Anilines: Acid-base properties

Aromatic amines (anilines) are less basic than aliphatic amines and therefore the anilinium salts are more acidic than the ammonium salts.

The resonance forms give the explanation because they show us that the electron lone-pair on the nitrogen is delocalized through the ring and thus less available to an external acid. Electron density calculations displays redder (higher density) carbons in ortho and para positions relative to the nitrogen as compared to benzene.

Substitution on the aromatic ring affects the acid-base properties of the aniline group. The following table collects experimental pKb values of different substituted anilines. From those pKb values, one can deduce the donor or acceptor electronic behavior of the listed groups.

("Click" on each substituent to get an explanation)

	ORTHO pKb	META pKb	PARA pKb
H	9.4	9.4	9.4
Me	9.6	9.3	8.9
OMe	9.5	9.8	8.7
CF3	12.9	10.8	11.2
F	10.0	10.4	9.3
Cl	11.4	10.5	10.0
Br	11.5	10.4	10.1
I	11.4	10.4	10.2
CN	13.0	11.2	12.3
PhCO	13.7	10.6	11.8
NO2	14.3	11.5	13.0

ACID-BASE PROPERTIES OF ANILINES