Amines (1): Preparation

Amines

AQA

Amines are compounds based on ammonia where alkyl or aryl groups replace the hydrogen atoms. This section includes their reactions as nucleophiles.

Preparation of Amines

Primary aliphatic amines can be prepared by the reaction of ammonia with halogenoalkanes and by the reduction of nitriles.

Aromatic amines, prepared by the reduction of nitro compounds, are used in the manufacture of dyes.

OCR:6.2.1b

(b) the preparation of:

.    (i)  aliphatic amines by substitution of haloalkanes with excess ethanolic ammonia and amines 


.    (ii)  aromatic amines by reduction of nitroarenes using tin and concentrated hydrochloric acid. 


Including formation of primary amines from ammonia and secondary/tertiary amines from amines.
See also reduction of nitriles (see 6.2.4 c).

Edexcel: Topic 18B 11 and 12

11. To understand, in terms of reagents and general reaction conditions, the preparation of primary aliphatic amines:

.    i  from halogenoalkanes 


.    ii  by the reduction of nitriles 


12. To know that aromatic nitro-compounds can be reduced, using tin and concentrated hydrochloric acid, to form amines

Preparation of aliphatic amines

1) Reaction of ammonia with a halogenoalkane (haloalkane)

Treat a primary haloalkane with ethanolic ammonia and “initially the conjugate acid of the corresponding primary amine is formed” to quote ROC Norman:

H₃N:↷CH₃—I → H₃N⁺—CH₃    +    I^—

The conjugate acid can then react with further ammonia to form the primary amine and the ammonium ion.

CH₃—NH₃⁺    +    NH₃   ⇋   CH₃—NH₂   +   NH₄⁺

However the primary amine that has just been formed will then react with a second molecule of the halogenoalkane and this reaction will lead to the formation of the secondary amine

CH₃—NH₂      +     CH₃—I    →     (CH₃)₂⁺NH₂   +  I^—

The secondary amine can them react with further excess haloalkane and produce the tertiary amine.

Usually a mixture of products finally forms so that we can say the alkylation of ammonia is a less than satisfactory method of the formation of amines.

A little modification of the method means that it can be used to form the primary amine.  A good yield of primary amine is obtained if a considerable excess of ammonia is used since to quote ROC Norman again “the amine formed is a relatively ineffective competitor for reaction with the halide”.

2) Reduction of the corresponding nitrile

Nitriles can be reduced catalytically using finely divided nickel and hydrogen under pressure at about 150^oC.

To prevent the formation of secondary amine, ammonia is added to the nitrile flow.

Benzyl cyanide (phenyl methyl nitrile) gives phenyl ethyl amine at 85% yield in liquid ammonia under pressure.

                           Ni/H₂/NH₃

PhCH₂CN            -------→                   PhCH₂CH₂NH₂

                           120^oC  120atm

This is reduction reaction the addition of hydrogen to the nitrile across its triple bond.

Preparation of aromatic amines

Aromatic amines can be prepared by the reduction of aromatic nitro compounds like nitrobenzene (C₆H₅NO₂) which forms phenylamine (or aniline) C₆H₅NH₂

This is one long preparation and complex involving steam distillation and purification of the organic product. 

The exam specs just expect you to know of the reaction and to be able to articulate the equation for the prep.

Next up we look at the properties of amines:..