Aliphatic Bases
completely ionised as there is no possibility, as with tertiary amines
etc., of reverting to an unionised form:
R 3 NH+sOH -+ R 3 N: + H 20
The effect of introducing electron-withdrawing groups, e.g. CI,
N0 2 , close to a basic centre is, naturally, to decrease the basicity, due
to their electron-withdrawing inductive effect (see substituted
anilines below, p. 53); thus
•F 3 C^
F 3 C-«-N:
/
F 3 C
- is virtually non-basic due to the three powerfully electron-withdraw
ing CF 3 groups. #
The change is particularly pronounced with groups such as
y>C=0 when these are adjacent to the basic centre for they are then
able to act via a mesomeric effect:
I p. I e • #
R—C^NH 2 *-> R—C=NH,
Thus amides are only very weakly basic in water (pKb for acetamide
= 14-5) and if two ^C=0 groups are present, the resultant imides,
far from being basic, are often sufficiently acidic to form alkali metal
salts, e.g. phthalimide:
The effect of delocalisation in increasing the basic strength of an
amine is seen in guanidine, HN=C(NH 2 ) 2 , which, with the exception
of the quaternary alkylammonium hydroxides above, is among the
strongest organic bases known, having too small a vKb in water for it