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Unformatted text preview: Electrophilic Aromatic Substitution The Nitration of Methyl Benzoate Aromatic compounds react with electrophiles to give products resulting in substitution of a hydrogen atom by the electrophile. This is in contrast to the reaction of alkenes with electrophiles, which typically give products of addition, rather than substitution. Br 2 FeBr 3 Br + HBr Br 2 Br Br The reaction of aromatic systems with electrophiles results in substitution because abstraction of an H + from the intermediate resonance stabilized carbocation restores aromaticity to the product. This aromaticity imparts ~ 36 kcal/mole stabilization energy compared to non-aromatic systems. If the aromatic ring is already substituted prior to the reaction with an electrophile, the nature of the substituent affects not only the rate of reaction, but the location of the attacking electrophile. In this weeks laboratory exercise, the aromatic ester, methyl benzoate, will be nitrated with nitric acid in the presence of concentrated sulfuric acid. The sulfuric acid is necessary because nitric acid alone is not a strong enough electrophile to react with most aromatic rings. The sulfuric acid protonates the nitric acid, and the resulting cation dehydrates, forming the nitronium ion which is the active electrophile in this reaction. HNO 3 + H 2 SO 4 N + O- OH 2 + O N + O O + O H 2 + HSO 4- C + Br H H path A path B Br- A Br- B Br Br Br aromatic- lower energy non-aromatic-higher energy (does not occur) & The nitronium ion reacts with methyl benzoate to produce the nitrated methyl benzoate as...
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