The free energy, enthalpy, and entropy changes for the neutralisation of some C-1-substituted 1,2-diaminoethanes

Abstract

Values of the thermodynamic functions ΔG°, ΔH° and ΔS° have been determined for the stepwise protonation of 1-methyl-, 1-ethyl-, 1-phenyl-, and 1,1-dimethyl-substituted 1,2-diaminoethanes. Free energy values were calculated from protonation equilibrium constants at zero ionic strength. These constants were obtained by extrapolation from concentration quotients measured for the ionic strength range 0·04–0·35M, using a glass-electrode technique. Enthalpy changes were determined calorimetrically. The basicities of the C-1-substituted diaminoethanes decrease with increasing chain length and branching for small C-1-substituents, despite the increasing inductive capacity of the substituent. A small thermistor calorimeter, and a novel method for calibrating the glass electrode to measure directly [H⁺] are described.