Q.Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E +

Concept: Electrophilic aromatic substitution reactivity — the rate depends on the electron-donating or -withdrawing nature of the substituents on the ring.

Reasoning:

1. Electron-donating groups (EDG) activate the ring toward electrophiles; electron-withdrawing groups (EWG) deactivate it. The more deactivated the ring, the slower the reaction.
2. For (a): Chlorobenzene has a weakly deactivating Cl (inductive withdrawal, but resonance donation). p-Nitrochlorobenzene has a strong EWG (NO₂) that deactivates further. 2,4-Dinitrochlorobenzene has two NO₂ groups, making it the least reactive. …

Why This Approach Works

Electrophilic aromatic substitution (EAS) is all about the electron density on the benzene ring. An electrophile (E⁺) is attracted to regions of high electron density. So, the more electron-rich the ring, the faster it reacts. Substituents already on the ring either push electrons toward the ring (activating, ortho/para-directing) or pull electrons away (deactivating, meta-directing). The key is to compare the net electronic effect of all substituents present.

For part (a), all compounds have a chlorine atom. Chlorine is a peculiar case: it withdraws electrons inductively (through the sigma bond) but donates electrons via resonance (through its lone pairs). The resonance effect usually wins, making chlorobenzene moderately activating. But when strong electron-withdrawing groups like nitro (-NO₂) are added, they dominate and deactivate the ring heavily.

For part (b), we compare toluene (methyl group is strongly activating) with two nitro-substituted toluenes. The methyl group pushes electrons, while the nitro group pulls them. The balance determines the overall reactivity.

Step-by-Step Solution

Part (a): Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene

1. Identify the substituents and their effects.

• Chlorobenzene: Only one substituent — Cl. It is an ortho/para director and a weak activator (due to resonance donation). The ring is moderately electron-rich.
• p-Nitrochlorobenzene: Two substituents — Cl (weak activator) and -NO₂ at the para position. The nitro group is a strong electron-withdrawing group (EWG) via both inductive and resonance effects. It deactivates the ring significantly. The chlorine’s activating effect is overwhelmed.
• 2,4-Dinitrochlorobenzene: Three substituents — Cl and two nitro groups at positions 2 and 4. Two strong EWGs pull electron density away from the ring even more aggressively. This compound is extremely deactivated.

2. Compare the electron density on the ring.

The more electron-withdrawing groups present, the lower the electron density, and the slower the reaction with an electrophile. So:

• Chlorobenzene has the highest electron density.
• p-Nitrochlorobenzene has lower density due to one nitro group.
• 2,4-Dinitrochlorobenzene has the lowest density due to two nitro groups.

3. Arrange in decreasing reactivity.

Thus, the order is: Chlorobenzene > p-nitrochlorobenzene > 2,4-dinitrochlorobenzene.

Part (b): Toluene, p-H₃C–C₆H₄–NO₂, p-O₂N–C₆H₄–NO₂

1. Identify the substituents. …