The urea–phenol(s) systems

Solid–liquid equilibrium has been studied in the binary systems formed by urea with phenol, o-, m-, p-cresol, 2,3-, 2,4-, 2,5-, 3,4-, 3,5-xylenol, and p-cumylphenol by the method involving crystal disappearance temperature measurements and by differential scanning calorimetry. The congruently melting (1:2) complex exists only in the urea–phenol system. The miscibility gap was found in three of the systems viz., urea–2,5-xylenol, –2,6-xylenol, and –p-cumylphenol. An incongruently melting complex and/or plateau-like section of the liquidus curve characterize the SLE diagrams of the other systems. The enthalpy of formation of the hydrogen bonded 1:1 complexes in the binary urea–phenol, o-, m-, p-cresol, and p-cumylphenol mixtures dissolved in 1,2-dichloroethane was evaluated via temperature IR measurements. The enthalpy was found to be about −30 kJ/mol, except for the urea–o-cresol (1:1) complex (−25 kJ/mol). The ab initio HF/6-31G** calculations of stabilization energies for urea–phenol complexes bonded through a linear hydrogen bond show very good agreement with the IR derived ΔH-values.