A Guidebook to Mechanism in Organic Chemistry

The Strengths of Acids and Bases

O

II

H C—O

H -H<

(V)

o

II

H C—O

\ / N

c

HAC-0-

H

A

» O

II

H C

\ / \

C OH

II

HO • C

\ / \

C H

O

(VI)

The second dissociation of fumaric occurs more readily than that
of maleic acid, however (pKaa is 4-38 compared with 6-23) because
of the greater difficulty in removing the proton from the negatively
charged cyclic system of the latter. Oxalic, malonic and succinic acids
are each weaker in their second dissociations than formic, acetic and
propionic acids, respectively, because the second proton has to be
removed from a negatively charged species containing an electron—
donating substituent, i.e.^0 O 2 C, which will destabilise the anion with
respect to the undissociated acid as compared with the unsubstituted
system:

O n©

©OjC-^-CHj^-C

1 ©

CH.

BASES
(0 Pfffc

The strength of a base, R 3 N, in water, may be denned by considering
the equilibrium: v

RjN:+ HOH ^ R 3 N:H+eOH

Then the equilibrium constant, in water, is given byr

[R 8 NH] [eOH]

Kb «

[RaN]

the concentration of water being taken as constant as it is present in
such large excess. As with Ka, Kb is, on the grounds of convenience,