150 



Essays in Biochemistry 



Table 3. Apparent Acidic Dissociation Constants of Some 5-amino- 

 tetrazole Derivatives in 50% Methanol * 



R 2 N— CN 4 H 



* All values taken from refs. 3, 10, 11, 12, 13. 



f Determined potentiometrically in water at 25°C. 



$ The first dissociation is as a strong organic acid, and K\ is approximately 

 1 X 10 _1 ; in the second dissociation, K* is 9 X 10 _7 . u Both in aqueous solu- 

 tion at 25°C. 



group is also very weak. Both of these observations may be attributed 

 to the type of resonance invoked for aniline. 8 The free pair of elec- 

 trons of the amino nitrogen may be involved in the resonance of the 

 attached ring system. In such a resonating system, the electronega- 

 tivity of the ring would be decreased and dissociation of a proton 

 from the ring would become more difficult causing a decrease in acidity. 

 Since charge separation imposes ammonium ion character on the 

 amino group, the basicity of this group should also be decreased. 

 Several forms of this type that may contribute to the resonance hybrid 

 are illustrated in scheme XL 



H H 



\ / 



N 



N 

 N= 



/ 



C 



N— II 



=N 



H 



H 



N 



+ 



H H 



\ / 



N+ 



C C 



/ \ / \ 



N N— H N N— H 



I I - I! I 



N= =N N- — N~ 



(XI) 



The effect upon the acidity of replacing the hydrogens of the amino 

 group with alkyl groups appears to be primarily steric in nature. The 



