Question

Find out the correct statement from the options given below for the above 2 reactions.

• A. Reaction (I) is of 1^st order and reaction (II) is of 2^nd order
• B. Reaction (I) and (II) both are 2^nd order
• C. Reaction (I) is of 1^st order and reaction (II) is of 1^st order
• D. Reaction (I) is of 2^nd order and reaction (II) is of 1^st order

Hint:

Remember that electron-donating groups (like -OCH3) increase the nucleophilicity of the aromatic ring, leading to a first-order mechanism, while electron-withdrawing groups (like -NO2) decrease the nucleophilicity, making the reaction mechanism slower.

Answer 1

The Correct Option is C

The problem involves analyzing the order of the two given chemical reactions. To determine the order of each reaction, we need to consider the type of nucleophilic substitution taking place in each case.

Reaction (I) involves the substrate with a methoxy group (OMe) at the para position. Reaction (II) involves the substrate with a nitro group (O₂N) at the para position.

Analysis:

1. Reaction (I):
   • The methoxy group is an electron-donating group, which stabilizes the carbocation intermediate formed during the reaction.
   • Given the presence of stabilizing groups, this reaction likely proceeds through an S_N1 mechanism, which is first order in nature.
2. Reaction (II):
   • The nitro group is an electron-withdrawing group, which suggests that this reaction proceeds through an S_N1 mechanism.
   • The electron-withdrawing nature of the nitro group facilitates leaving group departure, leading to a first-order reaction mechanism.

Therefore, both reactions are first order. The rate law for both reactions depends solely on the concentration of the substrate, not the nucleophile.

- In the reaction involving an electron-donating group like \(-OCH_3\), the electron density on the benzene ring is increased, leading to a faster nucleophilic attack. This makes the mechanism of reaction (I) 1^st order, as the rate depends on the concentration of the nucleophile only.

In the reaction involving an electron-withdrawing group like \(-NO_2\), the electron density on the benzene ring is decreased, which slows down the nucleophilic substitution and typically follows a 2^ndorder mechanism, where both the nucleophile and the substrate are involved in the rate-determining step. However, reaction (II) involves a 1^st order mechanism because the presence of \(-NO_2\) only partially influences the overall rate, which results in an overall first-order reaction. 
Thus, the correct answer is (3): Reaction (I) is of 1^st order and reaction (II) is of 1^st order.