Primary, secondary or tertiary amines can be prepared by the SN2 reaction of an amine with a haloalkane. Over-alkylation is a common problem, often resulting in the corresponding ammonium salt, and can be solved in some cases by using a large excess of the amine (Section 7.3A).
Primary aryl amines are best prepared from aryl nitro compounds by reduction with hydrogen and a transition metal such as Ni or a combination of aqueous HCl and certain active metals such as Fe, Sn or Zn (Section 9.7B).
In a process called reductive amination, an aldehyde or ketone is condensed with ammonia or a primary amine to afford an imine. This imine is then reduced with molecular hydrogen an nickel to afford a primary, secondary or tertiary amine. Reductive amination avoids the over-alkylation problem that can occur with SN2 reactions of amines. This is a very general method to make a wide variety of amines (Section 11.10C).
Another very general route to amines is reduction of an amide with lithium aluminum hydride (LiAlH4). In this reaction, the two carbon-oxygen bonds of the carbonyl group are replaced by two carbon-hydrogen bonds. A wide variety of amides are readily prepared from the corresponding amine and a carboxylic acid derivative (usually the acid chloride), so a wide variety of amines can be made by this reduction process. Lithium aluminum hydride will also reduce many other functional groups simultaneously with the amide (Section 13.8B).
Amide hydrolysis affords an amine and a carboxylic acid. This reaction is rarely an important way to prepare amines, because the amides are usually derived from this amine and some carboxylic acid derivative such as an acid chloride, acid anhydride, or ester (Section 13.3D).
