Question

Which of the following statements is correct? (1) $-{\mathrm{NO}}_2$ group activates the benzene ring for attack of electrophile at ortho and para position. (2) $-{\mathrm{NH}}_2$ group activates the benzene ring for attack of electrophile at ortho and para position. (3) Both-NO ${}_2$ group as well as $-{\mathrm{NH}}_2$ group activate the benzene ring for attack of electrophile at ortho and para position. (4) Neither $-{\mathrm{NO}}_2$ group nor $-{\mathrm{NH}}_2$ group activate the benzene ring for attack of electrophile at ortho and para position.

Short answer

The correct statement is (2).

Step-by-step solution

Analyze the impact of $-{\mathrm{NO}}_2$ group

The $-{\mathrm{NO}}_2$ group is an electron-withdrawing group. It decreases the electron density on the benzene ring, thus deactivating it towards electrophilic aromatic substitution. It directs electrophiles to the meta position, not ortho or para positions.

Analyze the impact of $-{\mathrm{NH}}_2$ group

The $-{\mathrm{NH}}_2$ group is an electron-donating group. It increases the electron density on the benzene ring, making it more reactive towards electrophilic aromatic substitution. It directs electrophiles to the ortho and para positions.

Evaluate the statements

1. The first statement is incorrect because $-{\mathrm{NO}}_2$ deactivates the benzene ring and directs to meta positions.2. The second statement is correct because $-{\mathrm{NH}}_2$ activates the benzene ring and directs to ortho and para positions.3. The third statement is incorrect as only $-{\mathrm{NH}}_2$ activates the ring, not both.4. The fourth statement is incorrect as $-{\mathrm{NH}}_2$ does activate the benzene ring.

Key concepts

Electron-donating groups

Electron-donating groups (EDGs) are substituents that donate electron density towards the benzene ring. This process happens through their lone pairs or by inductive effects. When a group donates electron density, it increases the overall electron density on the benzene ring. This makes the ring more reactive towards electrophiles, which are species that seek out electrons.
For example, the amino group $-{\mathrm{NH}}_2$ is an electron-donating group. By donating electrons, $-{\mathrm{NH}}_2$ activates the benzene ring, making it more enthusiastic about reacting with electrophiles. When the benzene ring is more reactive, the electrophile attacks can happen at specific positions. For $-{\mathrm{NH}}_2$, those positions are ortho (adjacent to the substituent) and para (opposite to the substituent).
In summary, electron-donating groups:

• Increase electron density on the benzene ring.
• Activate the benzene ring for electrophilic aromatic substitution.
• Direct electrophilic attacks to the ortho and para positions.

Electron-withdrawing groups

Electron-withdrawing groups (EWGs) do the opposite of EDGs. They pull electron density away from the benzene ring. This process happens through inductive effects or resonance. When an EWG such as $-{\mathrm{NO}}_2$ withdraws electron density, it deactivates the benzene ring.
Deactivated benzene rings are less attractive to electrophiles, so reactions are slower and occur less often. Moreover, EWGs usually direct electrophilic attacks to the meta position, which is not adjacent or opposite but one position removed from the substituent.
In summary, electron-withdrawing groups:

• Decrease electron density on the benzene ring.
• Deactivate the benzene ring, making it less reactive towards electrophiles.
• Direct electrophilic attacks to the meta position.

Substituent effects on benzene

The position where an electrophile attacks the benzene ring depends heavily on the nature of the substituents already present on the ring. This phenomenon is known as substituent effects. There are two main types of substituent effects: activating and deactivating.
Activating substituents, like electron-donating groups, increase the reactivity of the benzene ring towards electrophilic aromatic substitution and direct the incoming electrophile to the ortho and para positions. On the other hand, deactivating substituents, like electron-withdrawing groups, reduce the reactivity of the benzene ring and direct electrophiles mainly to the meta position.
Understanding substituent effects is crucial because they determine where new substituents will be added during chemical reactions. This knowledge helps chemists plan and execute reactions predictably and efficiently.
In summary, substituent effects on benzene:

• Determine the position where electrophiles attack the benzene ring.
• Can either activate or deactivate the benzene ring.
• Are essential for predicting outcomes in electrophilic aromatic substitution reactions.