Basicity of amines

Basicity of amines

In amines, there is a lone pair of electrons on nitrogen. The tendency of nitrogen to share these electrons with acids is responsible for the basic character of amines.

For comparison of the basic character of amines, the equilibrium constant of following reaction is a measure of their basic character.

Since [H₂O] is constant

The basically constant K_b = K_eq [H₂O]

Aliphatic amines with K_b between 10^-3 and 10^-4 are stronger bases then ammonia (K_b = 1.8 x 10^-5).

Like ammonia, amines are strong bases and react with mineral acids to form ammonium salts from which they can be liberated by treatment with a strong base like NaOH.

That alkylamines are stronger bases then ammonia can be expressed in terms of electron releasing inductive effect of alkyl groups. Alkyl groups by their electron releasing effect, concentrate electron density on nitrogen and hence make the lone pair of nitrogen more easily available for sharing with acids. Also the electron releasing effect of alkyl groups stabilizes the alkyl ammonium ion formed and hence shifts the equilibrium in forward direction making the alkylamines stronger bases than ammonia.

Thus the basic character of aliphatic amines should increase with increase of alkyl substitution. In the gasphase, the basic trend in nature is as expected i.e.,

tertiary amine > secondary amine > primary amine > ammonia.

However, the order of basicity in aqueous solution does not follow the expected trend and gets altered as revealed by their K_b values.

The basic strength in aqueous solution depends not only upon electron releasing effect but also upon steric effect and hydration effect.

a) Steric effect

Steric effect refers to the crowding of alkyl groups around N atom which hinders the attack of proton on the amine molecule and this decreases its basic strength. Since crowding of alkyl groups around N atom increases from 1^o to 3^o amines, the basic strength of amine should decrease in the order 1^o > 2^o> 3^o.

b) Hydration effect

Hydration effect refers to the stabilization of the protonated amine by water molecules. The water molecules from H - bonds with the protonated amine and release energy called hydration energy.

Greater the extent of H - bonding in protonated amine more will be its stabilization and consequently greater will be the basic strength of the corresponding amine.

Hydration due to hydrogen-bonding is maximum in monoalkyl ammonium ion (protonated cation of 1^oamine), it is less in dialkyl ammonia ion and still less in trialkyl ammonium ion. Thus basic strength should decrease from 1^o > 2^o > 3^o.

In 3^o amine, hydration is least, steric hinderance is maximum so least basic inspite of maximum inductive effect.

In 1^o amine, steric hinderance is least, hydration is maximum and inductive effect is minimum. So its basic strength is more than 3^o amine.

The resultant of all factors cause 2^o amine to be still more basic than 1^oamine.

So overall basic strength varies as 2^o > 1^o > 3^o.

Aromatic amines are weaker bases (Kb ~ 10^-9) than ammonia and aliphatic amines.

The less basic character of aniline can be explained on the following basis:

(a) Delocalization of one pair of electrons on the nitrogen atom

Amiline is a hybrid of the following resonating structures.

Thus the lone pair of electrons on nitrogen is withdrawn from it and is being partially shared with the benzene ring. Thus, in aniline the electron donating capacity of nitrogen for protonation is considerably decreased as compared to that of ammonia and aliphatic amines. Hence aniline is a weaker base than aliphatic amines and ammonia.

(b) Lower stability of the anilinium ion

Anilinium ion forward by aniline by accepting a proton is not stabilized by resonance.

Anilinium ion

does not form resonating structures similar to I, II, III and IV (in the case of aniline) 

due to absence of lone pair on N atom. Hence anilinium ion is less stable as compared to aniline. Therefore, aniline has less tendency to accept proton to form anilinium ion. This accounts for the lower basic strength of aniline.

An electron releasing group present in an aromatic amine ring especially in ortho / para position will stabilize the ammonium cation formed after the protonation of amine and hence increases the basic strength of aromatic amine.

The electron releasing groups (like -OCH₃, -CH₃, -NH₂ etc) enhance the availability of unshared electrons on nitrogen and increases the basic strength.

Electrons withdrawing groups like (-NO₂, -CN, -X etc) affect the stability of an aromatic ammonium cation and decreases the basic strength of parent aromatic amine.

The electron withdrawing groups decrease the availability of unshared electrons on nitrogen and thereby decrease the basic strength of aromatic amines.