Question

The decreasing order of reactivity of the following compounds towards nucleophilic substitution (${\mathrm{S}}_{\mathrm{N}}2$) is:

• A. $\left(\mathrm{III}\right)>\left(\mathrm{II}\right)>\left(\mathrm{IV}\right)>\left(\mathrm{I}\right)$
• B. $\left(\mathrm{IV}\right)>\left(\mathrm{II}\right)>\left(\mathrm{III}\right)>\left(\mathrm{I}\right)$
• C. $\left(\mathrm{II}\right)>\left(\mathrm{III}\right)>\left(\mathrm{IV}\right)>\left(\mathrm{I}\right)$
• D. $\left(\mathrm{II}\right)>\left(\mathrm{III}\right)>\left(\mathrm{I}\right)>\left(\mathrm{IV}\right)$

Answer 1

The correct option is C

$\left(\mathrm{II}\right)>\left(\mathrm{III}\right)>\left(\mathrm{IV}\right)>\left(\mathrm{I}\right)$

The explanation for the correct option:

Option (C): $\left(\mathrm{II}\right)>\left(\mathrm{III}\right)>\left(\mathrm{IV}\right)>\left(\mathrm{I}\right)$

1. Nucleophilic substitution is greatly affected due to the presence of the -I effect, -R effect.
2. Greater -I and -R effect, the greater is reactivity towards${\mathrm{S}}_{\mathrm{N}}2$ , mechanism
3. The first compound has no -I effect hence, it is the least reactive.
4. The second compound has a nitro group attached ortho and meta position which has the most -I effect.
5. The third compound has nitro groups attached at the para and meta position which has a less -I effect because the para position is less stable than the ortho position.
6. The fourth compound has nitro groups at the meta position which will show the least -I effect.
7. Hence, it is the correct option.

The explanation for the correct options:

Option (A): $\left(\mathrm{III}\right)>\left(\mathrm{II}\right)>\left(\mathrm{IV}\right)>\left(\mathrm{I}\right)$

1. The third compound has nitro groups attached at the para and meta position which has a less -I effect because the para position is less stable than the ortho position.
2. Hence, it is an incorrect option.

Option (B): $\left(\mathrm{IV}\right)>\left(\mathrm{II}\right)>\left(\mathrm{III}\right)>\left(\mathrm{I}\right)$

1. The fourth compound has nitro groups at the meta position which will show the least -I effect.
2. Hence, it is an incorrect option.

Option (D): $\left(\mathrm{II}\right)>\left(\mathrm{III}\right)>\left(\mathrm{I}\right)>\left(\mathrm{IV}\right)$

1. The first compound has no -I effect hence, it is the least reactive
2. Hence, it is an incorrect option.

Therefore, The decreasing order of reactivity of the compounds towards nucleophilic substitution (${\mathrm{S}}_{\mathrm{N}}2$) is $\left(\mathrm{II}\right)>\left(\mathrm{III}\right)>\left(\mathrm{IV}\right)>\left(\mathrm{I}\right)$