Question

How many pi bonds are in a `C_2H_4` molecule?

Answer 1

There is one `pi`-bond.

Explanation:

`sigma`-bonds are the first bond between atoms. `pi`-bonds are those built up on top. A double bond is a `sigma` and a `pi`; a triple bond is a `sigma` and 2 `pi` bonds.

`C_2H_4` is also known as ethene and has a double bond equivalent (DBE) of `1`.

DBE is calculated using the formula

`1/2 (A - X)`,

where `A` is the potential number of hydrogen and `X` is the actual number. `A` is calculated using

`2n + 2`

where `n` is the number of carbons in the molecule.

Therefore, when `n = 2`,

`1/2 (A - X) = 1/2 (6 - 4) = 1/2 * 2 = 1`

Although double bond equivalents are not necessarily double bonds, as they could be triple bonds or cyclic structures, in a molecule this small there is no opportunity for those.

Since double bonds have one `sigma` and one `pi`, this is the total number of `pi`-bonds: `1`.