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Experiment 1 - Experiment II Electrophilic Aromatic...

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Experiment II : Electrophilic Aromatic Substitution; Synthesis of 4-bromo-2-nitroaniline Synthesis Type : Multiple-step; Linear Synthetic Transformations Involved : Addition-Elimination (substitution: amide bond formation); Electrophilic Aromatic Substitution; Hydrolysis Laboratory Techniques Involved : Filtration (Chapter 3, p.25-6), Recrystallization (Chapter 3), TLC (Chapter 8), Melting Point Determination (Chapter 2), IR Spectroscopy (Chapter 11), NMR Spectroscopy (Chapter 13) 1) Introduction The target molecule ( II-05 ) can easily be obtained from inexpensive reagents and starting materials. The planning of the synthesis toward II-05 , using electrophilic aromatic substitution reactions, requires some knowledge about activating/deactivating groups, ortho-, meta-, para-directing groups, and the chemistry of functional groups attached to aromatic rings in general. NH 2 Br NO 2 II-05 This experiment employs one protection step, one deprotection step and two different electrophilic aromatic substitution reactions. It is sometimes necessary to protect certain functional groups, particularly if they would undergo side reactions with a particular reagent. In this case, unprotected aromatic amines would yield undesired products if treated with bromine or nitric acid. 2) Synthetic Strategy Amines are strongly activating groups in electrophilic aromatic substitution reactions. As aromatic amines generally are quite reactive functional groups, it is necessary to protect the aniline as the corresponding amide ( II-02 ). The advantages of having an acetyl group attached to the amine are fourfold. 1) The acetamide group is less reactive in electrophilic aromatic substitutions, 2) The protected group is more bulky, 3) The acetamide is still an activating/ortho- para directing group and 4) the protecting group is very easy to remove in high yield.
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Nitro groups on aromatic rings are in most cases introduced late in synthetic sequences.
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