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Chemistry
Explain the reasons behind the activation at the ortho and para positions and deactivation at the meta position during the nitration and halogenation of biphenyl. Additionally, provide an explanation for why biphenyl is more reactive than 2 , 2 2,2^(') dimethylbiphenyl in nitration.
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The nitration and halogenation of biphenyl involve electrophilic aromatic substitution reactions, where an electrophile attacks the aromatic ring. The reactivity of the aromatic ring depends on the electron density, which is influenced by the substituents attached to it. In the case of biphenyl, the second phenyl ring acts as a substituent on the first phenyl ring. The resonance effect of the second phenyl ring increases the electron density at the ortho and para positions, making them more susceptible to electrophilic attack. However, the meta position is deactivated due to steric hindrance caused by the presence of the second phenyl ring.
Comparing the reactivity of biphenyl and 2 , 2 2,2^(') dimethylbiphenyl in nitration, the resonance effect in biphenyl is stronger than the inductive effect of the methyl groups in 2 , 2 2,2^(') dimethylbiphenyl. Additionally, the methyl groups cause steric hindrance, making the ortho and para positions less accessible to electrophiles. This results in biphenyl being more reactive than 2 , 2 2,2^(') dimethylbiphenyl in nitration.
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