# Question #a16da

Apr 9, 2015

Well, benzene and toluene do not have the same molecular formula, since benzene is ${C}_{6} {H}_{6}$ and toluene is ${C}_{7} {H}_{8}$.

The difference between benzene and toluene is that the latter has a mthyl group attached to the ring. That is why, according to IUPAC nomenclature, toluene is called methylbenzene, or ${C}_{6} {H}_{5} C {H}_{3}$

As you can see, the two molecules have indeed the same number of pi bonds, 3 to be precise, but differ in the number of sigma bonds.

Toluene has 15 sigma bonds, while benzene has only 12 sigma bonds. The difference between the two comes from the fact that a sigma bond between a carbon atom and a hydrogen atom has been replaced by a sigma bond between two carbons.

One of the two is one the ring, while the other is part of the methyl group. SInce the methyl group has 3 sigma bonds of its own, the total number of sigma bonds for toluene will be 15.

You can think of this like

$\text{nr. sigma b. toluene" = "nr. sigma b. benzene" + "nr. sigma b. methyl}$

or

$\text{nr. sigma bonds toluene} = 12 + 3 = 15$.