Amines
After studying this section you should be able to:
- explain the relative basicities of ethylamine and phenylamine
- describe the reactions of primary amines with acids to form salts
- describe the formation of phenylamine by reduction of nitrobenzene
- describe the synthesis of an azo-dye from phenylamine
- describe the reactions of amines with halogenoalkanes and acyl chlorides
Aliphatic and aromatic amines
- Notice the similarity with ammonia.
Physical properties of amines
The amino group dominates the physical properties of short-chain amines.
Hydrogen bonding takes place between amine molecules, resulting in:
- higher melting and boiling points than alkanes of comparable relative molecular mass
- solubility in water – amines with fewer than six carbons mix with water in all proportions.
- As with other polar functional groups, the solubility of amines in water decreases with increasing carbon chain length as the non-polar contribution to the molecule becomes more important.
Amines as bases
![](/sites/default/files/styles/full_width/public/revisionworld/imce/aminesas%20bases.jpg?itok=3wko-b7b)
- Amines are the organic bases.
The basicity of aliphatic and aromatic amines
Aliphatic amines are stronger bases than ammonia; aromatic amines are substantially weaker.
Organic ammonium salts
![](/sites/default/files/styles/full_width/public/revisionworld/imce/organic%20ammonium%20salts.jpg?itok=i5Rrb_NP)
- Amines react by the usual ‘base reactions’ producing organic ammonium salts
- A proton, H+, is added to the amino nitrogen atom.
Preparation of amines
![](/sites/default/files/styles/full_width/public/revisionworld/imce/preparation%20of%20aliphatic%20amines.jpg?itok=eqIZ3V2_)
Other reducing agents can also be used:
- H2/Ni catalyst
- LiAlH4 in dry ether
- LiAlH4 is an almost universal reducing agent and works in most examples of organic reduction.
Preparation of amines from amides (Edexcel only)
![](/sites/default/files/styles/full_width/public/revisionworld/imce/amines%20from%20amides.jpg?itok=CJeyvhQy)
- This process is often called the Hofmann degradation and can be used to move down a homologous series.
The preparation of dyes from aromatic amines
Aromatic amines, such as phenylamine, are important industrially for the production of dyes. Modern dyes are formed in a two-stage synthesis:
- the aromatic amine is converted into a diazonium salt
- the diazonium salt is coupled with an aromatic compound such as phenol, forming an azo dye.
Formation of diazonium salts
![](/sites/default/files/styles/full_width/public/revisionworld/imce/formation%20of%20diazonium%20salts.jpg?itok=8vEKVq8e)
Using benzene for the synthesis of dyes
Benzene is used as the raw material for the synthesis of dyes.
A four-stage synthesis is shown below:
- benzene is nitrated to nitrobenzene
- nitrobenzene is reduced to phenylamine (see above)
- phenylamine is converted into a diazonium salt
- the diazonium salt is coupled with an aromatic compound such as phenol, forming an azo dye.
Reactions of amines with halogenoalkanes
![](/sites/default/files/styles/full_width/public/revisionworld/imce/preparation%20of%20a%20primary%20amine.jpg?itok=Wj4-VAJt)
Key points from AS - Nucleophilic substitution reactions of halogenoalkanes
- At each stage, a hydrogen atom is replaced by an alkyl group.
- Quaternary ammonium salts are used as cationic surfactants in fabric softeners.
Reactions of amines with acyl chlorides
![](/sites/default/files/styles/full_width/public/revisionworld/imce/reactions%20with%20acyl.jpg?itok=kKbyYtKX)
PROGRESS CHECK
![](/sites/default/files/styles/full_width/public/revisionworld/imce/progress%20check%204_0.jpg?itok=sRBJJGuq)