M.L. Frenkel

Thermochemistry of alkyl derivatives of urea

A thermodynamic study was carried out on urea and 12 of its alkyl derivatives by the methods of combustion calorimetry, vapor formation, and differential scanning calorimetry. The numerical data obtained permitted determination of the enthalpy of formation of the above substances under standard conditions and in the gas phase. An additive scheme was proposed for calculating the thermodynamic properties of alkyl substituted urea.

Additivity of the enthalpies of formation of urea derivatives in the crystalline state

Abstract An isothermally jacketed calorimeter was used to measure the energies of combustion of methylurea (I), ethylurea (II), 1,1-dimethylurea (III), 1,1-diethylurea (IV), isopropylurea (V), tertbutylurea (VI), 1,3-ditertbutylurea (VII), secbutylurea (VIII), butylurea (IX), tetramethylurea (X), urea (XI), phenylurea (XII), and 1,3-diphenylurea (XIII). Based on experimental enthalpies of combustion the standard molar enthalpies of formation ΔfHmo/(kJ·mol−1) were calculated to be I, −(332.78±0.75); II, −(357.76±0.74); III, −(319.06±0.68); IV, −(372.21±1.13); V, −(389.49±1.27); VI, −(414.73±0.87); VII, −(499.81±4.22); VIII, −(413.23±1.50); IX, −(419.48±3.23); X, −(262.17±1.06); XI, −(333.13±0.69); XII, −(218.57±2.38); and XIII, −(116.83±4.43). The possibility of applying calculation methods to estimate the enthalpies of formation of substituted urea derivatives in the condensed state is analysed and two versions of additive schemes for calculation of ΔfHmo are considered.

Thermodynamic properties of cyclohexanol and cyclohexanone

Abstract A flow system was used to study gas-phase equilibria of the cyclohexanol dehydrogenation: in the temperature range from 467.5 to 533.8 K over a catalyst consisting of copper and zinc oxides, and of hydrogen redistribution during in the temperature range 453.1 to 533.1 K over a magnesium oxide catalyst. The values of ΔrHmo and ΔrSmo obtained from the temperature dependences of the equilibrium constants were (63.4 ± 2.3) kJ · mol−1 and (119.9 ± 4.6) J · K−1 · mol−1 for reaction (1) and −(9.9 ± 1.9) kJ · mol−1 and −(5.8 ± 3.9) J · K−1 · mol−1 for reaction (2). Based on molecular and spectroscopic information, statistical calculations of the thermodynamic properties of cyclohexanol and cyclohexanone were made over the range 298.15 to 1000 K for the ideal-gas state, taking into account the conformations of the compounds. The calculations agree with all experimental thermodynamic properties. An original technique is suggested for calculation of the contributions of conformers to the heat capacity.

The effects of isomerism on the vibrational spectra and thermodynamic characteristics of biuret in the gas phase

Abstract A theoretical study of the IR spectra of cis- and trans-isomers of biuret, based on coupled calculations of vibrational frequencies as a valence force field approximation and of absorption band intensities by the CNDO/2 method, is reported. Calculated thermodynamic functions for biuret agreed with experimental thermochemical data. For calculating the vibrations of an isolated biuret molecule, the experimental vapour-phase IR absorption spectra were used.

Thermodynamics of diastereomeric transformations of alcohols with different carbon-skeleton structures

Abstract The gas-phase equilibria of diastereomers have been investigated in a flow system. The following reactions have been studied: (1), threo to- erythro form of 3-methylpentanol-2; (2), cis -to- trans form of 2-, 3-, and 4-methylcyclohexanol; (3), menthol-to-neomenthol; and (4), borneol-to-isoborneol. The values of ΔH r o and ΔS r o for (1) and (2) were calculated from the temperature dependence of the equilibrium constants making use of the least-squares method. The values of ΔG r o (503 K) for transformations (3) and (4) were also determined. It was shown that the enthalpy changes of the reactions increase in the alcohol series: acyclic-to-cyclic-to-bicyclic and this evidently is accounted for by the decrease of degrees of freedom of internal rotation. The determined dependence can be of a great importance for the development of additive schemes for calculating ΔH f o for cyclic compounds.