Question

How do you count sigma and pi bonds? For example, in the lewis structure azidothymidine there are 33 sigma and 5 pi bonds. How is this found?

Answer 1

Azidothymidine:

When you draw the purely skeletal structure, i.e. with no double or triple bonds, what is there counts as one sigma bond per single bond.

That gives you:

• Five `"C"-"O"` `sigma` bonds
• Six `"C"-"N"` `sigma` bonds
• Two `"N"-"N"` `sigma` bonds
• One `"N"-"H"` `sigma` bond
• One `"O"-"H"` `sigma` bond
• Seven `"C"-"C"` `sigma` bonds

And don't forget the implicit hydrogens!

• Eleven `"C"-"H"` `sigma` bonds

Now when you consider `pi` bonds, it is one additional `pi` bond for each additional bond for up to only double and triple bonds:

• Two `"N"-"N"` `pi` bonds
• One `"C"-"C"` `pi` bond
• Two `"C"="O"` `pi` bonds

So we have:

• `5 + 6 + 2 + 1 + 1 + 7 + 11 = 33` `sigma` bonds
• `2 + 1 + 2 = 5` `pi` bonds