian counters saying that the number of atoms in saturated rings or chains is always higher than in the corresponding unsaturated ones and, therefore, the “size” criterion can be applied more consistently if we give seniority to saturated structures.

(h)

cyclohexylbenzene (substitutive, PIN)

What if we have a system consisting of rings and chains? We can call the structure (i) hexylcyclopentane (ring is senior to chain) or 1-cyclopentylhexane (chain has greater number of skeletal atoms) but the preferred IUPAC name will be the former one [1, P-44.1.2.2]. Again, my colleague is not convinced: rings-senior-to-chains sounds like a matter of taste, she says.

(i)

hexylcyclopentane (substitutive, PIN) 1-cyclopentylhexane (substitutive)

Organic classes

Substitutive nomenclature employs an order of seniority for classes of organic compounds [1, P-4, Table 4.1]. As discussed earlier [4], it is the most senior characteristic group that gives a name to a class — and corresponding “suffix” to a systematic name. Thus, L-serine (c) is named substitutively (2S)-2-amino-3-hydroxypropanoic acid and not (1S)-1-carboxy-2-hydroxyethanamine because acids are senior to amines. If we take away a hydron from (c), we’ll get L-serinate (j), or (2S)-2-amino-3-hydroxypropanoate (anions > amines).

(j)	(k)	(l)

L-serinate (retained) (2S)-2-amino-3-hydroxypropanoate (substitutive) L-serinium (retained) (1S)-1-carboxy-2-hydroxyethanaminium (substitutive) L-serine zwitterion (retained) (2S)-2-ammonio-3-hydroxypropanoate (substitutive)

On the other hand, if we add a hydron to (c), we’ll get L-serinium (k). Its substitutive name is (1S)-1-carboxy-2-hydroxyethanaminium because cations are senior to acids. Finally, L-serine zwitterion (l) is sytematically named as (2S)-2-ammonio-3-hydroxypropanoate (anions > cations).

As you can see, there is no great structure change here, just moving a hydron around. There’s no intrinsic reason why the carboxy group –COOH should be senior to amino group –NH₂ but “junior” to the aminium group –NH₃⁺.

Compounds such as halothane (a) and (b) are at the bottom of the seniority order [1, P-41, Table 4.1]:

λ¹ Halogen compounds in the order F > Cl > Br > I

In substitutive nomenclature they don’t qualify to be expressed by “suffixes” even if there are no other characteristic groups. (I find it discriminatory, says my alien colleague.) The quoted element sequence explains why halothane is named 2-bromo-2-chloro-1,1,1-trifluoroethane and not 1-bromo-1-chloro-2,2,2-trifluoroethane (fluorine is senior to the rest of halogens and thus gets the lower locant). The substituents ‘bromo’, ‘chloro’ and ‘fluoro’ are ordered alphabetically.

Alphanumerical order

Which brings us to the last ordering criterion for today. The substituents that appear as “prefixes” are cited in alphabetical order. The Blue Book prefers the term alphanumerical order, “to convey the message that both letters and numbers are involved” (in ordering principles) [1, P-14.5].

In some cases, locants are assigned according to alphanumerical order [1, P-14.4 (g)]. E.g., preferred name for (m) is 1-chloro-2-nitrobenzene, not 1-nitro-2-chlorobenzene:

(m)

1-chloro-2-nitrobenzene (substitutive, PIN)

Herold [5] notes that we have to be careful when translating English systematic names because alphabetical order in other languages is often not the same. He exemplifies his point with 3-methyl-5-phenylpyridine: its correct translation to Spanish, Italian and Portuguese will be 3-fenil-5-metilpiridina. In both names the substituent cited first get lower locants. In case of 1-chloro-2-nitrobenzene (m), its Russian systematic name will be 1-нитро-2-хлорбензол because the alphabetical order of substituents is opposite to that of the English name.

You should have seen my colleague’s face when I came to this bit. And how does that work in Chinese, she inquired. Somewhat naïvely, she thought I speak every Earth language. We decided to stop right there because it was almost dinner time.

We never came back to discuss chemical nomenclature: she flew back to Arcturus stream early the following morning. I sent her an email, of the it-was-nice-to-meet-you persuasion, but I fear I won’t receive an answer in my lifetime.

