February 19, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
As would-be elucidators dive into an elucidation for an organic unknown, narrowing down a single molecular formula (MF) becomes vital in simplifying the elucidation. However, one hurdle quickly leads into another. As the total atom count adds up, the number of possible isomers increases just as much (See Table and Graph below).
I can’t imagine drawing by hand every possible isomer for C12H26 at 355. And even if I did, how would I determine the candidate structure? The only choice is to acquire additional data to help eliminate the poor candidates. In my experience, the best choices are 1D and 2D NMR, MSn, and/or single-crystal XRD data.
MFÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Number of Isomers
CH4Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 1
C2H6Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 1
C3H8Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 1
C4H10Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 2
C5H12Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 3
C6H14Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 5
C7H16Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 9
C8H18Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 18
C9H20Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 35
C10H22Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 75
C11H24Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 159
C12H26Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 355
C15H32Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 4,347
C20H42Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 366,319
C30H62Â Â Â Â Â Â Â Â Â Â 4,111,846,763
C40H82Â Â Â Â 62,491,178,805,831
C10H17Br2ClO2Â Â Â Â Â 50,502,293
C15H22O2Â Â Â Â Â Â 38,136,211,624
C15H20O1Â Â Â Â Â Â 37,568,150,635
C12H12O3Â Â Â Â Â Â 68,930,547,646
It’s worth looking at the comments in this post: http://www.chemspider.com/blog/how-many-structures-can-you-generate-from-a-molecular-formula.html
Especially the one by Eric Milgram.