464 



NIACIN 



The second group of isobestic points at 2465 A. and 2710 A. covering a 

 range of pH values from 4.4 to 13.0 is associated with the acid equihbrium 

 of nicotinic acid. 



+ H3O+ 



^^COOH ^/ ^— COO- 



+ H2O ^ 



The acid and base dissociation constants as calculated by two methods 

 are shown in Table IV. Ostwald^^ reported the acid dissociation constant 

 of nicotinic acid as Ka = 1.4 X 10"^ (25°). 



/. Chemical Properties 



The fact that nicotinic acid is an amino acid is often overlooked. Al- 

 though not an a-amino acid, it is nevertheless at once both a carboxylic 

 acid and an amine and, accordingly, exhibits the characteristic properties 



Table IV 

 Thermodynamic Dissociation Constants for Nicotinic Acid at an Average 



Temperature of 22° ± 2° 



Isoelectric 

 pKa Ka pKb Kb point 



Method 1 

 Method 2 



1 . 12 X 10- 

 1.23 X 10- 



10.45 

 10.45 



3.55 X 10-11 

 3.55 X 10-" 



4.25 

 4.23 



associated with the simultaneous presence of these two groups in a molecule 

 (although it does not form a zwitterion in solution^*). It is soluble in both 

 acids and bases, forming quaternary salts with the former and carboxylic 

 acid salts with the latter. It possesses an isoelectric point (pH 4.23-4.25). 

 Its high melting point is characteristic of amino acids. Its strength as an 

 acid exceeds its strength as a base. A 1 % aqueous solution has a pH of 

 about 3.5» 



( 1 ) Properties associated with its basic character 



Nicotinic acid, like other compounds containing a basic amino group, 

 forms quaternary ammonium compounds. Thus, hydrogen chloride con- 

 verts it to nicotinic acid hydrochloride, a white crystalUne solid readily 

 soluble in water (see Tables I and II) but insoluble in benzene and ether. 

 It has a melting point at 272° (with decomposition).^ 



-COOH 



+ HCl 



\N/ 



-COOH 



/ \ 

 H CI 



«5 W. Ostwald, Z. physik. Chem. 3, 369 (1889). 



