Biphenyl is two benzene rings joined by a single bond. The site of substitution for a biphenyl is determined by (1) which phenyl ring is more activated (or less deactivated), and (2) which position on that ring is most reactive, using the fact that a phenyl substituent is activating and ortho, para-directing.
a. Use resonance forms of a sigma complex to show why a phenyl substituent should be ortho, para-directing.
(i)

검증된 단계별 안내

Step 1: Begin by understanding the concept of ortho, para-directing groups. A phenyl substituent is activating because it donates electron density to the benzene ring through resonance and inductive effects. This increases the reactivity of the ring, particularly at the ortho and para positions relative to the substituent.

Step 2: Draw the resonance structures for the sigma complex formed when an electrophile attacks the ortho position of a benzene ring with a phenyl substituent. Show how the positive charge is delocalized across the ring and the phenyl substituent, stabilizing the intermediate.

Step 3: Repeat the process for the para position. Draw the resonance structures for the sigma complex formed when an electrophile attacks the para position. Again, show how the positive charge is delocalized across the ring and the phenyl substituent, stabilizing the intermediate.

Step 4: Compare the resonance structures for ortho and para substitution to those for meta substitution. For meta substitution, the positive charge cannot be delocalized onto the phenyl substituent, making the intermediate less stable.

Step 5: Conclude that the phenyl substituent is ortho, para-directing because the resonance stabilization of the sigma complex is greater at these positions compared to the meta position. This explains why electrophilic substitution preferentially occurs at the ortho and para positions.

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14m

주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Biphenyl Structure

Biphenyl consists of two benzene rings connected by a single bond. This structure allows for rotation around the bond, which can influence the reactivity of the rings. Understanding the biphenyl structure is crucial for predicting how substituents will affect the overall reactivity and orientation of further reactions, particularly electrophilic aromatic substitution.

Electrophilic Aromatic Substitution (EAS)

Electrophilic aromatic substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The directing effects of substituents, such as activating or deactivating groups, determine the position of substitution. In biphenyl, the presence of a phenyl group influences the reactivity and directs new substituents to the ortho and para positions due to resonance stabilization.

Resonance and Directing Effects

Resonance in aromatic compounds allows for the delocalization of electrons, which stabilizes the sigma complex formed during EAS. A phenyl substituent is ortho, para-directing because it can stabilize the positive charge developed in the sigma complex at these positions through resonance. This concept is essential for predicting the outcome of reactions involving biphenyl and similar aromatic systems.