Question

Which of the following compounds has difficulty in breaking of \(\mathrm{C}-\mathrm{X}\) bond?

• A. o-Nitrochlorobenzene
• B. m-Nitrochlorobenzene
• C. p-Nitrochlorobenzene
• D. 2, 4, 6-trinitrochlorobenzene

Hint:

Nitro groups at ortho or para positions activate the ring towards nucleophilic substitution; a meta nitro group does not.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept
Nucleophilic substitution in chlorobenzene is hard because the C-Cl bond has partial double bond character. An electron-withdrawing nitro group at the ortho or para position stabilises the intermediate carbanion by resonance, which makes the C-Cl bond easier to break.

Step 2: Apply to the Compounds
The ortho, para and 2,4,6-trinitro compounds all have nitro groups at activating positions, so their C-X bond breaks more easily.
In the meta isomer the negative charge of the intermediate is not placed on the carbon carrying the nitro group, so there is no extra stabilisation.

Step 3: Compare the options
The meta isomer is the least reactive, so its C-X bond is hardest to break.

Final Answer:
m-Nitrochlorobenzene has the most difficult C-X bond cleavage. \[ \boxed{\text{(B) }\text{m-Nitrochlorobenzene}} \]