Question

How many sigma and pi bonds are in xylene?

Answer 1

There are `18` `sigma` and `3` `pi` bonds.

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There are three isomers of xylene, but they all contain the same number of `sigma` and `pi` bonds.

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An easy way to count these is to treat the skeletal structure as all `sigma` bonds, and add on `pi` bonds for each double bond in the major resonance structure.

So, taking `m`-xylene arbitrarily:

• All hexagons have six sides, contributing 6 `sigma` bonds.
• Each `"C"-"H"` bond on the benzene ring adds one `sigma` bond, for a total of 4.
• The `"C"-"C"` bond with each methyl group counts as one `sigma` bond, contributing a total of 2 `sigma` bonds.
• The three `"C"-"H"` bonds on each methyl group contribute a total of 6 more `sigma` bonds.

So, we have a total of 18 `sigma` bonds.

Inside the benzene ring, you have delocalized `pi` electrons over six atoms. The major resonance structure has three `pi` bonds; one for each double bond.

So the result is 18 `sigma` and 3 `pi` bonds.