Dissociation of nitrophenols in pyridine. Thermodynamic dissociation constant and homoconjugation constant

Pyridine appears to be an interesting solvent for quantitative analysis of weak acids, but systematic studies concerning the dissociation mechanism of acid in this solvent have not yet been attempted. In the present paper, the thermodynamic dissociation constants of simple ion pair K(HA) and the formation constants of homoconjugate ion K(HA2-) have been determined by measuring the conductivity of nitrophenols and the solubility of the corresponding potassium salts in the presence of acids in pyridine. The limiting molar conductivity of pyridi nium ion λ∞(PyH+) must be known in order to clarify the dissociation equilibria of acid in pyridine. The value of λ∞(PyH+) at 25°C in pyridine was calculated to be 43.72 S·cm2·mol-1 from the conductometric data of pyridinium and tetrabutylammonium perchlorates (Table 1), in addition to the previous datum (λ∞(Bu4N+)=21.87 S·cm2·mol-1). The limiting molar conductivity of the homoconjugate ion was estimated on the basis of the conductivity of tetrabutylammonium m-nitrophenolate in acid solutions (Table 2). The values of K(HA2-) were determined by using the solubility data of potassium nitrophenolates in acid solutions and electrolytic conductivities of these saturated solutions. The dissociation constants K(HA) were determined from the conductometric data of nitrophenols and K(HA2-) values (Table 7). It was found that ( 1 ) K(HA) of nitrophenol becom es larger with an increase in the number of substituent groups, ( 2 ) the order of acid strength is picric acid>2, 4-dinitrophenol(2, 4-DNP) >2, 6-DNP >2, 5-DNP, and p-nitrophenol(p-NP)> ο-NP> m-NP, ( 3 ) the substitution of the nitro group at meta position depresses the dissociation of acid, while enhances the formation of the homoconjugate ions, and ( 4 ) picric acid, 2, 4-DNP and 2, 6-DNP are hardly homoconjugated.