Question

Consider the following compounds : Chlorobenzene (I), 2,4,6-trinitrochlorobenzene (II), 2,4-dinitrochlorobenzene (III), 4-nitrochlorobenzene (IV).
The correct order of increasing rate of nucleophilic substitution reactions of these compounds is :

• A. I < IV < III < II
• B. I < III < IV < II
• C. II < III < IV < I
• D. IV < III < II < I

Hint:

Each extra −NO₂ group speeds up the reaction, so order = nitro count.

Answer 1

The Correct Option is A

Step 1: Why nitro groups matter here.
Nucleophilic substitution on these rings runs through a negatively charged intermediate. Anything that helps spread out that negative charge will speed the reaction up. A nitro group is strongly electron withdrawing, so when placed at the ortho or para positions it steadies that intermediate beautifully.

Step 2: Count the nitro groups on each.
Chlorobenzene (I) has none. 4-nitrochlorobenzene (IV) has one. 2,4-dinitrochlorobenzene (III) has two. 2,4,6-trinitrochlorobenzene (II) has three.

Step 3: Link count to speed.
Each extra nitro group gives more help in stabilising the intermediate, so the rate climbs steadily as the number of nitro groups climbs. More groups means a faster reaction.

Step 4: Write the increasing order.
From the slowest (no nitro groups) to the fastest (three nitro groups):
\[ \text{I} < \text{IV} < \text{III} < \text{II} \]
\[ \boxed{\text{Option (A)}} \]