Which one of the following compounds undergoes predominantly \(S_N2\) reaction with aqueous \(NaOH\) in polar aprotic solvent?

1-(chloromethyl)-4-nitrobenzene

The correct answer is option 4. 1-(chloromethyl)-4-nitrobenzene.

The structures of the compounds given in question are :

Let us break down why 1-(Chloromethyl)-4-nitrobenzene undergoes predominantly \(S_N2\) reactions, while also explaining why the other options are less favorable for \(S_N2\) reactions.

In an \(S_N2\) reaction, a nucleophile attacks an electrophilic carbon atom that is bonded to a leaving group, resulting in a single-step, concerted mechanism where the bond to the leaving group breaks as the bond to the nucleophile forms. Key factors affecting \(S_N2\) reactivity include:

Steric Hindrance: Less sterically hindered substrates react more favorably in \(S_N2\) reactions. This means that the nucleophile can approach and attack the carbon more easily if there are fewer bulky groups around the carbon.

Leaving Group: A good leaving group is crucial. Chlorine (Cl) is a good leaving group, making it favorable for \(S_N2\) reactions.

Electronic Effects: Electron-withdrawing or electron-donating groups attached to the aromatic ring can influence the nucleophilic attack by stabilizing or destabilizing the transition state. Electron-withdrawing groups generally make the carbon more electrophilic and more susceptible to nucleophilic attack.

Analysis of Each Compound

1. 1-(Chloromethyl)-4-methoxybenzene

Contains a methoxy group (\(-OCH_3\)) attached to the benzene ring.

Effect of Methoxy Group: The methoxy group is an electron-donating group through its +M (mesomeric) effect. This increases the electron density on the benzene ring, including on the carbon attached to the chlorine, making the carbon less electrophilic and thus less prone to nucleophilic attack. The methoxy group also introduces some steric effects.

Conclusion: While the chloromethyl group is a good leaving group, the electron-donating methoxy group reduces the reactivity of the chloromethyl carbon toward \(S_N2\) reactions.

2. 4-(Chloromethyl)benzenamine

Contains an amine group (\(-NH_2\)) attached to the benzene ring.

Effect of Amino Group: The amine group is an electron-donating group. It increases the electron density on the ring and the chloromethyl carbon, making the carbon less electrophilic and less susceptible to nucleophilic attack. Additionally, the amino group introduces steric effects that can further hinder the \(S_N2\) process.

Conclusion: The electron-donating effect and steric hindrance from the amino group make this compound less favorable for \(S_N2\) reactions.

3. 1-(Chloromethyl)benzene

Contains only the chloromethyl group attached to the benzene ring with no additional substituents.

Effect: This compound has no additional substituents affecting the electron density around the chloromethyl group. The carbon attached to chlorine is relatively unhindered and electrophilic enough for nucleophilic attack.

Conclusion: It is a good candidate for \(S_N2\) reactions because the chloromethyl group is a good leaving group and there are no additional groups to significantly alter the reactivity.

4. 1-(Chloromethyl)-4-nitrobenzene

Contains a nitro group (\(-NO_2\)) attached to the benzene ring.

Effect of Nitro Group: The nitro group is a strong electron-withdrawing group through its -M (mesomeric) and -I (inductive) effects. This reduces the electron density around the carbon attached to the chlorine, making it more electrophilic and more susceptible to nucleophilic attack. The nitro group stabilizes the transition state of the \(S_N2\) reaction, enhancing the rate of nucleophilic attack.

Conclusion: The strong electron-withdrawing effect of the nitro group makes the chloromethyl carbon more electrophilic and thus more reactive in \(S_N2\) reactions compared to the other compounds.

Summary

1-(Chloromethyl)-4-nitrobenzene is the compound that will undergo \(S_N2\) reactions most readily because the nitro group significantly increases the electrophilicity of the carbon attached to the chlorine, making it more susceptible to nucleophilic attack. The other compounds either have electron-donating groups that decrease electrophilicity or additional steric hindrance that impedes the \(S_N2\) process.

Thus, the correct answer is: 4. 1-(Chloromethyl)-4-nitrobenzene.