Question

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)

Accepted Answer

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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Key Concepts

Biphenyl Structure

Electrophilic Aromatic Substitution (EAS)

Resonance and Directing Effects