![]() (see left side of Figure 1), it has not been clarified entirely. Although, proposed reaction mechanism for current MDA synthesis is presented in Ref. However, neither of the catalytic MDA productions have gone beyond the laboratory stage. To avoid the use of corrosive hydrochloric acid and the formation of large amount of salt solution as waste, several attempts had been made to replace the current technology with catalytic process using solid acids, zeolites, delaminated materials, ionic liquids, or ion exchange resins as catalysts. The most commonly applied industrial MDA synthesis is the reaction of aniline with formalin in the presence of hydrochloric acid under mild (60–110 ☌) reaction conditions. The acid strength of the key intermediates such as N-methylenebenzeneanilium, PABAH +, 4-methylidenecyclohexa-2,5-diene-1-iminium, and AMH + was estimated by relative p K a calculation. AMH + can be formed through loose transition state, but it becomes kinetic dead-end, while formation of significant amount of 2,4-MDA is plausible through low-lying transition state. Competing reactions of the 4,4′-MDA formation is also thermodynamically analyzed such as the formation of 2,4-MDAH +, 3,4-MDAH +. Based on the gas phase calculations standard enthalpy of formation, entropy and heat capacity values were evaluated using G3MP2B3 results for intermediates The proposed mechanism was critically evaluated and important side reactions are considered: the competition of formation of protonated p-aminobenzylaniline (PABAH +), protonated aminal (AMH +) and o-aminobenzylaniline (OABAH +). Then G3MP2B3-SMD results in aqueous and aniline solutions were compared to the gas phase mechanism. ![]() In this work, a molecular mechanism consisted of eight consecutive, elementary reaction steps from anilines and formaldehyde to the formation of 4,4′-MDA in acidic media is proposed using accurate G3MP2B3 composite quantum chemical method. Molecular understanding of the 4,4′-MDA formation can provide strategy to prevent from side reactions. ![]() The most commonly applied industrial synthesis of 4,4′-methylene diphenyl diamine (4,4′-MDA), an important polyurethane intermediate, is the reaction of aniline and formaldehyde. ![]()
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