Chapter 20 (Lecture #23)

Chapter 20 (Lecture #23) - Synthesis of Amines Nucleophilic...

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Synthesis of Amines Nucleophilic Substitution Reactions: Direct Alkylation Method The nucleophilic substitution of alkyl halides with ammonia is a general synthesis of primary amines. The reaction may be carried out in aqueous or alcoholic (to improve the solubility of RX) solutions of ammonia. All the usual structural limitations of an S N 2 reaction apply. CH 3 CH 2 CH 2 CH 2 Br NH 3 CH 3 CH 2 CH 2 CH 2 NH 3 Br - + butyl ammonium bromide CH 3 CBr CH 3 CH 3 NH 3 while CH 3 C=CH 2 CH 3 isobutene + NH 4 Br - + NH 3 : + R-X RNH 3 + X - aminium salt base (-HX) RNH 2 : primary amine
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Side Products in the Direct Alkylation Synthesis of Primary Amines RX NH 3 RNH 3 X - + 1 o -aminium salt NH 3 (-HX) RNH 2 : primary amine RX R 2 NH 2 X - + 2 o -aminium salt NH 3 (-HX) R 2 NH secondary amine RX NH 3 (-HX) R 3 N tertiary amine A serious problem in synthesizing primary amines by the direct alkylation reaction is the formation of secondary and tertiary amines, as shown in the scheme below. While the direct alkylation synthesis of primary amines is simple, it provides products contaminated by small amounts of secondary and tertiary amines. When primary amines are needed that are free of such contaminants, the Gabriel synthesis is used. R 3 NH X - + 3 o -aminium salt
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This synthesis utilizes the anion of phthalimide as a nucleophile in the reaction with alkyl halides. Alkaline hydrolysis of the N-alkylated phthalimide yields the primary amine free of secondary and tertiary amines.
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