The Nitration of Benzene

Benzene reacts slowly with hot concentrated nitric acid in an electrophilic aromatic substitution reaction to form nitrobenzene. This reaction is potentially dangerous, however, because nitric acid is a strong oxidizing agent that often explodes in the presence of any material that readily oxidizes. A safer, faster, and more convenient synthesis employs a mixture of concentrated nitric acid and concentrated sulfuric acid. The concentrated sulfuric acid acts as a catalyst allowing nitration to take place more readily at more moderate temperatures.

The nitronium ion (⊕NO2) is the electrophile in the nitration of benzene to form nitrobenzene. Although concentrated nitric acid produces the nitronium ion by itself, the equilibrium is so far to the left that the process is slow. Adding concentrated sulfuric acid to the reaction mixture increases the concentration of the nitronium ion, thereby increasing the rate of the nitration reaction. The nitronium ion forms via a pathway similar to the first step in the dehydration of an alcohol.

After the nitronium ion forms, it reacts with benzene to form the σ complex, the first step of the electrophilic aromatic substitution reaction. This step is slow because the σ complex is not aromatic. Additionally, the σ complex is higher in energy than the benzene and the nitronium ion.

In the next step of the mechanism, the σ complex loses a proton to form nitrobenzene. This step is rapid because the loss of a proton allows the molecule to become aromatic again.

Chemists tested whether the loss of a proton is the fast step or the slow step of an electrophilic aromatic substitution by replacing the hydrogens in benzene with deuterium and then running the reaction. Deuterium (2H abbreviated as D) is an isotope of hydrogen (1H) that contains not only one proton in its nucleus but also one neutron. Thus, deuterium has twice the mass of hydrogen. Because the bond energy between a pair of atoms changes in proportion to the masses of the isotopes involved in that bond, the C—D bond is higher in energy than the C—H bond. This isotope effect is observable in the IR spectrum. The IR absorption of the C—H bond in benzene is approximately 3050 cm–1; whereas the C—D bond of deuteriobenzene is about 2150 cm–1.

Because breaking a C—D bond requires more energy than breaking a C—H bond, a reaction whose rate-determining step involves breaking a C—H bond proceeds more slowly when deuterium is present. Thus, replacing C6H6 with C6D6 results in a reduction of the nitration rate if the breaking of a C—H bond is the rate-determining step. With the electrophilic aromatic substitution reaction, chemists measured no difference in the rate of reaction between C6D6 and C6H6. This shows that the rate-determining step is the formation of the σ complex, not the step that breaks the C—H bond.