288 



2. MALE ATE 



Ionization of Maleic Acid 



The pKa of maleic acid (1.67) is much lower than that for fumaric 

 (2.85) or succinic (3.95) acid; thus the first proton is dissociated very 

 readily from maleic acid and the second proton with difficulty. These unique 

 properties have stimulated many hypotheses for the mechanisms involved, 

 and from these has come a better understanding of the structure of maleic 

 acid which is of some importance in considerations of its action on enzymes. 

 Electrostatic interactions between the cis carboxylate groups have been 

 invoked to explain the aberrant p^^'s. Certainly bringing the carboxylate 

 groups closer together (as in the series of succinic-malonic-oxalic acids) 

 decreases p/C^ , and the propinquity imposed by the cis form must play 

 a role, but the groups in maleate are not closer than in malonate and so 

 some additional factor must be considered. This was provided by the crys- 

 tallographic study of Shahat (1952), who demonstrated that crystals of 

 maleic acid are made up of layers of nearly flat molecules, the structure of 

 each molecule being as represented in Fig. 2-1. Adjacent molecules in the 

 layers are bound through hydrogen bonds, while the' layers are held together 

 by van der Waals' forces. The unequal distances of the C — bonds are 

 probably due to hydrogen bonding between the atoms, this effectively 

 making a seven-membered ring. It is interesting, in passing, that the 

 C — C distances are approximately equivalent and correspond to around 

 25-30% double-bond character, indicating a fair degree of resonance. The 

 hydrogen bond is a strong one and abnormally short (most are 2.7-2.8 A). 

 These observations stimulated the publication of six communications in 

 1953 on the role of hydrogen bonding in the ionization of maleic acid. 



L. Hunter (1953) reasoned that if such a hydrogen bond were retained 

 in ionizing solvents, it would tend to stabilize the H-maleate^ ion; the 



2,46 A 



1.21 A 



H H 



Fig. 2-1. Structural characteristics of maleic acid. (From Shahat, 1952. 



