Subtractive names

In compositional and additive nomenclatures, we build the names more or less from scratch. The more complex the structure, the longer the name. With substitutive nomenclature, we take the names of parent hydrides or functional parents and modify them adding the names of substituent groups. Once again, the complexity of the name increases with complexity of the structure. In case of both skeletal replacement and functional replacement, a small or no increase in structural complexity still leads to longer names.

Yet there are opposite situations.

(a)	(b)

demethylmirtazapine (trivial + subtractive) 1,2,3,4,10,14b-hexahydropyrazino[2,1-a]pyrido[2,3-c][2]benzazepine mirtazapine (trivial) 2-methyl-1,2,3,4,10,14b-hexahydropyrazino[2,1-a]pyrido[2,3-c][2]benzazepine

You might have noticed that, while the substitutive name of (a) is shorter than that of (b), the reverse is true for their trivial names. This is because the structure of mirtazapine (b) contains a methyl group which is lost in (a), hence ‘demethyl’ bit in demethylmirtazapine.

This method of naming is known as subtractive.

Functional replacement nomenclature

Organic molecules are often thought of as comprising a skeleton, for example a chain or a ring, adorned by functional groups. Having just read about skeletal replacement, you might think that functional replacement has something to do with those functional groups. It’s only logical. But you’d be mistaken.

Let’s name the structure (a):

(a)

[CS(SH)2] trithiocarbonic acid (common + functional replacement) carbonotrithioic acid (functional replacement) sulfidodisulfanidocarbon (additive)

Of course, its formula [CS(SH)₂] gives us a clue: we can call such an entity additively sulfidodisulfanidocarbon. But there is another way to do it.