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SolutionsPreparation

1. Choose the method

Propose a method to prepare each amine from the compound given:

  1. Benzylamine from benzyl chloride
  2. Cyclohexylamine from cyclohexanone
  3. Pentylamine from hexanamide
  4. Propylamine from 1-nitropropane
  1. Gabriel synthesis (potassium phthalimide and then hydrazine), or NaN3 followed by LiAlH4.
  2. Reductive amination: NH3 and H2/Ni (or NaBH3CN).
  3. Hofmann rearrangement (Br2, NaOH): one carbon is lost.
  4. Reduction of the nitro group (H2/catalyst, or Zn/HCl).

2. One more or one less?

Starting from 1-bromobutane, how would you prepare butylamine and pentylamine?

Butylamine (same number of carbons): Gabriel synthesis, or sodium azide and reduction. Pentylamine (one more carbon): NaCN gives pentanenitrile, which LiAlH4 reduces to pentylamine.

Conversely, the Hofmann rearrangement of an amide shortens the chain by one carbon.

3. Why not with ammonia?

Why is the reaction of 1-bromobutane with ammonia not a good way to prepare butylamine?

Because the butylamine formed is more nucleophilic than ammonia and competes with it for the halide: a mixture of primary, secondary and tertiary amines and quaternary ammonium salt is obtained. Not even a large excess of ammonia completely prevents polyalkylation.

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