When X is an electron donating substituent, stabilization of the positive charge results.  How does the positive charge affect the compound's stability??? Why is meta position the most stable??? Why is it least stable when positive charge on the carbon  that bears the substituent X????

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When X is an electron donating substituent, stabilization of the positive charge results.  How does the positive charge affect the compound's stability??? Why is meta position the most stable??? Why is it least stable when positive charge on the carbon  that bears the substituent X????

The overall directing and rate effects of a substituent can be classified into three groups: ortho-para-directing activators; ortho-para-directing deactivators; and meta-directing deactivators. Any substituent that activates the aromatic ring is an ortho-para-director. Figure 3 shows the resonance forms of the arenium ion associated with a monosubstituted aromatic system. Electrophilic attack at either the ortho- or para-position places a positive charge on the carbon that bears the substituent X, indicated by resonance forms C and E, respectively. When X is an electron-donating substituent, stabilization of the positive charge results. This stabilization is not possible when attack occurs at the meta-position.

**Diagram Explanation:**

The diagram illustrates electrophilic substitution reactions on an aromatic ring with a substituent X. It shows three types of attacks: ortho, para, and meta.

- **Ortho Attack:** The electrophile (E⁺) attacks the position adjacent to the substituent X. This leads to the formation of resonance structures labeled A, B, and C. In these structures, the positive charge can be stabilized by resonance with the electron-donating group X.

- **Para Attack:** The electrophile attacks the position opposite the substituent X, resulting in resonance structures labeled D, E, and F. Here, the positive charge is also stabilized by resonance interaction with X.

- **Meta Attack:** In this scenario, the electrophile attacks a position one carbon away from X. The structures labeled G, H, and I depict this reaction, where the positive charge is not stabilized by X due to the lack of direct resonance interaction.

The stabilization of the positive charge is optimal in ortho and para attacks when X is electron-donating, making these positions more favorable for substitution.
Transcribed Image Text:The overall directing and rate effects of a substituent can be classified into three groups: ortho-para-directing activators; ortho-para-directing deactivators; and meta-directing deactivators. Any substituent that activates the aromatic ring is an ortho-para-director. Figure 3 shows the resonance forms of the arenium ion associated with a monosubstituted aromatic system. Electrophilic attack at either the ortho- or para-position places a positive charge on the carbon that bears the substituent X, indicated by resonance forms C and E, respectively. When X is an electron-donating substituent, stabilization of the positive charge results. This stabilization is not possible when attack occurs at the meta-position. **Diagram Explanation:** The diagram illustrates electrophilic substitution reactions on an aromatic ring with a substituent X. It shows three types of attacks: ortho, para, and meta. - **Ortho Attack:** The electrophile (E⁺) attacks the position adjacent to the substituent X. This leads to the formation of resonance structures labeled A, B, and C. In these structures, the positive charge can be stabilized by resonance with the electron-donating group X. - **Para Attack:** The electrophile attacks the position opposite the substituent X, resulting in resonance structures labeled D, E, and F. Here, the positive charge is also stabilized by resonance interaction with X. - **Meta Attack:** In this scenario, the electrophile attacks a position one carbon away from X. The structures labeled G, H, and I depict this reaction, where the positive charge is not stabilized by X due to the lack of direct resonance interaction. The stabilization of the positive charge is optimal in ortho and para attacks when X is electron-donating, making these positions more favorable for substitution.
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