Question

Identify the correct order of reactivity for the following pairs towards the respective mechanism




Choose the correct answer from the options given below :

• A. (B), (C) and (D) only
• B. (A), (B) and (D) only
• C. (A), (C) and (D) only
• D. (A), (B), (C) and (D)

Hint:

For SN₂, steric hindrance matters: \(1^◦ > 2^◦ > 3^◦\). For SN₁, carbocation stability is key. EDGs activate aromatic rings for electrophilic substitution, while EWGs activate for nucleophilic substitution.

Answer 1

The Correct Option is D

The given question asks to identify the correct order of reactivity for different chemical pairs in their respective mechanisms. Let's examine each one step-by-step:

1. Option (A): S_N2 Mechanism

   

   In an S_N2 reaction, the substrate reacts with a nucleophile in a single step where steric hindrance plays a significant role. The reactivity decreases with an increase in branching at the carbon atom attached to the leaving group. Thus, 1-bromo-2-methylpropane (less hindered) is more reactive than 2-bromo-2-methylpropane (more hindered). Hence, this order is correct.

2. Option (B): S_N1 Mechanism

   

   In an S_N1 reaction, the substrate forms a carbocation intermediate. The reactivity increases with the stability of the carbocation formed. Benzyl carbocations are stabilized by resonance. Thus, 2-bromo-2-methyl-2-phenylpropane generates a more stable carbocation than 1-bromo-2-phenylethane, making the first compound more reactive. Therefore, this order is correct.

3. Option (C): Electrophilic Substitution

   

   Electrophilic substitution reactions on benzene rings are influenced by substituents. Electron-withdrawing groups (EWG) like nitro groups deactivate the ring towards electrophilic attacks, whereas electron-donating groups (EDG) like halogens can slightly activate or direct the electrophile to ortho/para positions. Therefore, chlorobenzene is more reactive than nitro-chlorobenzene. This order is correct.

4. Option (D): Nucleophilic Substitution

   

   In nucleophilic aromatic substitution, EWG like nitro groups increase the reactivity by making the ring more susceptible to attack by nucleophiles due to their electron-withdrawing nature. Thus, 2,4-dinitrobromobenzene is more reactive than bromobenzene. This order is correct.

Conclusion: Each option reflects the correct order of reactivity for the given mechanisms. Therefore, the correct answer is (A), (B), (C), and (D).