Animated Solution for Chemistry - Organic Chemistry: The increasing order of reactivity of the following compounds towards aromatic electrophilic substitution reaction is

Imagine an electrophile as an incredibly "electron-hungry" species. It is desperately searching for a rich source of electrons to satisfy its deficiency. When it approaches a benzene ring, its decision to attack—and how fast it attacks—depends entirely on how "wealthy" that ring is in terms of electron density.

This brings us to the core principle of Electrophilic Aromatic Substitution (EAS): Reactivity is directly proportional to the electron density on the benzene ring.

Groups attached to the benzene ring act as either generous donors or greedy thieves. - Activating Groups (Electron Donating Groups - EDG): These groups pump electrons into the ring, making it highly attractive to electrophiles. - Deactivating Groups (Electron Withdrawing Groups - EWG): These groups suck electrons out of the ring, leaving it impoverished and unappealing to electrophiles.

Inductive vs. Mesomeric Effects

To determine whether a group is a donor or a thief, we must analyze the tug-of-war between two primary electronic effects: 1. Inductive Effect ($I$): Operates through sigma bonds based on electronegativity differences. 2. Mesomeric/Resonance Effect ($M$): Operates through pi bonds and lone pairs, allowing for the actual delocalization of electrons.

Let's analyze our four contenders to see who wins this tug-of-war.

1. Benzonitrile (Compound D): The Ultimate Thief The cyano group ($-CN$) is a ruthless electron-withdrawing group. Nitrogen is highly electronegative, pulling electrons via the $-I$ effect. Furthermore, the pi bond between carbon and nitrogen allows it to withdraw pi electrons from the ring via resonance ($-M$ effect). Because both effects work in the same direction to drain the ring, benzonitrile is strongly deactivated.

2. Chlorobenzene (Compound A): The Deceptive Halogen Halogens are tricky. Chlorine has lone pairs, which means it can donate electrons via resonance ($+M$). However, chlorine is also highly electronegative, pulling electrons away through the sigma bond ($-I$). For halogens, the inductive pull is stronger than the resonance donation ($-I>+M$). Therefore, chlorobenzene is a net loser of electrons and is weakly deactivated.

3. Toluene (Compound C): The Subtle Donor The methyl group ($-C{H}_3$) doesn't have any lone pairs to share. Yet, it manages to enrich the ring through two subtle mechanisms: the $+I$ inductive effect and, more importantly, hyperconjugation ($+H$). These effects push electron density into the ring, making toluene weakly activated.

4. Anisole (Compound B): The Generous Benefactor The methoxy group ($-OC{H}_3$) features an oxygen atom directly attached to the ring. While oxygen is electronegative ($-I$), its ability to donate a lone pair directly into the ring's pi system via resonance is incredibly powerful. Here, the resonance donation completely overwhelms the inductive pull ($+M>-I$). This floods the ring with electrons, making anisole strongly activated.

Now that we have profiled each compound, arranging them in increasing order of reactivity is straightforward. We start with the most impoverished ring and end with the wealthiest:

- Least Reactive: Benzonitrile (D) — Strongly Deactivated - Next: Chlorobenzene (A) — Weakly Deactivated - Next: Toluene (C) — Weakly Activated - Most Reactive: Anisole (B) — Strongly Activated

Therefore, the correct increasing order is $D<A<C<B$.