Amines

Preparation of Amines

Preparation of Amines

It is possible to prepare amines in two ways:

  • nucleophilic substitution of a halokane which results in primary, secondary and tertiary amines and their salts
  • reduction from nitriles which results in primary amines only

Preparation of amines from haloalkanes

Haloalkanes are able to react with nucleophiles. If the nucleophile used is ammonia then the products are amines. Using this method it is possible to produce primary, secondary and tertiary amines as well as quaternary ammonium salts. However, this depends on the relative quantities used of the haloalkane and ammonia. In this respect, it is not regarded as a good method for preparing primary amines.

The first step for the reaction proceeds as follows:

NH3 + C-Cl ? C-NH2-H + Cl

The second step depends on which reactants are present in excess.

Primary amines

In the presence of excess ammonia, another ammonia molecule will remove the H forming a primary amine and ammonia chloride.

Therefore, primary amines can be created with the addition of excess ammonia to a haloalkane:

R-X + 2NH3 ? R-NH2+NH4+X

Primary amines will also be formed if ammonia and haloalkanes are mixed with a ratio of 1:1. In this scenario the chances are that HCl will be the second product because the Cl will pull off the additional H:

R-X + NH3 ? R-NH2 + HX

Secondary amines

In the presence of excess haloalkane, a secondary amine is formed when the primary amine attacks the remaining haloalkane.

Therefore secondary alkanes can be created by reacting:

  • haloalkanes and ammonia in a ratio of 2:1:

? 2R-X + NH3 ? R-NH-R+ 2HX

  • haloalkanes with a primary amine in a ratio of 1:1:

? R1-X + R2-NH2 ? R1-NH-R2 + HX

Tertiary amines

Once formed, a secondary amine is capable of attacking any haloalkane present to create a tertiary amine.

Therefore, tertiary amines are produced by reacting:

  • haloalkanes and ammonia in a ratio of 3:1:

? 3R-X + NH3 ? R3N+ 3HX

  • haloalkanes with a secondary amine in a ratio of 1:1:

? R1-X + R2-NH-R3 ? R1R2R3N+ HX

Quaternary ammonium salts

Any haloalkane remaining once the tertiary amine is formed will be attacked by this amine to produce a quaternary ammonium salt.

Therefore, quaternary ammonium salts can be produced by reacting:

  • haloalkanes and ammonia in a ratio of 4:1:

? 4R-X + NH3 ? [R4N]+X + 3HX

  • tertiary amines and haloalkanes in a ratio of 1:1:

? R1-X + R2R3R4N ? [R1R2R3R4N]+X

Preparation of primary amines from nitriles

It is possible to create primary amines through the treatment of nitriles with a strong reducing agent like LiAlH4:

R-CN + 4[H] ? R-CH2NH2

It is not possible to prepare either secondary or tertiary amines like this: the only product is the primary amine. This makes this method efficient in creating good primary amine yields.