Nomenclature roundup

Here are the main types of chemical nomenclature in a nutshell. Well, the post turned out to be a bit longer than I expected. So let’s say “chemical nomenclature in a coconutshell”.

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.

Additive again

As we know, additive nomenclature is widely used in inorganic chemistry. It could be employed for organic structures too, although in a very specific and limited way [1].

I mentioned earlier that the subtractive ‘dehydro’ operation (as in ‘didehydro’, ‘tetradehydro’ etc.) is the opposite of additive ‘hydro’ operation, i.e. hydrogen addition to unsaturated carbon—carbon bond (as in ‘dihydro’, ‘tetrahydro’ etc.).

(a)	(b)	(c)

naphthalene (trivial, parent hydride) 1,2-dihydronaphthalene (trivial + additive) tetralin (trivial) 1,2,3,4-tetrahydronaphthalene (trivial + additive)

For example, adding two hydrogen atoms to carbons 1 and 2 of naphthalene (a) gives us the structure (b) which we call, logically enough, 1,2-dihydronaphthalene; adding two more hydrogens to carbons 3 and 4 we get (c), 1,2,3,4-tetrahydronaphthalene. Easy!

Yet something does not sound quite right here.

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)

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.