Which of the following compounds undergo predominantly \(S_N2\) reaction with \(NaOH\) in a polar aprotic solvent?

1. 1-(chloromethyl)-4-methoxybenzene
2. 4-(chloromethyl)benzanamine
3. 1-(chloromethyl)benzene
4. 1-(chloromethyl)-4-nitrobenzene

Choose the most appropriate answer from the options given below:

D only

The correct answer is option 4. D only.

The \(S_N2\) reaction is a bimolecular nucleophilic substitution reaction in which the nucleophile \((Nu^-)\) attacks the electrophilic carbon atom \((C^+)\) of the substrate \((RX)\) to form a new bond. The leaving group \((X^-)\) then departs from the molecule.

In polar aprotic solvents, \(S_N2\) reactions are favored for compounds with strong nucleophiles and good leaving groups. The nucleophile \(NaOH\) is a strong nucleophile, and the leaving group \(Cl^-\) is a good leaving group. However, the other compounds listed have substituents that can interfere with the \(S_N2\) reaction.

• 1-(chloromethyl)-4-methoxybenzene: The methoxy group \((-OCH_3)\) is an electron-donating group that can make the carbon atom less electrophilic and therefore less susceptible to attack by the nucleophile.
• 4-(chloromethyl)benzanamine: The amino group \((-NH_2)\) is an electron-donating group that can also make the carbon atom less electrophilic. Additionally, the amino group can act as a base and abstract a proton from the nucleophile, making it less reactive.
• 1-(chloromethyl)benzene: Benzene is an aromatic ring that is relatively unreactive. The SN2 reaction is not favored for aromatic compounds.
• 1-(chloromethyl)-4-nitrobenzene: The nitro group \((-NO_2)\) is an electron-withdrawing group that can make the carbon atom more electrophilic. However, the nitro group is also a meta directing group, which means that it directs the nucleophile to attack the meta position rather than the ortho or para position. This can lead to the formation of unwanted side products.

Therefore, the only compound that is likely to undergo predominantly \(S_N2\) reaction with \(NaOH\) in a polar aprotic solvent is 1-(chloromethyl)-4-nitrobenzene.

Here is a summary of the reactivity of the four compounds:

 OR