A Guidebook to Mechanism in Organic Chemistry

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