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Allene versus Carbonyl

January 12, 2010

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.

Allene13C_13C_Jan112010

 

 

In the example below, there are an odd number of sp2 carbon atoms along with 1 oxygen atom and 6 hydrogen atoms.

 

Allene13C_Oddsp2_Jan112010

 

 

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.

 

Allene13C_2Possibilities_Jan112010

 

2 Replies to “Allene versus Carbonyl”

  1. Hello, can you tell me why allene has an unexpected paramagnetic term in nmr?In several texts the 232.8 has been reported for it. Can you tell me the reason of it?? thanks.

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