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. And this something is ‘o’ in ‘hydro’.

Normally, in substitutive nomenclature hydrogen is implied but not named unless there is a real need for it. When we say ‘chloro’, it means that a hydrogen atom is substituted by a chlorine atom; when we say ‘oxo’, it means that two hydrogens are substituted by one oxygen, and so on. Using the same logic with ‘hydro’ though would mean that a hydrogen atom is substituted by another hydrogen atom, which does not make much sense^*. So here is an exception: if hydrogen is named in ‘o’ fashion, it is being added.

This also means that addition of elements other than hydrogen to organic structures must be expressed differently. The most common method is... to use binary names. Yes, yet another kind of binary names.

(d)

1a,7b-dihydronaphtho[1,2-b]oxirene (fused ring + additive) naphthalene 1,2-oxide (trivial + additive)

Consider the structure (d). Fused ring nomenclature [2] provides it with a rather frightening name 1a,7b-dihydronaphtho[1,2-b]oxirene. Or it could be named as a derivative of naphthalene (a) with oxygen “added” to carbons 1 and 2: naphthalene 1,2-oxide. Which one do you like more?

We can extend this method to include halogen atoms.

(e)	(f)	(g)

1,2,3,4,5,6-hexachlorocyclohexane (substitutive) benzene hexachloride (trivial + additive) cyclohexane (parent hydride) benzene (trivial, parent hydride)

For instance, (e) could be named substitutively as a derivative of cyclohexane (f): 1,2,3,4,5,6-hexachlorocyclohexane, or additively as benzene hexachloride^†. In this latter name, we add six chlorine atoms to benzene (g), with concomitant saturation of three formal double bonds.

When oxygen and other chalcogens are “added” to heteroatoms, binary names are often the only sensible names for the resulting structures^‡.

(h)	(i)

N,N-dimethylaniline (substitutive) N,N-dimethylaniline N-oxide (substitutive + additive)

(j)	(k)

dibenzo[b,d]thiophene (fused ring) dibenzo[b,d]thiophene 5-oxide (fused ring + additive)

*	It would make sense though if we are talking about a specific isotope of hydrogen. For example, we can name the molecule CHD2T substitutively “dideuterio(tritio)methane”.
†	I also saw this structure named “benzene 1,2,3,4,5,6-hexachloride” and there’s nothing wrong with it. However, we can add only one chlorine to each carbon in the ring anyway, so the locants here are superfluous. (We still need them in cases of benzene di-, tri- and tetrachloride.) Likewise, we can give cyclohexane (f) an additive name “hexahydrobenzene” which is totally unambiguous. Contrast that with the substitutive name “1,2,3,4,5,6-hexachlorocyclohexane” where we really need ‘1,2,3,4,5,6’. Each carbon atom in cyclohexane ring has two hydrogen atoms attached, and each of them can be potentially substituted by chlorine. So “hexachlorocyclohexane” can refer to any of a variety of isomers, e.g. 1,1,2,3,3,4-hexachlorocyclohexane or 1,1,3,3,5,5-hexachlorocyclohexane.
‡	Alternatively, an atom of O, S, Se or Te added to a heteroatom could be indicated by ‘oxido’, ‘sulfido’, ‘selenido’ or ‘tellurido’, respectively [1]. This sounds very much like additive nomenclature that I know and love, but it doesn’t look like this method caught on with organic chemists.

References

1. Bünzli-Trepp, U. Systematic Nomenclature of Organic, Organometallic and Coordination Chemistry. EPFL Press, 2007, pp. 36—37.
2. Moss, G.P. (1998) Nomenclature of fused and bridged fused ring systems (IUPAC Recommendations 1998). Pure and Applied Chemistry 70, 143—216.