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

Concept: In this kind of reaction a negatively charged intermediate is formed on the ring. Electron-pulling $\mathrm{-NO_2}$ groups, when placed at the ortho or para spots, help to steady that negative charge and make the reaction go faster. So more nitro groups means a faster reaction.

Step 1: Count the nitro groups
Compound I (chlorobenzene) has 0 nitro groups, IV (4-nitro) has 1, III (2,4-dinitro) has 2, and II (2,4,6-trinitro) has 3.

Step 2: More groups means faster
Each $\mathrm{-NO_2}$ group steadies the negative intermediate a little more, so as the number of nitro groups goes up, the rate goes up too.

Step 3: Write the increasing order
From the slowest (fewest nitro groups) to the fastest (most nitro groups): \[ \text{I} < \text{IV} < \text{III} < \text{II} \] That is option (A).

Answer: Option (A), I < IV < III < II.