*	I assumed that my colleague was a “she”, although I can’t be completely sure. She never introduced herself stating her gender; come to think about it, neither did I. I addressed her as “you” and she did likewise. It didn’t cause any problem because nobody else in the room spoke Modern English.
†	More specifically, between livermorium and nitrogen (Lv > H > N); in earlier recommendations, between polonium and nitrogen [2, p. 260, Table VI].

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. 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. Hartshorn, R.M., Hellwich, K.-H., Yerin, A., Damhus, T. and Hutton, A.T. (2015) Brief guide to the nomenclature of inorganic chemistry (IUPAC technical report). Pure and Applied Chemistry 87, 1039—1049.
4. 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.
5. Herold, B. (2013) Lost in nomenclature translation. Chemistry International 35, no. 3, 12—15.

Planar chirality

In most organic chemistry textbooks, double bond cis/trans isomerism is exemplified by alkenes. It is also observed in cycloalkenes such as cyclooctene that can exist as either cis (a) or trans (b) isomer:

(a)	(b)

(Z)-cyclooctene (PIN) cis-cyclooctene (E)-cyclooctene (PIN) trans-cyclooctene

To the trans isomer, there is a twist — and the pun is fully intended. Have a look at the structures (c) and (d) (or at their 3-D models here, Fig. 2 and Fig. 3, respectively).

(c)	(d)

(1E,1P)-cyclooct-1-ene (PIN) (E,P)-cyclooctene (E,Rp)-cyclooctene (1E,1M)-cyclooct-1-ene (PIN) (E,M)-cyclooctene (E,Sp)-cyclooctene

Inorganic chains and rings

Let’s name a simple inorganic chain (a):

(a)

1,2-dinitrosodioxidane (substitutive) bis(nitrosyloxygen)(O—O) (additive) 2,5-diazy-1,3,4,6-tetraoxy-[6]catena (ICR)

The shortest systematic name I can think about is 1,2-dinitrosodioxidane, based on the parent hydride dioxidane (aka hydrogen peroxide). Alternatively, we can emphasise the structure’s symmetry by naming it as a dinuclear entity, bis(nitrosyloxygen)(O—O).

Or we can have a go at it employng yet another type of nomenclature developed for inorganic chains and rings (ICR): 2,5-diazy-1,3,4,6-tetraoxy-[6]catena [1, 2 IR-7.4]. What’s going on here?

Phane names

Have a look at the structure (a).

(a)

calix[4]arene (trivial) pentacyclo[19.3.1.13,7.19,13.115,19]octacosa-1(25),3(28),4,6,9(27),10,12,15(26),16,18,21,23-dodecaene (von Baeyer) 1,3,5,7(1,3)-tetrabenzenacyclooctaphane (phane)

Applying von Baeyer nomenclature, we get a horrendously long and unwieldy name ‘pentacyclo[19.3.1.1^3,7.1^9,13.1^15,19]octacosa-1(25),3(28),4,6,9(27),10,12,15(26),16,18,21,23-dodecaene’. I think it’s a crime to name a beautifully symmetrical structure like (a) in such a fashion. Can’t we create a name that states the obvious: (a) is a big cycle containing four benzene rings?

Ring assemblies

How shall we call the structure (a)?

(a)

biphenyl (trivial) 1,1′-biphenyl (ring assembly, PIN) phenylbenzene (substitutive)

We can name it substitutively, i.e. substituting one hydrogen atom in the parent hydride benzene with phenyl group: phenylbenzene. This name, however, does not reflect the obvious symmetry of the molecule.

Similar story with (b) whose substitutive name, cyclopentylidenecyclopentane, is barely pronounceable.

Spiro names

Observe the structure (a). Doesn’t it look like our old friend housane after a tornado? It kept its roof but only just.

(a)

spiro[2.3]hexane spirohexane

Let us number it in the following fashion:

(a)

The atom 3 is a quaternary carbon while the rest are secondary carbons. Or, using the graph theory language, we can say that in the graph (a) the degree of vertex 3 is 4 and the degrees of the rest of vertices are 2. The atom 3 is also known as a spiro atom [1, SP-0] while the whole structure is an example of spiro union.

