6.2.1 (a) Basicity of Amines

Syllabus

(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

{Comparison of basicity of different Amines not required.}

{Restricted to inorganic acids.}

What does this mean?

Image result for primary secondary and tertiary amines

All Amines (except Quaternary Salts) have a lone pair of electrons on the Nitrogen atom.

Lone pairs can accept H⁺ ions - making Amines Alkaline when dissolved in water, just like Ammonia

:NH_3(aq) + H⁺_(aq) --> NH₄⁺_(aq)

CH₃NH_2(aq) + H⁺_(aq) --> CH₃NH₃⁺_(aq)

CH₃NHCH_3(aq) + H⁺_(aq) --> CH₃NH₂CH₃⁺_(aq)

Although Tertirery amines don't dissolve because they have no N-H bond and so can't Hydrogen bond to water.

Alternatively, we can say that soluble amines are alkalis because they hydrolise water forming OH^- ions.

We no longer seem to be required to explain why Secondary Amines are more basic than Primary Amines, or why Primary Amines are more basic than Ammonia.

But you do still need to remember the order below:

SECONDARY AMINE > PRIMARY AMINE > AMMONIA>AROMATIC AMINES

You can think of the ability of an Amine to be a base as its ability to attract and accept an H⁺ ion.

Alkyl groups (R) push electrons away from themselves (we don't need to know why).

The N atom on the Primary Amine above has a "slightly bigger" lone pair than the N atom in Amine because there is one alkyl group pushing electrons towards it. This means that it is better able to attract and accept an H⁺ ion and so is a slightly better base.
The N atom on the Secondary Amine above has a "slightly bigger" lone pair than the N atom in a Primary Amine because there are two alkyl groups pushing electrons towards it. This means that it is better able to attract and accept an H⁺ ion and so is a slightly better base.
You'd think that Tertiary Amines would make excellent bases but, without any N-H bonds, Tertiary Amines are insoluble and so can't act as bases.

This is also true in secondary and tertiary amines.

It's unlikely that an A level examiner will ask a question that requires you to judge the solubility of an amine without providing any data.

But it would be good to recall that no insoluble amine of any type can be a base.

Phenylamine can just about dissolve and has a lone pair on the N atom.

So, it can add as a base.

But the lone pair on the N atom partly delocalises into the benzene ring, making it less available even than the lone pair on the N atom in Ammonia.

Consequently, it is only a very poor base.

Reaction of Amines with Acids

If you're happy writing equations for ammonia reacting with acids then writing the equivalent for an amine will be easy.

For a typical primary amine like methylamine:

NH₃ + HCl --> NH₄Cl becomes CH₃NH₂ + HCl --> CH₃NH₃Cl

NH₃ + HNO₃ --> NH₄NO₃ becomes CH₃NH₂ + HNO₃ --> CH₃NH₃NO₃

2NH₃ + H₂SO₄ --> (NH₄)₂SO₄ becomes 2CH₃NH₂ + H₂SO₄ --> (CH₃NH₃)₂SO₄

For a typical secondary amine like N-methyl methylamine:

NH₃ + HCl --> NH₄Cl becomes (CH₃)₂NH + HCl --> (CH₃)₂NH₂Cl

NH₃ + HNO₃ --> NH₄NO₃ becomes (CH₃)₂NH + HNO₃ --> (CH₃)₂NH₂NO₃

2NH₃ + H₂SO₄ --> (NH₄)₂SO₄ becomes 2(CH₃)₂NH + H₂SO₄ --> ((CH₃)₂NH)₂SO₄

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