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How to know the basic character of amine in aqueous solution.

How to know the basic character of amine in aqueous solution.

Grade:11

1 Answers

anil kumar
25 Points
6 years ago
 
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.
action of amine with water
Since [H2O] is constant
The basically constant Kb = Keq [H2O]
 
Aliphatic amines with Kb 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.
formation of ammonium salts from amines with acids
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.
basic character of aliphatic amines
However, the order of basicity in aqueous solution does not follow the expected trend and gets altered as revealed by their K
b values.
basicity order of amines
The basic strength in aqueous solution depends not only upon electron releasing effect but also upon steric effect and hydration effect.
 

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