The influence of hydration on base strength. Part V. Hydrazines and oxyamines. Heats of solution and thermodynamics of ionization of the 20 methyl and ethyl substituted hydrazines. Isomerization equilibria for unsymmetrical hydrazinium ions. Substituent constants (σ* values) for amino-groups

Abstract

Calorimetric heats of solution in water and in 0·1 M-hydrochloric acid are presented for the 20 methyl and ethyl substituted hydrazines. Their difference is given as the heat of neutralization of each hydrazine. The base strengths (pK_a) of these hydrazines were determined at five temperatures between 10 and 40° and at several ionic strengths, permitting extrapolation to values at zero ionic strength (pK_a⁰) and calculation of the heats of ionization. An equation is presented by means of which the pK_a⁰ data are correlated with a root mean square deviation ca. 0·15 pK unit in terms of inductive substituent constants (Taft σ*) and hydration parameters dependent on the number of hydrogens on each nitrogen of a hydrazinium ion. It is shown how the equation can be used to calculate heats of ionization, changes in heat capacity on ionization of amines and hydrazines, and equilibrium constants for tautomerization of hydrazinium ions. The changes in heat capacity appear to be only a function of the difference between the σ* constants for hydrogen and the substituents replacing it. Calculated equilibrium constants for the tautomerization: R¹R²NHNR³R⁴⇌ R¹R²NNHR³R⁴ are presented for 26 unsymmetrical hydrazines, including phenylhydrazine. Published pK_a⁰ data for six oxyamines are correlated with a root mean square deviation of 0·08 pK unit.