Question

What are the sigma and pi bonds in cumulene?

Answer 1

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.