Question #a16da

1 Answer
Stefan V.
Apr 9, 2015

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#

http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter_02%3A_Introduction_to_organic_structure_and_bonding_II/Section_2.4%3A_Solubility,_melting_points_and_boiling_points

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 The difference between the two comes from the fact that a sigma bond between a carbon atom and a hydrogen atom has been replaced by a sigma bond between two carbons.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

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