A Theoretical Study on the Phosgenation of 2,4-Toluenediamine (2,4-TDA)

Abstract

Industrially relevant phosgenation mechanisms of 2,4-toluenediamine (2,4-TDA) were investigated using G3MP2B3 model chemistry. Six reaction pathways had been explored, which resulted in the formation of toluene diisocyanate (2,4-TDI) including different scenarios of the ‘phosgenations first’ and ‘consecutive phosgenations’ mechanisms in both gas and condensed phases. Two possible ‘phosgenations first’ mechanisms show superior to the others in terms of energy, regardless of which phases are considered. Due to the o-dichlorobenzene (ODCB) solvation, the reaction barriers are dramatically reduced compared to the gas-phase reaction mechanism and the solvent effect can be described by linear relationship. Standard enthalpy of formation value was also recommended for 2,4-TDA (59.3 kJ/mol) and 2,4-TDI (−94.1 kJ/mol), as well as for the gas-phase intermediates (IM).

An Ab Initio Investigation on Relevant Oligomerization Reactions of Toluene Diisocyanate (TDI)

“…A similar trend was also observed for the reaction enthalpy of the 1-2-dimers (20.0 kJ/mol vs. 16.0–27.4 kJ/mol). As has been seen in the case of phosgenation of TDA [ 6 ], the solvent effect on the reaction enthalpies can be described as a linear relationship: Δ H 0 G3MP2B3 (ODCB) = 1.11Δ H 0 G3MP2B3 (gas)—6.5 kJ/mol, although the fitted parameters are different here (system dependent). According to this, the presence of the solvent makes the reactions more exothermic than they are in the gas phase.…”

“…The Δ f,298.15K H 0 (g) readings were obtained with an atomization scheme (AS) using the standard enthalpy of formation for atoms given at CCCBDB [ 33 ]. The accuracy of the protocol for measuring the enthalpy of formation has been proven for similar systems [ 6 , 23 ].…”

“…Due to the nitration, a mixture of 2- and 4-nitrotoluene isomers is formed first and this mixture is then further nitrated in a continuous process which mainly results in the formation of 2,4- and 2,6-dinitro-toluene isomers. Subsequent hydrogenation transforms the nitro groups into amine groups forming toluene diamines (TDA) which are treated further and converted to TDI isomers via phosgenation [ 6 ]. However, it should be noted that 2,4-TDI and 2,6-TDI are two commercial isomers among the six possible TDI structures and 80/20 mixtures of these isomers have the highest industrial importance [ 5 ].…”

“…This work aims to further explore the reaction mechanisms of dimerization of 2,4-TDI and 2,6-TDI using an accurate and robust G3MP2B3 model chemistry (which has an average absolute deviation of 5.23 kJ/mol for 299 energies of the G2/97 test set [ 21 ]) and implicit SMD solvent model, a combination which has already been successfully used for similar systems [ 6 , 22 , 23 ]. Furthermore, it also studies the mechanism for the formation of the TDI trimer based on G3MP2B3 and B3LYP/6-31G(d) calculations, which are described in detail in the next section.…”

An automated method for graph‐based chemical space exploration and transition state finding

“…The reaction mechanisms are in good agreement with those reported in previous studies. [64][65][66][67][68][69] By inspecting activation energies in paths connecting the reactant and the product for the chemical systems shown in Figure 4 in their respective reaction networks, we have found that the minimum energy paths agree with the expected ones described to allow for posterior successful optimizations of transition states with the higher DFT-B3LYP level of theory.…”