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mukul chakravarty Grade:
        

RNH3+  R2NH2+  R3NH+    NH4+  WHICH OF THESE IS MOST STABLE IN WATER AND WHY?

6 years ago

Answers : (3)

SAGAR SINGH - IIT DELHI
879 Points
										

Dear mukul,









 


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 (Kb = 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 Kb 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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Sagar Singh


B.Tech IIT Delhi












6 years ago
Tarun Saxena
29 Points
										

if we take these comp. in water then NH4+ is most stable


the reason being , as all these comp. get H-bonded in water and +ve charge is max. on NH4+ b'cz of no e- pushing group.


hence NH4+ get max. H-bonded.

6 years ago
mohit sengar
39 Points
										

as saagar replied was absolutely correct but as i think in this case all the four compounds have h+ ion already so the stability in water should be reversed acc to the answer given by saagar

6 years ago
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