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Basicity of Amines

Amines are considered to be the alkyl- or aryl derivative of ammonia (NH3).  Amines can be categorized on the basis of the number of alkyl or aryl groups substituted for the hydrogen.

In other words, on the degree of substitution at the nitrogen atom of ammonia.Usually, aryl substituted ammonia are called aryl amines or aromatic amines. Among alkyl substituted ammonia (alkyl amines), we have the following categories.

  1. Primary amines
  2. Secondary amines
  3. Tertiary amines

Primary amine

They have a general formula of
Primary Amine ,
the characteristic group here being
Characteristic Group of Primary Amine .

Only one of the hydrogen atoms is substituted by another alkyl or an aryl group.

Secondary amines

They have a general formula of 
Secondary Amine ,
 the characteristic group here being
Characteristic Group of Secondary Amine.

Two of the hydrogen atoms are substituted by other alkyl or an aryl groups.

Tertiary amines

They have a general formula of
  Tertiary Amine ,
the characteristic group here being
Characteristic Group of Tertiary Amine.

All the hydrogen atoms are substituted by other alkyl or an aryl groups.

Basicity of amines

Amines are bases due to the lone pair of electrons on the nitrogen atom of amines. These react with water to form hydroxyl ions.
Bases are those species that donate OH- ions (hydroxyl ions). The more easily is the availability of the hydroxyl ions; more is the basicity of the amine.  

Let us now, explore and diagnose the order of basicity in amines.

Ammonia and alkyl amines

The nitrogen atom in both alkyl amines and ammonia has a lone pair of electrons. In the case of alkyl amines, the presence of the alkyl group makes the amine more basic.  

Lone Pair of Electrons

This can be explained on the electron releasing property of alkyl groups.

Alkyl Group in Amine

Due to this property of the alkyl group attached to the nitrogen of the amine, the electron density around the nitrogen atom increases due to which its electron releasing ability increases. Due to the greater and easier release of electrons in the alkyl amine, the molecule becomes more basic than ammonia. 

In ammonia, you have no such releasing group attached. So, obviously the basicity of ammonia is much less than that of an alkyl amine.

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Primary Secondary Tertiary Amines

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When we compare the basicity of the aliphatic amines, we would expect the basicity of tertiary amines to be the greatest, followed by secondary amine and then primary amine. 

But this is not so. The order of basicity is

NH3< primary amine ~ tertiary amine < secondary amine

This “unexpected” order of basicity can be understood from the below stated reasons:

Steric hindrance

The size of an alkyl group is more than that of a hydrogen atom. So, an alkyl group would hinder the attack of a hydrogen atom, thus decreasing the basicity of the molecule. So, the more the number of alkyl groups attached, lesser will be its basicity. 

So, the decreasing order of basicity will be:

Primary amine > secondary amine > tertiary amine 

Solvation of ions

When amines are dissolved in water, they form protonated amines. Also, the number of possibilities of hydrogen bonding also increases. More the number of hydrogen bonding, more is the hydration that is released in the process of the formation of hydrogen bonds. 

The more the hydration energy of the molecule, more is the stability of the amine. Experiments have indicated, in terms of hydration energy, that the tertiary amines are the least stable followed by the secondary amines followed by the primary amines. 

This can be explained on the basis of the following reactions.

In primary,

RNH2 + H2O $\rightarrow$ RNH3+ + OH-

In secondary,

R2NH + H2O  $\rightarrow$ R2NH2+ + OH-

In tertiary,

R3N + H2$\rightarrow$ R2NH+ + OH-

So, the number of hydrogen bonds possible when primary amines are dissolved in water is the greatest, implying that they are most stable species of amine, the least being the tertiary amines.

The combined effect of the pushing effect of the alkyl group (+I effect), steric hindrance and the salvation of amines causes the basicity order to be: (basicity of tertiary is almost the same as that of primary).

NH3< primary amine ~ tertiary amine < secondary amine

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