It is possible to synthesize 1-bromo-4-nitrobenzene by reacting ...

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It is possible to synthesize 1-bromo-4-nitrobenzene by reacting bromobenzene with a mixture of nitric and sulfuric acids. However, 1-bromo-4-nitrobenzene is not obtained if nitrobenzene is reacted with Br2/FeBr3 . Explain the difference.

Question 2

2,4,6-trinitrophenol is a stronger acid than phosphoric acid. Another name for this compound is picric acid. Explain the acidity of picric acid.

[Li Jun]

Answer to Question 1

Bromine is an ortho-para director. The para isomer preferentially forms on nitration due to the steric hindrance of the bulky Br group. The result is a good yield of 1-bromo-4-nitrobenzene. NO2 is a meta director and preferentially forms the meta isomer on bromination. The resulting product is the meta isomer or 1-bromo-3-nitrobenzene.

Answer to Question 2

Picric acid's acidity is partly due to the inductive effect of the three electron-withdrawing nitro groups. In addition, substitution of a nitro group at the ortho and para positions increases resonance stabilization because the negative charge of the phenoxide ion can be delocalized onto an oxygen of the nitro group. These two effects of the three nitro groups increase the acidity of picric acid substantially.

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Gracias!

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