Amines (3): Nucleophilic reactions

EDEXCEL

Topic 18B:9.

To understand the reactions of primary aliphatic amines, using butylamine as an example, with:

.    i  water to form an alkaline solution 


.    ii  acids to form salts 


.    iii  ethanoyl chloride 


.    iv  halogenoalkanes 


.    v  copper(II)ions to form complex ions 


AQA

Nucleophilic properties of Amines

Amines are nucleophiles.

The nucleophilic substitution reactions of ammonia and amines with halogenoalkanes to form primary, secondary, tertiary amines and quaternary ammonium salts.

The use of quaternary ammonium salts as cationic surfactants.

The nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides and acid anhydrides.

Students should be able to outline the mechanisms of:

.    these nucleophilic substitution reactions 


.    the nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides. 


Reactions of Amines

Let’s first look at these simple reactions of amines with butylamine  C₄H₉NH₂ as our example:

1. With water

C₄H₉NH₂    +    H₂O          ⇌          [C₄H₉NH₃]⁺        +      OH^—

Like ammonia, butylamine is a water soluble weak base and forms butylammonium ( [C₄H₉NH₃]⁺ ) ions in aqueous solution.

2. With acids to form salts

C₄H₉NH₂    +    HCl          ⟶           [C₄H₉NH₃]⁺        +      Cl^— 

3. With ethanoyl chloride

C₄H₉NH₂        +    CH₃COCl         ⟶      C₄H₉NHCOCH₃       +   H⁺     +   Cl^—   

This very fast room temperature reaction forms N-butyl ethanamide.  The picture below shows this reaction and in particular the evolution of the white fumes of the amine chloride.

4. With excess halogeno alkanes under pressure and at 120^oC

C₄H₉NH₂         +    CH₃Cl         ⟶      [C₄H₉NH₂CH₃ ]⁺     +   Cl^—   

Then methyl groups successively replace hydrogen atoms in the amine salts:

[C₄H₉NH₂CH₃ ]⁺     +   CH₃Cl   ⟶    [C₄H₉NH(CH₃)₂]⁺     +   Cl^—   

then

[C₄H₉NH(CH₃)₂]⁺     +   CH₃Cl   ⟶    [C₄H₉N(CH₃)₃]⁺     +   Cl^—   

The final product is the quaternary amine salt: trimethyl butyl ammonium chloride.

5. With copper(II) ions to form complex ions

Butylamine acts as if it were ammonia so that the nitrogen lone pair forms a dative covalent bond with the transition metal ion.  4 moles of the amine react with one mole of the copper ion as in the ammonia reaction and produce a square planar complex ion.

4C₄H₉NH₂      +      Cu²⁺    ⟶        [Cu(C₄H₉NH₂)₄ ]²⁺

In the equation above, I have removed all state symbols and other unnecessary ions (e.g. sulphate SO₄^2—) in order to show up more clearly the formation of the complex. 

In the photo above of the reaction in aqueous solution the pale blue copper ion solution turns the deeper blue of the copper complex.

Let’s now recap the two significant mechanisms here:

First the mechanism for the reaction between a halogenoalkane and an amine:

The reaction depends on the nucleophilic character of the amine i.e. that it carries a lone pair of electrons on its nitrogen atom.  It is this lone pair that attacks the electropositive carbon atom in the halogenoalkane in step one.  Excess amine is then protonated to leave the free secondary amine and the amine salt: 

In the diagram above note that the amine is shown with a negative charge rather than a lone pair:

Second, the mechanism for the reaction between an amine and an acyl chloride

This again illustrates the nucleophilic character of the amine because of its nitrogen lone pair.

In the illustration below I have kept to the ammonia molecule for simplicity

But you can see that the first step is the nucleophilic attack on the electropositive carbon atom of the —COCl group. 

This attack leaves a species with a charge separation: the oxygen atom carries a negative charge and the nitrogen atom a positive charge. 

Therefore, the second step is the resolution of this charge separation as they come together.  In doing so, a molecule of HCl is eliminated. 

So an addition step is followed by an elimination step.

Hence this is called an addition-elimination mechanism.

This leaves a new molecule where the ammonia or amine group has replaced the labile chlorine atom. 

Here is summary chart for the substitution mechanism:

In the next blog, we’ll look at amides and amino acids.