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

Welcome to this fascinating exploration of Electrophilic Aromatic Substitution ($Ar{S}_{E}2$)! This is a cornerstone concept in organic chemistry, and mastering it will unlock your ability to predict the outcomes of countless reactions.

Imagine the benzene ring as a rich reservoir of $\pi$-electrons. In an electrophilic aromatic substitution reaction, this electron-rich ring acts as a nucleophile. It actively seeks out and attacks an electron-deficient species, known as an electrophile ($E^{+}$).

The fundamental rule here is simple: the higher the electron density on the benzene ring, the faster it will react. Groups attached to the ring can either pump electrons in (activating the ring) or pull electrons out (deactivating the ring).

Let's break down the three compounds given in our problem to see how their substituents influence the ring's reactivity.

1. Toluene (Compound II): The Activator Look at the methyl group ($-C{H}_3$) in toluene. Alkyl groups are classic Electron Donating Groups (EDG). They push electron density into the ring through two mechanisms: hyperconjugation ($+HPC$) and the inductive effect ($+I$). Because it enriches the ring with electrons, toluene is highly activated and will react the fastest among our choices.

2. Acetophenone (Compound III): The Strong Deactivator Now, consider acetophenone. The acetyl group ($-COC{H}_3$) features a carbonyl carbon directly bonded to the benzene ring. The highly electronegative oxygen atom pulls electron density away from the ring through a powerful resonance effect ($-R$) and an inductive effect ($-I$). This severe electron withdrawal makes the ring highly electron-deficient, rendering it the least reactive.

3. Chlorobenzene (Compound I): The Halogen Anomaly Halogens like chlorine present a unique and often tricky scenario. Because chlorine has lone pairs of electrons, it can donate them to the ring via resonance ($+R$). However, chlorine is also highly electronegative, meaning it pulls electrons away through the inductive effect ($-I$).

Here is the crucial catch: for halogens, the $-I$ effect dominates over the $+R$ effect. Therefore, chlorine is a net electron withdrawer. It deactivates the ring, but because of the opposing $+R$ effect, it is not as strongly deactivating as the $-R$ effect seen in the acetyl group.

Putting it all together, we can establish the increasing order of reactivity based on the net electronic effects:

Strong Deactivator ($-R,-I$) $<$ Weak Deactivator ($-I>+R$) $<$ Activator ($+HPC,+I$)

This translates to: Acetophenone (III) $<$ Chlorobenzene (I) $<$ Toluene (II)

Therefore, the correct option is (a) III $<$ I $<$ II.

Always remember to carefully evaluate the balance of inductive and resonance effects when predicting the reactivity of substituted benzenes!