Amines (2): Base properties

AQA

Amines: Base properties

Amines are weak bases.

The difference in base strength between ammonia, primary aliphatic and primary aromatic amines.

Students should be able to explain the difference in base strength in terms of the availability of the lone pair of electrons on the N atom.

OCR

6.2.1 Amines

Learning outcomes

Learners should be able to demonstrate and apply their knowledge and understanding of:

Basicity and preparation of amines

(a) the basicity of amines in terms of proton acceptance by the nitrogen lone pair and the reactions of amines with dilute acids, e.g. HCl(aq), to form salts

Edexcel

Topic 18B:10. To understand reasons for the difference in basicity of ammonia, primary aliphatic and primary aromatic amines given suitable data

Amines: Base properties

Amines are weak bases and commonly found in the natural world e.g. in fish: ethylamine C₂H₅NH₂

or on the battlefield from decaying flesh: cadaverine H₂NCH₂CH₂CH₂CH₂CH₂NH₂. 
Cadaverine is a diamine as it contains two amino groups. 
This molecule has a very bad smell and it is why you see men on the battlefield collecting the bodies of the fallen with a mask to keep out the smell.  See the photo below from the Vietnam War:

Amines owe their basic properties to the lone pair of electrons on the nitrogen atom.  As with ammonia so amines can form strong bonds to hydrogen ions (protons H⁺) when in aqueous solution. 

           H

           ⏐

CH₃—N:     +      H+     ➝      CH₃—NH₃⁺

           ⏐

           H

The bond amines form with protons is a dative covalent bond.

The ease with which the lone pair can bond to the proton and hence the base strength of the compound amine depends upon the groups attached to the nitrogen.

Aromatic rings tend to reduce the availability of the lone pair of electrons because the electrons become part of the delocalised π system, whereas aliphatic groups like CH₃— tend to be electron pushing and so increase the availability of the lone pair. 

So phenylamine C₆H₅NH₂ is a weaker base than ammonia.

C₆H₅NH₂        +          H⁺      ➞      [C₆H₅NH₃]+     pK_b   9.38

But methylamine CH₃NH₂ is a stronger base than ammonia (lower pK_b).

CH₃NH₂        +          H⁺      ➞      [CH₃NH₃]+     pK_b   3.36

Reaction with dilute acids

If amines are weak bases then they will react with dilute mineral acids.

So reaction with dilute hydrochloric acid forms the hydrochloride of the amine

CH₃NH₂        +          HCl    ➞      [CH₃NH₃]⁺   +   Cl^—  

The product is methyl ammonium chloride or methylamine hydrochloride.  These compounds form stable crystalline salts soluble in water and useful in synthesis.

Again take the case of aniline

C₆H₅NH₂        +          HCl        ➞      [C₆H₅NH₃]⁺ Cl^—   

The product is aniline hydrochloride of phenylammonium chloride

Similar acid base reactions to produce salts take place with sulphuric and nitric acids.

In my next post, we’ll look at the properties of the nucleophilic nitrogen atom in amines.