January 12, 2010
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
Functional groups such as allene (C=C=C) and cumulene (C=C=C=C) present an interesting challenge in an elucidation project, especially when an elucidator is not expecting them. The 13C chemical shifts for allene carbons are typically expected at 80, 200 and 100 ppm +/- ~20 ppm per shift. The confusion, or better described as bias, arises at treating a 13C chemical shift of 200 ppm as a carbonyl (C=O) rather than considering the possibility of an allene group.
In the example below, there are an odd number of sp2 carbon atoms along with 1 oxygen atom and 6 hydrogen atoms.
The example leads into two possible candidates: an allene in 1-(ethenyloxy)propadiene ethenyl propadienyl ether or an alkene plus a carbonyl group in penta-1,4-dien-3-one.