Fused ring names

(a)

furan (trivial, retained) oxole (Hantzsch-Widman)

Knowing that the structure (a) is called furan, let’s name the structure (b).

(b)

2-nitrofuran (substitutive)

Easy: 2-nitrofuran.

Keeping that in mind, what kind of structure do you think corresponds to the name ‘2-benzofuran’?

von Baeyer names

Here’s a cute little structure:

(a)

housane (trivial) bicyclo[2.1.0]pentane (von Baeyer)

Drawn like this, (a) looks like a little house and, indeed, is known as a housane. Alexander Senning called this structure “the poor man’s housane” [1, p. 77] while referring to pentaprismane as “the rich man’s housane” [1, p. 78]. Of course, there is a systematic name too.

Bicycles

How many rings has the structure (a)?

(a)

diphenyl ether (functional class) 1,1′-oxydibenzene (multiplicative) phenoxybenzene (substitutive)

Why, there are two, you’ll say. Anybody can see that. And you’ll be right.

What about (b) then?

(b)

norbornane (trivial) bicyclo[2.2.1]heptane (von Baeyer)

The many names of crowns

What is the best way to name the structure (a)?

(a)

1,4,7-trioxonane (Hantzsch-Widman) 1,4,7-trioxacyclononane (replacement) cyclo[tri(oxyethylene)] (organic macrocycle) 9-crown-3 (Pedersen) 9<O3coronand-3> (Vögtle-Weber)

The general naming method is skeletal replacement applied to the corresponding carbocyclic parent hydride, in our case cyclononane, thus 1,4,7-trioxacyclononane. Or we can use extended Hantzsch-Widman (H-W) system and call it 1,4,7-trioxonane. For rings with up to ten members, the H-W names are preferred [1, p. 96].

What about the structures (b)—(d) then? Since all of these rings have more than ten members, we cannot use H-W system, so we have to give them replacement names: 1,4,7,10-tetraoxacyclododecane (b), 1,4,7,10,13-pentaoxacyclopentadecane (c), 1,4,7,10,13,16-hexaoxacyclooctadecane (d). Rather long, completely unambiguous, and very boring.

Hantzsch-Widman names

Are you tired of carbocycles? Let’s have some ring diversity, I say.

Structures that contain two or more different elements in a ring are called heterocyclic. Perhaps because “heteroatom” is really an organic chemistry concept, the word “heterocycle” is commonly (mis)understood as “organic heterocycle”, that is, a carbocycle where at least one carbon atom is replaced by an heteroatom. I blame organic chemists for that.

For a small number of five- and six-membered organic heterocycles the trival names are retained to be used as parent hydride names. Note that, although “trivial” in chemical parlance means “non-systematic”, there is a system to most of those names. For instance, we can see that imidazolidine (a) is a fully saturated version of 1H-imidazole (b):

(a)	(b)

imidazolidine 1H-imidazole

Mancude rings and annulenes

What do the structures (a), (b) and (c) have in common?

(a)	(b)	(c)

[18]annulene cyclooctadeca-1,3,5,7,9,11,13,15,17-nonaene (PIN) 1,3,5,2,4,6-triazatriphosphinine thiophene

Well, it is obvious that they all are rings. Also, apart from hydrogens in (a) and (c), they have no side chains. Otherwise, they are quite different. The structure (a) is a hydrocarbon. The ring (b) is purely inorganic while (c) is an organic heterocycle. What else?

You can see that in all these structures single bonds alternate with double bonds. Ring systems like this are referred to as mancude, which is an abbreviation of the “maximum number of non-cumulative double bonds”.

Alicyclic monocycles

Now let us have a look at monocyclic hydrocarbons, starting with cycloalkanes. By the way, I think this term is a bit misleading: cycloalkanes indeed contain cycles but are not alkanes because these latter, by definition, are acyclic. Gold Book defines cycloalkanes as “saturated monocyclic hydrocarbons (with or without side chains)”, where “side chains” are alkyl groups. The general molecular formula of cycloalkanes, with or without side chains, is C_nH_2n. I wish there was an elegant collective term for cycloalkanes-with-no-side-chains, or unsubstituted cycloalkanes, because only this subset of cycloalkanes can be used as parent hydrides in systematic organic nomenclature; I am not aware of any. Here, I will refer to unsubstituted cycloalkanes as ‘cycloalkane parents’^*.

