Measurement and Prediction of Thermochemical Properties:  Improved Increments for the Estimation of Enthalpies of Sublimation and Standard Enthalpies of Formation of Alkyl Derivatives of Urea

This work has been undertaken in order to obtain data on vapor pressures of alkyl derivatives of urea and to revise the group additivity values necessary for predicting their sublimation enthalpies and standard enthalpies of formation at the reference temperature <i>T</i> = 298.15 K. Molar enthalpies of sublimation of urea, 1-methylurea, 1-<i>n</i>-propylurea, 1-<i>n</i>-butylurea, 1-<i>sec</i>-butylurea, 1-<i>tert</i>-butylurea, 1,1-dimethylurea, 1,1-diethylurea, 1,3-dimethylurea, and 1,3-diethylurea were obtained from the temperature dependence of the vapor pressure measured by the transpiration method. Thermochemical investigations of 16 alkyl derivatives of urea available in the literature were collected and combined with our own experimental results to obtain their reliable standard molar enthalpies of formation at <i>T</i> = 298.15 K in the condensed or in the gaseous state. Ab initio calculations of alkyl derivatives of urea have been performed using G3(MP2), and results from the homodesmic reactions are in acceptable agreement with experiment. New results help to resolve uncertainties in the available thermochemical data on homologues of the alkylureas studied.