CHEMICAL PROPERTIES 291 



The isomerization of maleic acid to fumaric acid occurs at a very slow 

 rate in aqueous solution. Kailan (1919) kept a solution of maleic acid in 

 the dark and at room temperature for 5 years, and found that only 4% 

 had been transformed into fumaric acid. Davies and Evans (1956) studied 

 the kinetics of the reaction but their work was done at elevated tempera- 

 tures. However, even at 120° it requires 10-20 hr for a 25% conversion 

 of maleic to fumaric acid. An activation energy of 21 kcal/mole for the 

 uncatalyzed reaction was obtained, which is of the same order of magnitude 

 as the 25 kcal/mole activation energy found for the isomerization of di- 

 methylmaleate (Davies and Evans, 1955). The isomerization involves a 

 heterolytic mechanism and is mainly second order. The reaction is accel- 

 erated by increase in temperature, ultraviolet irradiation, acid, and vari- 

 ous catalysts. One may conclude that, under ordinary conditions, solutions 

 of maleate are quite stable. 



Thiols are able to catalyze many cis-trcms isomerizations by means of 

 thiyl radicals. Morgan and Friedmann (1938 a) observed that when mal- 

 eate is allowed to react with various thiols at 37° and pH 7.4, a certain 

 amount of fumarate is formed. Olefinic double bonds and thiols which can 

 react to form addition products are apparently necessary for this catalysis, 

 since cinnamate is not isomerized by glutathione. Knox and Edwards 

 (1954) also found that maleylacetoacetate is isomerized to fumarylaceto- 

 acetate by glutathione, a reaction of possible importance in the metabolism 

 of homogentisate. Waley (1962) postulated that the R — -S" anion may be 

 the reactive species, the intermediate formed being a carbanion. The in- 

 termediate can, in the case of maleate, either be converted into the trans 

 form or pick up a proton to form the addition compound. The relative 

 rates will determine how much maleate will be isomerized to fumarate. 

 Such reactions must be borne in mind when maleate is in the presence of 

 thiols, and may be important in the metabolism of maleate in the tissues. 



Purification and Determination 



Maleic acid obtained commercially is usually not sufficiently pure for 

 most work and may contain some fumaric and malic acids. It may be dis- 

 solved in warm water, treated with activated charcoal, and crystallized 

 by cooling. Two crystallizations usually suffice to give a satisfactory prep- 

 aration with a constant melting point. It may also be crystallized from 

 acetone, as was done by Shahat (1952) for his crystallographic work. It 

 has been stated that the only method giving pure maleic acid, free from 

 fumaric acid, is to dissolve several times redistilled maleic anhydride in 

 distilled water and crystallize the maleic acid by concentration in vacuo 

 (Parks and Huffman, 1930) and this is perhaps the preferred method. Maleic 

 acid is probably best detected and determined in a mixture of acids obtained 

 by fractionation of biological material by two-dimensional chromatography 



