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

(A) SN2 Reaction:

In SN2 reactions: 

• The nucleophile attacks the substrate from the backside.
• A transition state forms where the nucleophile and leaving group are partially bonded to the carbon atom.
• Steric hindrance plays a crucial role. The reactivity order is: Primary (1°) > Secondary (2°) > Tertiary (3°)
• The less substituted carbon is more accessible to the nucleophile, making the given order correct.

(B) SN1 Reaction:

In SN1 reactions:

• The reaction proceeds through a carbocation intermediate.
• The rate-determining step is the formation of the carbocation.
• Benzylic carbocations are highly stable due to resonance with the aromatic ring.
• The given order is correct as the benzylic carbocation is more stable.

(C) Electrophilic Substitution:

In electrophilic aromatic substitution:

• Electron-donating groups (EDGs) activate the aromatic ring by increasing electron density, making it more reactive to electrophiles.
• Electron-withdrawing groups (EWGs) deactivate the ring by reducing electron density.
• Chlorine (Cl) is weakly deactivating, while nitro (NO₂) is strongly deactivating.
• Hence, the given order is correct.

(D) Nucleophilic Substitution:

In nucleophilic aromatic substitution:

• EWGs at ortho and para positions to the leaving group stabilize the intermediate by withdrawing electron density.
• The nitro group (NO₂) is strongly electron-withdrawing compared to bromine (Br), enhancing reactivity for nucleophilic substitution.
• The given order is correct.

Conclusion:

(A), (B), (C), and (D) are all correct.