4 1. MALONATE 



is approximately —7 kcal/mole but the activation energy is 27.9 kcal/mole 

 (Gelles, 1956). Thus at physiological temperatures one may consider mal- 

 onate as completely stable. Nevertheless, there are enzyme systems which 

 catalyze the decarboxylation (see page 227) and one should be reasonably 

 certain in the use of malonate that it is stable in the system investigated, 

 since the formation of acetate might well confuse the results. 



Molecular Structure 



In crystals of malonic acid the molecules are arranged in zigzag chains 

 with the carboxyl groups linked through two hydrogen bonds (Goedkoop 

 and MacGillavry, 1957). The following bond parameters were observed 

 (the two values refer to the two carboxyl groups, since the molecule in the 

 crystal is apparently not symmetrical): C — C — C angle = 110^; C — C 

 distance = 1.54, 1.52; C— distance = 1.29, 1.31; C=0 distance = 1.24, 

 1.22; 0— C— angle = 128^, 128°; and H-bond distance = 2.68-2.71. 

 The malonate ion in solution would probably deviate somewhat from this 

 configuration but not a great deal inasmuch as malonate is fixed in a rather 

 rigid structure, because the C — C — C angle is determined by the electronic 

 tetrahedral orbitals and can be distorted only with difficulty. The carbox- 

 ylate groups can rotate around the C — C axis but they presumably ster- 

 ically interfere with each other when both lie in the plane of the molecule, 

 since the centers of the oxygen atoms would be 2.2 A apart and the van 

 der Waals' radius of the oxygen atom is 1.4 A (Goedkoop and MacGillavry, 

 1957). In malonic acid crystals one carboxyl seems to be in the molecular 

 plane and the other is at right angles; in the malonate ion it may well be 

 that neither is in the C — C — C plane. However, another factor must be 

 considered; it is possible that in solution there is intramolecular hydrogen 

 bonding (Gelles, 1956), at least for the hydrogen malonate ion. When the 

 carboxyl group ionizes, the equivalence of the structures: 



R-cf R_c--° 



allows a greater resonance than in the unionized state (equivalent to an 

 extra 8 kcal/mole energy), and this high resonance would indicate an inter- 

 mediate structure in which the center of negative charge lies midway be- 

 tween the two oxygen atoms. Although keto-enol tautomerism occurs 

 (Hofling et al., 1952) in the esters of malonic acid: 



O OH 



-CH^-C-OEt ^ -CH=C-OEt 



it is probably not significant in the malonate ion because it would reduce the 

 electronic resonance. 



