Do phase transition temperatures Tmp and Tbp obey linear free energy relationships?

Abstract

Normal melting point of a solid is the temperature at which it changes its state from solid to liquid. At the melting point the solid phase and the liquid phase exist in equilibrium. And normal boiling point of a liquid is a property at which the vapor pressure of the liquid becomes equal to the atmospheric pressure. The four types of intermolecular forces: hydrogen bonding, ionic forces, Van der Waals dipole-dipole interactions and Van der Waals dispersion forces (London forces) and sometimes the polarizability affect the melting and boiling points. Hydrogen bonding is one of the key factors that largely affect both melting and boiling points of solids and liquids respectively having functional groups such as OH, NH2 and a most electronegative atom F. And they are also affected by polar electronic effects of the substituents and by the size of the molecule due to the presence of the van der Waals attractions. Using Lindemann's equation Tmp = 4π2mν2c2a2kB and strong foundation of Trouton's two empirical rules ∆Slatent=∆HlatentxDensity273+Tmpand ΔSvaporization = ∆Hvaporization273+Tbp, Taft Linear Free Energy Relationship (LFER) is applied to the temperature of phase transitions (solid to liquid and liquid to vapor) of alkyl alcohols, alkyl amines, alkyl fluorides and aliphatic hydrocarbons. Two loci are observed in each case one with a negative slope for electron donating substituents and the other with a positive slope for electron withdrawing substituents with a minimum at CH3 substituent (Taft σ* = 0.00). The decreasing trends in both the melting and the boiling points with decrease in electron donating power of substituents and the increasing trends in both the melting and the boiling points with increase in electron withdrawing power of substituents are explained with two interpretations of hydrogen bonding in alcohols, amines and fluorides yielding the same dimers.