Animated Solution for Chemistry - Organic Chemistry: Presence of a nitro group in a benzene ring

The benzene ring is a beautiful, symmetric cloud of $\pi$-electrons. However, attached to it is the nitro group ($-{\text{NO}}_2$), which is a chemical heavyweight.

The nitrogen atom is bonded to two highly electronegative oxygen atoms. This makes the nitrogen atom extremely electron-hungry, carrying a formal positive charge. As a result, the $-{\text{NO}}_2$ group exerts a powerful electron-withdrawing effect on the benzene ring.

First is the Inductive Effect ($-I$). Because nitrogen is more electronegative than the carbon atoms of the ring, it pulls the $\sigma$-electrons through the single bond towards itself.

Second, and more importantly, is the Mesomeric Effect ($-M$). The $\pi$-electrons of the benzene ring are drawn out of the ring and into the nitro group through resonance. If you draw the resonance structures of nitrobenzene, you will see that the double bonds shift out of the ring, placing a formal positive charge on the ortho and para positions.

Because the electron density of the ring is severely depleted, we say the ring is electron-deficient.

Now, imagine an electrophile ($E^{+}$) approaching this molecule. An electrophile is positively charged and is desperately looking for a rich source of electrons to attack. When it approaches nitrobenzene, it finds a ring that has been drained of its electrons, with partial positive charges (${\delta }^{+}$) actively repelling it at the ortho and para positions.

Therefore, the $-{\text{NO}}_2$ group makes the benzene ring much less reactive towards electrophiles compared to a plain benzene ring. We conclude that the $-{\text{NO}}_2$ group deactivates the ring towards electrophilic substitution.