Reactions of amines

​​In a nutshell

Amines are basic as they can accept a proton. This is why they are able to form alkaline solutions. Amines can undergo nucleophilic substitution reactions due to the lone pair of electrons on the nitrogen atom. They can also act as ligands, taking part in ligand substitution to form complex ions.

Equations

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

​$$haloalkane \ + \ ammonia \rightarrow amine \ + \ ammonium \ salt \\ \ \\acyl \ chloride \xrightarrow{ conc. \ amine}amide \ + \ ammonium \ salt \\ \ \\amine \ + \ water \rightleftharpoons ammonium \ ion \ + \ hydroxide \ ion \\ \ \\amine \ + \ acid \rightarrow ammonium \ salt$$​​

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Amines as bases

Amines are weak bases as they are able to accept a proton. The lone pair of electrons on the nitrogen atom donates both electrons to form a dative covalent bond with a $$H^+$$​ ion. 

The basic strength of the type of amines is dependant upon the electron density around the nitrogen atom and what the group attached to the nitrogen is. The greater the electron density around the nitrogen, the more basic the amine is.

Example 

Strength of different amines as bases and nucleophiles.

​$$Primary \ aliphatic \ amines > ammonia > aromatic \ amines$$​​

Primary aliphatic amines have an alkyl group feeding in electrons towards the nitrogen. This increases the electron density around the nitrogen making it 'more nucleophilic' and available for electron donation. On the other hand, benzene pulls the lone pair towards itself making it partially delocalised. This decreases the electron density of the nitrogen atom and makes the lone pair less available for a nucleophilic attack.

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Reactions of amines

Amines are able to act as nucleophiles due to the lone pair of electrons on the nitrogen atom.  They can undergo many nucleophilic substitution reactions. 

​The more substituted an amine is, the more nucleophilic it becomes, due to the feeding of electrons from the alkyl groups towards the nitrogen atom. This increases the electron density around the nitrogen and leads to multiple substitutions.

Alkylation

Amines react with haloalkanes via nucleophilic substitution reactions.

Acylation

Amines are acylated to form N-substituted amides. 

​$$acyl \ chloride \xrightarrow{ conc. \ amine}amide \ + \ ammonium \ salt$$​​

Example 

Reaction between ethanoyl chloride and methylamine gives rise to two products. This is because the methylamine can be protonated to form an ammonium salt.

Step 1 

​$$ethanoyl \ chloride \xrightarrow{methylamine} N-methylethanamide \ + \ hydrogen \ chloride \\ \ \\CH_3COCl \xrightarrow{CH_3NH_2} CH_3CONHCH_3 \ + \ HCl$$​​

Step 2

​$$methylamine \xrightarrow{HCl} methylammonium \ chloride\\ \ \\CH_3NH_2 \xrightarrow{HCl} [CH_3NH_3]^+Cl^-$$​​

Overall equation

​$$ethanoyl \ chloride \ + \ methylamine \rightarrow N-methylethanamide \ + \ methyl ammonium \ chloride \\ \ \\CH_3COCl \ + \ 2CH_3NH_2 \rightarrow CH_3CONHCH_3 \ + \ [CH_3NH_3]^+Cl^-$$​​

Neutralisation

As amines are bases they can be neutralised by acids to form ammonium salts. 

​$$amine \ + \ acid \rightarrow ammonium \ salt$$​​

Example 

$$ethylamine \ + \ hydrochloric \ acid \rightarrow ethylammonium \ salt \\ \ \\C_2H_5NH_2 \ + \ HCl \rightarrow C_2H_5NH_3^+Cl^-$$​​

Alkaline solution

Small amine molecules are water soluble as the amine group is able to form hydrogen bonds with the water molecules. 

The larger the amine molecules, the greater the number of van der Waals forces between the molecules. Therefore more energy is required to break these bonds. The greater number of van der Waals forces means there is interference with hydrogen bonding so the larger amines are not able to form hydrogen bonds with water molecules, making them less water soluble compared to the smaller amines.

Upon dissolving, amines produce an alkaline solution. This is because some amine molecules become protonated by a hydrogen ion from a water molecule, forming an ammonium salt. This also leads to the formation of hydroxide ions, hence an alkaline solution.

​$$amine \ + \ water \rightleftharpoons ammonium \ ion \ + \ hydroxide \ ion \\$$​​

Example 

​$$methylamine \ + \ water \rightleftharpoons methylammonium \ ion \ + \ hydroxide \ ion \\CH_3NH_2 \ + \ H_2O \rightleftharpoons CH_3NH_3^+ \ + \ OH^-$$​​

Complex ions

Copper $$(II)$$ sulfate solution is pale blue, which contains the complex $$[Cu(H_2O)_6]^{2+}$$. The copper ions form coordinate bonds with the water molecules.

Addition of a small amount of butylamine solution to copper $$(II)$$ sulfate solution leads to the formation of a pale blue precipitate of the complex $$[Cu(OH)_2(H_2O)_4]$$. Excess butylamine leads to the precipitate to dissolve, forming a deep blue solution of the complex $$[Cu(CH_3(CH_2)_3NH_2)_4(H_2O)_2]^{2+}$$.

The same reaction occurs with other amine molecules. However, the shape adopted by the complex ion may be different for larger amines due to the limited space around the central metal ion. ​​