During the nitration of toluene, one or more of the hydrogen atoms in the benzene ring are replaced by a nitro group. The number of hydrogens that are replaced by the nitro groups depends on the conditions of the reaction, such as temperature.
Nitration of toluene requires a mixture of concentrated nitric acid and concentrated sulfuric acid. The nitric acid is the donor for the nitro group, and sulfuric acid acts as a catalyst to the reaction. Sulfuric acid also absorbs the water that forms over the course of the reaction. The nitro group bonds with the benzene ring, and through the rearrangement of bonds, it replaces a hydrogen atom in the benzene ring. This mechanism of substituting a hydrogen atom for a nitro group is called an electrophilic substitution.
Toluene reacts much faster than benzene in nitration reactions because the methyl group pushes the electrons toward the ring, thus increasing its reactivity and making it more susceptible to electrophilic substitution. Depending on the position at which the nitro group is added, relative to the methyl group in toluene, two isomers can be formed in this reaction. Since methyl groups are 2,4-directing, a mixture of 2-nitromethylbenzene or 4-nitromethylbenzene is formed. A small quantity of dinitrotoluene forms as well, but it is harder to form trinitrotoluene. This is because adding nitro groups to the benzene ring of toluene decreases its reactivity.