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  Preparation of 2º & 3º - Amines Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Preparation of 2º & 3º-Amines

Of the six methods explained earlier, three are appropriate for the preparation of 2º and/or 3º-amines. These are: 
          (i) Alkylation of the sulfonamide derivative of a 1º-amine. provides 2º-amines.
          (ii) Reduction of alkyl imines and dialkyl iminium salts. provides 2º & 3º-amines.
          (iii) Reduction of amide derivatives of 1º & 2º-amines. provides 2º & 3º-amines.

instances showing the application of these methods to the preparation of purticular amines are displayed in the diagram below. The sulfonamide procedure employed in the first example is identical in concept to the phthalimide example #2 existing in the earlier diagram. In both cases the acidity of the nitrogen reactant (ammonia or amine) is greatly improved by conversion to an imide or sulfonamide derivative. Nucleophilic conjugate base of this acidic nitrogen species is then arranged by treatment with sodium or potassium hydroxid  and this goes through an SN2 reaction with a 1º or 2º-alkyl halide. At last, the activating group is removed by hydrolysis (phthalimide) or reductive cleavage (sulfonamide) to give the needed amine. The phthalimide technique is only helpful for preparing 1º-amines, whereas the sulfonamide process might be employed to make either 1º or 2º-amines.

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Illustrations #2 & #3 use the carbonyl reductive amination reaction (method #4 in the preceding table. This versatile technique may be employed to prepare all classes of amines (1º, 2º & 3º), as displayed here and above. A weak acid catalyst is essential for imine formation, which occurs by amine addition to the carbonyl group that giving a 1-aminoalcohol intermediate, followed by loss of water. Final reduction of the C=N double bond may be carried out catalytically (Pt & Pd catalysts may be employed in place of Ni) or chemically (by NaBH3CN). The enamine or imine intermediates are generally not isolated, but are instantaneously reduced to the amine product.

Other general method for preparing all classes of amines makes use of amide intermediates, simply made from amines or ammonia by reaction with carboxylic acid chlorides or anhydrides. These stable compounds might be isolated, stored and identified prior to the final reduction. illustrations #4 & #5 demonstrate applications of this technique. As with the previous method, 1º-amines give 2º-amine products, and 2º-amines provide 3º-amine products.
The last instance (#6) displays how 4º-ammonium salts may be prepared by repeated (exhaustive) alkylation of amines.

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