A problem that has long interested chemists has been to calculate the number of isomers for an alkane of any given carbon content. Whilst there appears to be no simple mathematical relationship between the number of isomers and the carbon content, it is possible to calculate this number iteratively counting the number of isomers of each fragment into which the alkane can be decomposed. This approach has been encoded into an algorithm for calculating the number of isomers of large alkanes - It is estimated that C₁₆₇H₃₃₆ has more isomers than the universe has particles!

We have shown that in fact not all structural isomers are possible - high level ab initio calculations show that the C16 and C17 fragments below, if they were synthesized, would be unstable and rapidly undergo homolytic dissociation at room temperature. Certainly C17 cannot be made, and it is likely that C16 can only exist at very low temperatures for a short time.

This factor has been taken into account in the development of IsoCount which disallows the unstable substructures. The Applet calculates the number of isomers removing any containing the C17 substructure, or using the more stringent condition of removing the C16 substructure. The java source code is available.

Robert S. Paton and Jonathan M. Goodman

Exploration of the Accessible Chemical Space of Acyclic Alkanes

R. S. Paton and J. M. Goodman J. Chem. Inf. Model. 2007, 47, 2124-2132.

DOI: 10.1021/ci700246b

What Is the Smallest Saturated Acyclic Alkane that Cannot Be Made?

K. M. N. de Silva and J. M. Goodman J. Chem. Inf. Model. 2005, 45, 81-87.

DOI: 10.1021/ci0497657

What is the Longest Unbranched Alkane with a Linear Global Minimum Conformation?

J. M. Goodman J. Chem. Inf. Comput. Sci. 1997, 37, 876-878.

DOI: 10.1021/ci9704219