Preparation of amine
Amines can be prepared in two ways: by nucleophilic substitution of a haloalkane and by reduction from nitriles. The preparation from a haloalkane results in a mixture of primary, secondary and tertiary amines and their salts. The preparation from a nitrile results in the formation of primary amines only.
Preparation of amines from haloalkanes
Haloalkanes react with nucleophiles. If ammonia is used as the nucleophile then amines are formed. It is possible to make primary, secondary and tertiary amines and also quaternary ammonium salts by this method, depending on the relative quantities of ammonia and the haloalkane used. It is therefore not considered an effective method for the preparation of primary amines.
The mechanism for the reaction is nucleophilic substitution and it proceeds as follows:
The second step will vary depending on which of the reactants is present in excess.
If the reaction takes place in excess ammonia, the H will be removed by another ammonia molecule:
A primary amine is formed and ammonium chloride is the other product.
So primary amines are formed by the addition of excess ammonia to a haloalkane:
R-X + 2NH3 --> R-NH2 + NH4+X-
Primary amines will also be formed if haloalkanes and ammonia are mixed in a 1:1 ratio. In this case HCl is likely to be the second product as the Cl- will pull off the extra H:
R-X + NH3 ---> R-NH2 + HX
If the reaction takes place in excess haloalkane, the primary amine will attack the remaining haloalkane to form a secondary amine:
So secondary amines are formed by reacting haloalkanes and ammonia in a 2:1 ratio or by reacting a haloalkane with a primary amine in a 1:1 ratio:
2R-X + NH3 ---> R-NH-R + 2HX
R1-X + R2-NH2 ----> R1-NH-R2 + HX
If any haloalkane is present after the secondary amine has been formed, it can be attacked by the secondary amine to form a tertiary amine:
So tertiary amines are formed by reacting haloalkanes and ammonia in a 3:1 ratio or by reacting a haloalkane with a secondary amine in a 1:1 ratio:
3R-X + NH3 ---> R3N + 3HX
R1-X + R2-NH-R3 ----> R1R2R3N+ HX
Quaternary ammonium salts
If there is any haloalkane left after the tertiary amine has been formed, it will be attacked by the tertiary amine to form a quaternary ammonium salt:
So quaternary ammonium salts can be made by reacting haloalkanes and ammonia in a 4:1 ratio or by reacting tertiary amines and haloalkanes in a 1:1 ratio.
4R-X + NH3 ---> [R4N]+X- + 3HX
R1-X + R2R3R4N ---> [R1R2R3R4N]+X-
Preparation of primary amines from nitriles
Primary amines can be made by treating nitriles with a strong reducing agent such as LiAlH4:
R-CN + 4[H] ---> R-CH2NH2
Eg. CH3CH2-CN + 4[H] ---> CH3CH2-CH2NH2
Secondary and tertiary amines cannot be prepared in this way; the primary amine is the only product. As a result this preparation produces a very good yield of the primary amine.
Drafted by Eunice (Chemistry)