A2-Level Amines and Amides

  • If a halogenoalkane is reacted with an excess of ammonia, a primary amine is formed.

  • When insufficient ammonia is present to undergo substitution with all the halogenoalkane molecules, the primary amines that did form undergo further substitution and can become secondary, tertiary and quaternary amines.

  • Once four substitutions have happened, the nitrogen has four bonds and becomes positively charged. This quaternary ammonium ion is attracted to negative halide ions and a quaternary ammonium salt forms

QUICK NOTES

Producing Secondary and Tertiary Amines

 

Primary amines are produced by the nucleophilic substitution of ammonia with a halogenoalkane. During this mechanism, an excess of ammonia is used that reacts with all the halogenoalkanes and an ammonium-halide salt is formed.

 

 

 

If only a small amount of ammonia is used, not all the halogenoalkanes can undergo the nucleophilic substitution reaction with the ammonia. This means a mixture of primary amines and halogenoalkane molecules is produced.

 

 



 

The nitrogen atom in an amine group has a lone pair of electrons and can behave as a nucleophile in the same way as ammonia.

The halogenoalkanes that did not have a chance to react with the ammonia now react with the primary amines present.

 

 








 

The mechanism is exactly the same as the one that formed a primary amine from the halogenoalkane and ammonia, the only difference is an amine is substituted with a halogen rather than ammonia with a halogen.

 

 




 

The secondary amine formed is able to react with another halogenoalkane in a further nucleophilic substitution reaction.

 

This time, producing a tertiary amine.






 

 

Again, the tertiary amine formed is able to react with another halogenoalkane in a further substitution, this time however the nitrogen atom ends up with four bonds. The nitrogen gains a positive charge as a result and a ‘quaternary ammonium salt’ is formed.

 

 

No further substitutions can occur once a quaternary ammonium ion is produced, as there are no more hydrogen atoms that can be lost and the ion is unable to act as a nucleophile.