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The number of isomers that exist for any particular chemical structure is a favourite question early on in a chemistry course; have you ever noticed that the number of carbon atoms is seldom above four?

The prediction of isomer numbers is not a trivial problem; simple combinatorial theory cannot be used since there are chemical requirements superimposed on the mathematical ones. Thus for the alkanes:

 
Number of carbons Number of isomers
8 18
9 35
10 75
11 159
12 355
13 802
14 1,858
15 4,347
20 366,319
25 36,797,588
30 4,111,846,763
40 62,491,178,805,831

I promise there will be no questions on tetracontane.

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