MCQ Detailed View

When a substituent such as chlorine (Cl) is attached to a benzene ring, it influences the electron density of the ring through two main electronic effects: inductive effect (I effect) and resonance effect (R effect).

Chlorine is a highly electronegative atom. Due to its electronegativity, chlorine pulls electron density through the sigma bond away from the benzene ring. This withdrawal of electron density is called the –I effect (negative inductive effect). As a result, the electron density on the benzene ring decreases, making the ring slightly deactivated toward electrophilic substitution reactions.

At the same time, chlorine has lone pairs of electrons on its outer p-orbitals. These lone pairs can overlap with the π system of the benzene ring. This overlap donates electron density back into the ring, specifically at the ortho and para positions. This is known as the +R effect (positive resonance effect). Thus, chlorine acts as an electron-donating group through resonance.

The combined effect of chlorine is unique. The –I effect tends to withdraw electron density from the entire ring, making benzene less reactive overall. However, the +R effect increases electron density at the ortho and para positions, making substitution more likely at these sites compared to the meta position. This explains why chlorobenzene undergoes electrophilic substitution reactions mainly at the ortho and para positions, even though chlorine is an overall deactivating group.

Examples of such reactions include the formation of ortho- and para-chloronitrobenzene when chlorobenzene is nitrated.

In summary, chlorine attached to a benzene ring shows a –I effect due to electronegativity and a +R effect due to lone pair resonance. This combination explains its directing influence and reduced reactivity compared to unsubstituted benzene.