Question

Question #a16da

Answer 1

Well, benzene and toluene do not have the same molecular formula, since benzene is `C_6H_6` and toluene is `C_7H_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_6H_5CH_3`

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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 The difference between the two comes from the fact that a sigma bond between a carbon atom and a hydrogen atom has been replaced by a sigma bond between two carbons.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

`"nr. sigma b. toluene" = "nr. sigma b. benzene" + "nr. sigma b. methyl"`

or

`"nr. sigma bonds toluene" = 12 + 3 = 15`.