Aliphatic and aromatic amines

​​In a nutshell

Amines are derivatives of ammonia. Aliphatic amines can be produced via nucleophilic substitution and reduction. Aromatic amines can be formed by reducing aromatic nitro compounds.

Equations

These are general chemical equations. You will need to be able to use these when you write equations for specific substances.

​$$haloalkane \ + \ ammonia \rightarrow amine \ + \ ammonium \ salt \\ \ \\nitrile \ +\ hydrogen\xrightarrow[2. \ dil. \ acid]{1. \ LiAlH_4, \ dry \ ether} primary \ amine \\ \ \\nitrile \xrightarrow[high \ temperature \ and \ pressure]{metal \ catalyst} primary \ amine \\ \ \\aromatic \ nitro \ compound \xrightarrow[2. \ NaOH]{1. \ Sn, \ conc. \ HCl, \ reflux} aromatic \ amine \ + \ water$$​​

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Types of amines

Amines are derivates of ammonia. ​They are classed as primary, secondary or tertiary amines.

Primary amines have one alkyl group attached to the nitrogen. Secondary amines have two alkyl groups attached to the nitrogen. Tertiary amines have three alkyl groups attached to the nitrogen.

A positively charged ammonia or amine is called an ammonium ion or a quaternary ammonium ion.

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Aliphatic amines

Haloalkanes

Aliphatic amines can be produced via nucleophilic substitution, by heating a haloalkane with excess ethanolic ammonia.

$$haloalkane \ + \ ammonia \rightarrow amine \ + \ ammonium \ salt$$​​

Example 

​$$1-bromopropane \ + \ ammonia \rightarrow propylamine^* \ + \ ammonium \ bromide \\C_3H_7Br \ + \ NH_3 \rightarrow C_3H_7NH_2 \ + \ NH_4^+Br^-$$​​

* also known as $$propan-1-amine$$​ (IUPAC name) and $$1-aminopropane$$.

A drawback of using haloalkanes is that a mixture of the types of amines is obtained. The reason for this is that the nitrogen atom of the amines acts as a nucleophile therefore takes part in nucleophilic substitution reactions where possible, leading to multiple substitutions occurring. This gives rise to multiple products.

Nitriles

Aliphatic primary amines can be be produced by reducing a nitrile.

$$nitrile \xrightarrow[2. \ dil. \ acid]{1. \ LiAlH_4, \ dry \ ether} primary \ amine$$​​

Example 

​$$acetonitrile \xrightarrow[2. \ dil. \ acid]{1. \ LiAlH_4, \ dry \ ether} ethylamine \\ \ \\CH_3CN \xrightarrow[2. \ dil. \ acid]{1. \ LiAlH_4, \ dry \ ether}CH_3CH_2NH_2$$​​

However, lithium aluminium hydride is very expensive to use on an industrial scale. Instead, catalytic hydrogenation is used. Nitriles are reduced to primary amines using hydrogen gas, in the presence of a metal catalyst such as nickel or platinum.

​$$nitrile \ +\ hydrogen \xrightarrow[high \ temperature \ and \ pressure]{metal \ catalyst} primary \ amine$$​​

Example 

​$$acetonitrile \ +\ hydrogen \xrightarrow[high \ temperature \ and \ pressure]{cat. \ Ni} ethylamine \\ \ \\CH_3CN \ +\ 2H_2\xrightarrow[high \ temperature \ and \ pressure]{cat. \ Ni}CH_3CH_2NH_2$$​​

Aromatic amines

Aromatic amines can be produced by reducing nitro compounds.

​$$aromatic \ nitro \ compound \xrightarrow[2. \ NaOH]{1. \ Sn, \ conc. \ HCl, \ reflux} aromatic \ amine \ + \ water$$​​

Example 

​$$nitrobenzene \xrightarrow[2. \ NaOH]{1. \ Sn, \ conc. \ HCl, \ reflux} phenylamine \ + \ water \\ \ \\C_6H_5NO_2 \xrightarrow[2. \ NaOH]{1. \ Sn, \ conc. \ HCl, \ reflux} C_6H_5NH_2 \ + \ 2H_2O \\ \ \\ or \\ \ \\C_6H_5NO_2 \ + \ 6[H] \xrightarrow[2. \ NaOH]{1. [H],\ reflux} C_6H_5NH_2 \ + \ 2H_2O$$​​

[H] = a reducing agent