A Note on Complexity and Isomerism

My textbook has a table with information that I did not include in my last post, but that may improve understanding of isomerism. In case it is not obvious, the number of isomers grows with the size of a molecule. In my last post, I showed the two isomers of butane. Larger alkanes have even more, because with more atoms, there are more ways to rearrange them. Small alkanes are easy to understand in this regard. A hydrocarbon with one carbon has no isomerism. The same is true for two or three carbons. When we get to four, as already demonstrated, there are two possibilities: a straight chain and one with a branch. Five carbons means three isomers. With seven carbons, we get nine isomers, which is still manageable, but then add a single carbon and there are eighteen isomers. The table ends with icosane (C₂₀H₄₂), which has 366,319 constitutional isomers.

And that is just acyclic alkanes. There are so many other things to consider, that the complexity is staggering. And that is why we have a systematic method of naming molecules. Anything else would get pretty impractical.