Reaction Mechanisms and Rate Constants of Auto‐Catalytic Urethane Formation and Cleavage Reactions

Abstract

The chemistry of urethanes plays a key role in important industrial processes. Although catalysts are often used, the study of the reactions without added catalysts provides the basis for a deeper understanding. For the non‐catalytic urethane formation and cleavage reactions, the dominating reaction mechanism has long been debated. To our knowledge, the reaction kinetics have not been predicted quantitatively so far. Therefore, we report a new computational study of urethane formation and cleavage reactions. To analyze various potential reaction mechanisms and to predict the reaction rate constants quantum chemistry and transition state theory were employed. For validation, experimental data from literature and from own experiments were used. Quantitative agreement of experiments and predictions could be demonstrated. The calculations confirm earlier assumptions that urethane formation reactions proceed via mechanisms where alcohol molecules act as auto‐catalysts. Our results show that it is essential to consider several transition states corresponding to different reaction orders to enable agreement with experimental observations. Urethane cleavage seems to be catalyzed by an isourethane, leading to an observed 2nd‐order dependence of the reaction rate on the urethane concentration. The results of our study support a deeper understanding of the reactions as well as a better description of reaction kinetics and will therefore help in catalyst development and process optimization.

Connecting Gas-Phase Computational Chemistry to Condensed Phase Kinetic Modeling: The State-of-the-Art

“…Gertig et al [ 312 ] performed a computational study of urethane polymerization/depolymerization coupled with the experimental study of urethane cleavage for non-catalytic systems, assuming the reaction mechanisms presented in the literature. As the authors aimed to study a broad range of reaction conditions, i.e., temperature, alcohol concentration, and reaction medium, they employed the COSMO-RS solvation model for a higher accuracy.…”

“… Global reaction of urethane bond formation being the basis of polyurethane step-growth polymerization. Mechanism of autocatalysis by alcohol molecules as explored by Gertig et al [ 312 ]. …”

Catalyzed Reaction of Isocyanates (RNCO) with Water

“…1,[34][35][36][37][38][39][40][41][42] A key reaction involved in polymer formation is alcoholysis via the HNCO species to produce carbamates. 42,43 The rate of this reaction can be significantly modified in many ways by changing substituents on the RNCO or ROH reactants, [44][45][46] autocatalysis of the reactants, [47][48][49][50][51][52] solvent selection, 50 or via other more efficiently designed catalysts [53][54][55][56] such as organotin. 57,58 It is well established that HNCO will react with water across either the N − − C or C − − O bonds, producing carbamate and imidic acid, respectively.…”

“…In addition to these works, there are plenty of examples in the literature of specific R 1 NCO + R 2 OH reactions studied at various levels of theory. 43,50,51,69,70 All of this previous research emphasizes the importance of clearly understanding how substituted RNCO species influence the electronic structure and energetic landscape of the RNCO + H 2 O reactions. Despite its significant importance to industrial chemistry, the literature lacks a comprehensive and reliable theoretical benchmark for this system and a detailed analysis of the relationship between the isocyanate substituents and the electronic structure features of these reactions.…”

“…Third, we systematically study how substituents influence the electronic structure of important cooperative catalytic pathways (i.e. multiple water or RNCO molecules) that previous research [50][51][52] has suggested as very important for these systems. Finally, our sophisticated ab initio predictions are analyzed with Natural Bond Order analyses to provide detailed understanding of the electronic structure manifest in each process in order to characterize relationships that could generalize to larger or more complex isocyanate reactions.…”