Chains and rings

After hours spent looking in my books and searching the internet, I came to the conclusion that chemists talk about chains and rings without explaining what they mean. The only definition I found so far, viz. that of Gold Book, is specific for polymers and seems to be too complex to be used in general chemical nomenclature:

The whole or part of a macromolecule, an oligomer molecule or a block, comprising a linear or branched sequence of constitutional units between two boundary constitutional units, each of which may be either an end-group, a branch point or an otherwise-designated characteristic feature of the macromolecule.

(1)

On the other hand, general dictionary definitions of (chemical) chains are not precise enough. For example, Collins English Dictionary defines chain (chemistry) as

two or more atoms or groups bonded together so that the configuration of the resulting molecule, ion, or radical resembles a chain.

(2)

whereas Merriam-Webster says that it is

a number of atoms or chemical groups united like links in a chain.

(3)

So chain (chemistry) is like a chain. Is it?

Multiplicative names

Have a look at the structure (a).

(a)

N,N-bis(carboxymethyl)glycine (substitutive) N,N,N-tris(carboxymethyl)amine (substitutive) 2,2′,2″-nitrilotriacetic acid (multiplicative)

Its preferred IUPAC name (PIN) is N,N-bis(carboxymethyl)glycine. This is a classical substitutive name based on functional parent glycine (b). However this name fails to convey the structural feature of (a) that is obvious even to a non-chemist, viz. its threefold symmetry. Alternatively, we can think of (a) as a tertiary amine, that is, ammonia in which each hydrogen atom is substituted by a carboxymethyl group: N,N,N-tris(carboxymethyl)amine. This name clearly tells us that there are three identical groups, but it probably will be frowned upon because carboxy group is senior to amino.

It also can be given a name that reflects both symmetry and the fact that it is a carboxylic acid.

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)

Cyclo and seco

The prefix ‘cyclo’ is used in chemical names to indicate a ring structure. In additive nomenclature, this prefix is usually italicised and followed by a hyphen, as we have seen for polynuclear entities such as cyclo-tri-μ-oxido-tris(dioxidotungsten). On the other hand, in substitutive nomenclature ‘cyclo’ is not italicised, there is no hyphen, and no other prefixes could be inserted between ‘cyclo’ and the root, as in cyclopropane.

Somewhat confusingly, this prefix is also employed in skeletal modification nomenclature when an additional ring is created [1]. In the names generated thus, ‘cyclo’ has to be preceded by the locants of the skeletal atoms that form a new bond.

(a)	(b)

cycloartane (trivial) 9β,19-cyclolanostane (‘cyclo’) lanostane (trivial, fundamental parent)

Homonames

Both skeletal replacement and ‘nor’-type subtractive naming methods can be considered subtypes of skeletal modification nomenclature. And there are more.

Consider homocysteine (a):

(a)	(b)

homocysteine (trivial + ‘homo’ addition) 2-amino-4-sulfanylbutanoic acid (substitutive) cysteine (trivial) 2-amino-3-sulfanylpropanoic acid (substitutive)

Nornames

A variant of subtractive nomenclature employs the prefix ‘nor’. Probably the most famous example is the neurotransmitter noradrenaline aka norepinephrine (a):

(a)	(b)

noradrenaline (trivial + subtractive) norepinephrine (trivial + subtractive) 4-[(1R)-2-amino-1-hydroxyethyl]benzene-1,2-diol adrenaline (trivial) epinephrine (trivial) 4-[(1R)-1-hydroxy-2-(methylamino)ethyl]benzene-1,2-diol

These names are derived from the trivial names of (b), adrenaline or epinephrine. In this particular example, the meaning of ‘nor’ is the same as ‘demethyl’.

Skeletal replacement nomenclature

So, in substitutive nomenclature we take a parent hydride or a functional parent, whose name form a root of a term to be created, and replace the hydrogens with substituents, whose names become prefixes or suffixes. Can we do the same with other atoms apart from hydrogens, for the naming purposes?

Yes we can.

(a)	(b)

pentaethylene glycol (trivial) 3,6,9,12-tetraoxatetradecane-1,14-diol (replacement + substitutive) tetradecane