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Question
Which of the following compounds undergoes nuclephilic substitution reaction most easily?
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Solution
The electron-withdrawing or electron-donating groups influence the nucleophilic substitution reaction. The substitution reaction is enhanced by the electron-withdrawing group, whereas the nucleophilic attack is decreased by the electron-donating group.
The nitro group (−NO2) in p-nitrochlorobenzene is an electron-withdrawing group. When in the para position, the electron density is withdrawn to the nitro group, resulting in extended conjugation. The electron density on the ring is reduced as a result of this. As a result, the bond between the chlorine and the carbon atom (from the ring) weakens. When a more reactive nucleophile, such as OH−, attacks the p-nitrochlorobenzene, the nucleophilic substitution reaction is easily accomplished.
When the aromatic ring is attached to the electron-withdrawing group, electron density shifts towards the electron withdrawing group, making the aromatic ring electrophilic and hence easy to attack by nucleophiles.
When an aromatic ring is attached to an electron-donating or releasing group, electron density shifts towards the aromatic ring, making it nucleophilic and making it difficult for nucleophiles to approach it. The ring can easily undergo the electrophilic substitution reaction in such instances.
Methoxy is an electron donating group. It contributes its electron density to the aromatic ring when it is attached to it. As a result, the electron density surrounding the ring's (existing) nucleophilic groups increases. The existing nucleophile is difficult to knock off because of the high electron density. In the presence of an electron-donating group, nucleophilic substitution reactions are difficult to perform. The nucleophilic substitution process is difficult to perform on p-methylchlorobenzene.
The decreasing order of the ease towards the nucleophilic substitution reaction is as shown below:
The p-chloronitrobenzene can easily undergo the nucleophilic substitution reaction.
The nitro group (−NO2) in p-nitrochlorobenzene is an electron-withdrawing group. When in the para position, the electron density is withdrawn to the nitro group, resulting in extended conjugation. The electron density on the ring is reduced as a result of this. As a result, the bond between the chlorine and the carbon atom (from the ring) weakens. When a more reactive nucleophile, such as OH−, attacks the p-nitrochlorobenzene, the nucleophilic substitution reaction is easily accomplished.
When the aromatic ring is attached to the electron-withdrawing group, electron density shifts towards the electron withdrawing group, making the aromatic ring electrophilic and hence easy to attack by nucleophiles.
When an aromatic ring is attached to an electron-donating or releasing group, electron density shifts towards the aromatic ring, making it nucleophilic and making it difficult for nucleophiles to approach it. The ring can easily undergo the electrophilic substitution reaction in such instances.
Methoxy is an electron donating group. It contributes its electron density to the aromatic ring when it is attached to it. As a result, the electron density surrounding the ring's (existing) nucleophilic groups increases. The existing nucleophile is difficult to knock off because of the high electron density. In the presence of an electron-donating group, nucleophilic substitution reactions are difficult to perform. The nucleophilic substitution process is difficult to perform on p-methylchlorobenzene.
The decreasing order of the ease towards the nucleophilic substitution reaction is as shown below:
The p-chloronitrobenzene can easily undergo the nucleophilic substitution reaction.
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