Basic trendsof amines
- Bacisity of amines
Like ammonia, amines dissolve in water to form alkaline solution.
RNH2 + H2O ↔ RN+H3 + OH-(aq)
The strength of the alkaline solution is measured by the function Kb
The higher the Kb the stronger the base.
The ability to form alkaline solution, by amines, is due to the presence of a lone pair of electron on the nitrogen atom.
(i) Groups (such as alkyl groups) that donate electrons increase the electron density of the lone pair on the nitrogen atom. This increases the ability of alkylamine to attract a proton from water to release hydroxide ions.
Thus, secondary amines are stronger bases than primary amines than ammonia because secondary amine has two electron donating groups, primary amines has one, whereas, amines have none.
However, tertiary amines are weaker bases than either secondary or primary amines because their iminium ions are poorly solvated or hydrated.
(ii) Groups that withdraw electrons from nitrogen atoms like phenyl group, make amines weaker bases because they reduce the ability of the lone pair of electron on the nitrogen atom and its ability to attract a proton from water.
For the same reason ethylmethylamine is a stronger base than propylamine than trimethylamine than phenyllamine
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