Predictions of the enthalpies of protonation of amines. Log K, ΔH, and ΔS values for the protonation of ethylenediamine and tri-, tetra-, penta-, and hexa-methylenediamine

Abstract

Values of the thermodynamic functions ΔG, ΔH, and ΔS for the protonation of the polymethylenediamines H₂N[CH₂]_nNH₂(n= 2–6), i.e. ethylenediamine and tri-, tetra-, penta-, and hexa-methylenediamines have been determined, by means of potentiometric and calorimetric measurements at 25° in 0·5M-potassium nitrate solution. The variations in the values of the thermodynamic functions along the series have been interpreted in terms of the following : (i) the effect of the substituents NH₂ or NH₃⁺, (ii) the length of the aliphatic chain, (iii) solute–solvent hydrogen-bond formation, and (iv) the rigidity of the aliphatic chain. A formula for predicting the heats of protonation of aliphatic mono- and di-amines is proposed.