Theoretical Study of the Thermal Decomposition of Urea Derivatives

Abstract

An extensive theoretical study of the thermal decomposition of alkyl- and phenylureas, which are widely used in the pesticides, pharmaceuticals, and materials industries, has been carried out using electronic structure calculations and reaction rate theories. Enthalpies of formation and bond dissociation energies (BDE) of 11 urea derivatives have been calculated using different levels of theory (CBS-QB3, CCSD­(T)­/CBS­//M06-2X­/6-311++G­(3df,2pd), and CBS-QM062X) according to the size of the system. Potential energy surfaces for the unimolecular decomposition pathways of these urea derivatives were also systematically computed for the first time. Several pericyclic reactions can be envisaged, as a function of the size and the nature of the N substituents, and all of these pathways were explored. Our calculations show that these compounds are solely decomposed by four-center pericyclic reactions, yielding substituted isocyanates and amines, and that initial bond fissions are not competitive. Based on the set of urea derivatives studied, a new reaction rate rule for their thermal decomposition was defined and involves the nature of the transferred H atom (primary or secondary/alkyl or benzyl) and the nature of the N-atom acceptor (primary, secondary, or tertiary). This new reaction rate rule allows us to determine the product branching ratios in the thermal decomposition of a given urea derivative and its total rate of decomposition. Applications on urea derivatives used in the chemical industry are presented and illustrate the usefulness of this new rate rule that allows to predict the previously unknown thermal decomposition kinetics of a large number of these compounds.

Combustion in a Sustainable World: From Molecules to Processes

“…Niu et al used theoretical methods to analyze the destruction channels of several nitrate esters in both the gas and liquid phases to build a kinetic model for predicting the stability and decomposition behavior of such compounds . Another nitrogen-containing system that has been studied concerns urea , a compound widely used in the pharmaceutical, pesticide, and materials industry . Honorien et al conducted a theoretical study of the thermodynamics and decomposition behavior of 11 urea derivatives .…”

“…40 Another nitrogen-containing system that has been studied concerns urea, a compound widely used in the pharmaceutical, pesticide, and materials industry. 41 Honorien et al conducted a theoretical study of the thermodynamics and decomposition behavior of 11 urea derivatives. 41 As a result, a rate rule could be established to predict product branching ratios and further important details in the thermal decomposition kinetics of such compounds, of use in assessing associated environmental and health effects.…”

“…41 Honorien et al conducted a theoretical study of the thermodynamics and decomposition behavior of 11 urea derivatives. 41 As a result, a rate rule could be established to predict product branching ratios and further important details in the thermal decomposition kinetics of such compounds, of use in assessing associated environmental and health effects. 41 Yet another element is present in environmentally hazardous f luorine-containing H,C,O,F compounds whose thermochemistry and kinetics have been subjected to similar methods as detailed above.…”

“…41 As a result, a rate rule could be established to predict product branching ratios and further important details in the thermal decomposition kinetics of such compounds, of use in assessing associated environmental and health effects. 41 Yet another element is present in environmentally hazardous f luorine-containing H,C,O,F compounds whose thermochemistry and kinetics have been subjected to similar methods as detailed above. 42,43 Such "forever chemicals" represent an important class of persistent pollutants from uses such as flame retardants, surface coatings, food packaging, and refrigerant fluids that can be thermally destroyed by incineration.…”

Preparation of polysilsesquioxane-based CO2 separation membranes with thermally degradable succinic anhydride and urea units

“…A possible mechanistic interpretation of this phenomenon is presented in Figure 5b. It is known that monosubstituted urea undergo thermal degradation to form isocyanate with the liberation of NH3 [26,27]. The hydrolysis of the isocyanate unit with water results in the formation of amine and CO2.…”