Depto. Química Orgánica

ACID-BASE PROPERTIES OF AMINES

Image Link UAM Excelencia UAM Facultad Ciencias UAM
15/07/2017
page counter
Where I am Problems and exercises Molecular panel Home
Video: Acidez y basicidad de Aminas
Amines are moderately basic and the corresponding ammonium salts - their conjugated acids - are moderately acidic.
Image
Image
Image The higher the basicity, the more shifted the equilibrium to the right, i.e. to the formation of HO-, the higher Kb and the lower pKb.
Image The higher the acidity, the more shifted the equilibrium to the right, i.e. to the formation of H+, the higher Ka and the lower pKa.
Image
pKa and pKb values are related by the water ionic product. Therefore, from the pKa of an acid, the pKb of its conjugated base can be calculated by substracting 14, and viceversa.
How would you understand the pKb variations observed in this series?
Not that easy!
Click in this box to get an explanation.
Image
How would you understand the pKa variation observed in this series?
Not that easy as well!
Click in this box to get an explanation.
Image
Amines with at least a N-H group are feably acidic.
In logic contrast, their conjugated bases - metal amides - are extraordinarily strong.
Image
Image
Acidity of alkanes is minimal (pKa > 50).
Amines acidity is also low (pKa=35) but ca. 15 orders of magnitude higher.
That means that the N- anion is more stable than the C- carbanion coming from an alkane.
This can be understood in terms of the higher electronegativity of the nitrogen as compared to carbon.
Aromatic amines
- anilines -
are less basic than the corresponding amines.
As a consequence, anilinium salts are more acidic than the corresponding ammonium salts.
Image