Branched hydrocarbons

How can we name the structure (a)?

(a)

[Sn(CH3)3H] hydridotrimethyltin (additive) trimethylstannane (substitutive)

We can give it an additive name ‘hydridotrimethyltin’. Alternatively, based on the mononuclear hydride stannane SnH₄, we can call it ‘trimethylstannane’. Easy.

Now let’s replace the central tin atom with carbon, which results in the structure (b):

(b)

CH(CH3)3 isobutane (trivial) 2-methylpropane (substitutive)

Can’t we name it trimethylmethane? Why, of course we can. You may recall a similar substitutive name for the structure (c). However, as far as the naming of alkanes is concerned, a different method is used. Namely, the name is based on longest chain. Using the graph theory language, if our structure is a tree G, we are looking for a subgraph G′ that is the longest path. In the case of (b) the longest chain is propane (d), so the preferred IUPAC name (PIN) for (b) is 2-methylpropane.

I’ll tell you why I dislike this name. First, the locant ‘2’ is superfluous: there is only one way to attach methyl group to propane without converting the longest chain to butane. Second, the name does not reflect the obvious fact that there are three identical groups bonded to the central atom. My internal coordination chemist is not happy!

(c)	(d)

triphenylmethane (substitutive) 1,1′,1″-methanetriyltribenzene (multiplicative) H3C–CH2–CH3 propane (parent hydride)

Likewise, the only difference between the structures (e) and (f) is the central atom. So, the name tetramethylmethane for (f) makes every sense but, alas, is not the one recommended by IUPAC. Instead, the PIN is 2,2-dimethylpropane. I guess by now you understand why it doesn’t make me ecstatic either.

(e)	(f)

Pb(CH3)4 tetramethyllead (additive) tetramethylplumbane (substitutive) C(CH3)4 neopentane (trivial) 2,2-dimethylpropane (substitutive)

The reason why the current system is used is the sheer number of alkane isomers. There are only two C₄H₁₀ isomers, so it’s easy enough to give them trivial names, butane and isobutane. Since butane is an n-alkane, it used to be called ‘n-butane’, just to make sure that we talk about unbranched isomer. There are three C₅H₁₂ isomers, viz. pentane (or n-pentane), isopentane, and neopentane. For C₁₀H₂₂, there are 75 possible isomers; for C₁₅H₃₂, there are 4347 isomers; for C₂₀H₄₂, already 366,319 isomers [1]. Of them, only few are as beautifully symmetrical as (b) and (f), so the longest-chain method is most logical, although it might not generate the most attractive, or even the shortest, names. For example, the structure (g) is named 3-ethyl-3-methyldecane, not 2,2-diethylnonane, simply because decane is longer than nonane.

(g)

3-ethyl-3-methyldecane (substitutive)

If there are two or more chains of equal length, which one has to be chosen as the main one, for nomenclature purposes? Well, there are rules for that. For instance, the structure (h) is named 3-ethyl-2,7-dimethyloctane, not 6-isopropyl-2-methyloctane, because of the Rule A-2.6(d) that “the chain having the least branched side chains” is the senior.

(h)

3-ethyl-2,7-dimethyloctane (substitutive)

What about branched alkenes and other unsaturated hydrocarbons? The principle remains the same: start with the longest chain [2]. Thus, the structure (i) is named 3-methylideneheptane, not 2-ethylhex-1-ene^*. And if there are two or more chains of equal length, there are seniority rules for that too, of different degrees of arbitrariness.

(i)

3-methylideneheptane (substitutive)

*	In earlier recommendations, however, unsaturation was senior to chain length [3], thus 2-ethylhex-1-ene would be preferred to 3-methylideneheptane.

References

1. Sloane, N.J.A. Number of n-node unrooted quartic trees; number of n-carbon alkanes C_nH_2n+2 ignoring stereoisomers. On-Line Encyclopedia of Integer Sequences, sequence A000602.
2. 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.
3. Panico, R., Powell, W.H. and Richer, J.-C. A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993). Blackwell Science, 1993.