# How many sigma and pi bonds are in xylene?

Jun 21, 2016

There are $18$ $\sigma$ and $3$ $\pi$ bonds.

There are three isomers of xylene, but they all contain the same number of $\sigma$ and $\pi$ bonds. 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 $\text{C"-"H}$ bond on the benzene ring adds one $\sigma$ bond, for a total of 4.
• The $\text{C"-"C}$ bond with each methyl group counts as one $\sigma$ bond, contributing a total of 2 $\sigma$ bonds.
• The three $\text{C"-"H}$ 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.