CHEMICAL PROPERTIES 619 



Hydrogen bonding of the type suggested by Forsen and Nilsson (1961) is 

 represented; this undoubtedly occurs in the pure form but may not in 

 aqueous solution. Proton magnetic resonance and infrared spectra suggest 

 structure (b), but the other forms are not excluded. The restriction of the 

 acetyl group rotation may derive from such hydrogen bonding. 



Ionization 



It is important with regard to the action of dehydroacetate on enzymes 

 and its penetration into cells to determine the predominant forms in aqueous 

 solution at physiological pH. It is usually stated that dehydroacetic acid is 

 a very weak acid, and Wolf and Westveer (1950) remarked that it would 

 exist primarily in the ionized state at pH 9. The acidic property is the result 

 of enolization and each of the enolic forms shown above could lose a proton 

 to form the corresponding anion. However, once this occurs, resonance be- 

 tween the three forms is possible, stabilizing the anion and increasing the 

 acidity and, furthermore, producing a more diffuse negative charge. In order 

 to determine the state of the inhibitor under physiological conditions, titra- 

 tions were done, starting at pH 3.64 (the pH of a saturated solution). It 

 was found that two equivalents of base are taken up between pH 4.2 and 

 6.4, with a mean pK^ of 5.20, so that around pH 7 dianions of the a-pyran 

 and y-pyrone type must be present. 



H,C ^O^ ^O H,C 





O 



-CH, 



General Properties 



Dehydroacetic acid has an absorption peak around 313 m// (Calvin et al., 

 1941), sublimes at 109°, is fairly soluble in organic solvents but poorly sol- 

 uble in water (around 0.25% at 37°). The sodium salt, however, is quite 

 soluble in water (33%) (Wolf, 1950) and sufficiently stable in solution for 

 most purposes. It may be catalytically hydrogenated with PtOa to give the 

 corresponding 3-ethyl compound (Malachowski and Wanczura, 1933), or 

 with Ni under pressure to yield more completely hydrogenated forms, cor- 

 responding to the uptake of 3-5 moles of Hg (Adkins et al., 1931). 



Synthesis 



Dehydroacetic acid was first obtained by Geuther (1866) by distilHng the 

 ethyl ester of acetoacetate, and Conrad (1874) found that a reasonable yield 

 could be obtained by heating this substance under pressure. The present 



