What are the sigma and pi bonds in cumulene?

1 Answer
Truong-Son N.
Mar 31, 2016

Cumulenes are a class of compounds with #"C"="C"# chains directly connected (like #cdotcdotcdot="C"="C"="C"="C"=cdotcdotcdot#).

I can't assume that you are necessarily talking about a particular cumulene, but given the examples in the wikipedia page, I'm going to assume that you are talking about a cumulene that only contains the #"C"="C"# chain, but nothing crazy like #("CO")_5"M"="C"="C"="C"="C"="C"="C"="C"-("NMe"_2)_2# like I would find in my textbook chapter on metallacumulenes. :)

In that case, there would be one #sigma# bond and one #pi# bond for each #"C"="C"# connection, as a pure double bond would contain one #sigma# and one #pi# bond. There would also be one #sigma# bond for each terminal #"C"-"H"#.

If we were to take butatriene as an example (coincidentally also called cumulene... see what I mean?), then, seeing as it is:

#\mathbf("H"_2"C"="C"="C"="CH"_2)#

...there are three #"C"-"C"# #pi# bonds, three #"C"-"C"# #sigma# bonds, and four #"C"-"H"# #sigma# bonds overall.

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