The nitration of nitrobenzene at room temperature gives 1,3-dinitrobenzene, but not 1,3,5-trinitrobenzene, despite the use of excess reagents. Why is the reaction selective?
I think that under ordinary circumstances, at room temperature, some trinitrobenzene is formed, but not very much. The addition of a -nitro to the ring makes is much less reactive, and when two have been added, the ring becomes virtually unreactive.
The selective nitration of nitrobenzene at room temperature, leading to the formation of 1,3-dinitrobenzene instead of 1,3,5-trinitrobenzene, can be explained by considering the reactivity and stability of the intermediate species involved.
To understand this, let's first look at the reaction mechanism. The nitration of nitrobenzene involves the substitution of a nitro group (-NO2) onto the benzene ring using a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) as the nitrating agent. The reaction is typically carried out at low temperatures to minimize the formation of unwanted byproducts.
In the initial step of the reaction, the nitric acid molecule is protonated by the sulfuric acid, leading to the formation of the nitronium ion (NO2+). This nitronium ion then attacks the electron-rich benzene ring, resulting in the formation of an arenium ion or sigma complex.
The arenium ion is an intermediate species in which the benzene ring has a positive charge at the carbon atom to which the nitro group is being added. At this point, the arenium ion is stabilized by resonance, which distributes the positive charge throughout the aromatic ring.
Now, let's consider the reactivity and stability of the intermediate species involved. Adding a nitro group to the benzene ring decreases the electron density of the ring due to the electron-withdrawing nature of the nitro group. This decreased electron density reduces the reactivity of the ring towards further nitration.
When two nitro groups have been added (forming 1,3-dinitrobenzene), the electron density of the benzene ring is even further reduced. As a result, the ring becomes virtually unreactive toward further nitration. The strong electron-withdrawing effect of the two nitro groups stabilizes the ring by delocalizing the positive charge across the entire ring system.
On the other hand, the formation of 1,3,5-trinitrobenzene requires the addition of a third nitro group to the already substituted 1,3-dinitrobenzene. However, due to the high electron-withdrawing nature of the two nitro groups in 1,3-dinitrobenzene, the reactivity of the ring is greatly diminished. The benzene ring becomes highly deactivated towards further nitration, preventing the addition of a third nitro group.
In summary, the selectivity of the nitration of nitrobenzene at room temperature can be attributed to the progressive decrease in reactivity of the benzene ring as nitro groups are added. The addition of two nitro groups stabilizes the ring and reduces its reactivity toward further nitration, limiting the formation of 1,3,5-trinitrobenzene.