R. Zsanett Boros, László Farkas, Károly Nehéz, Béla Viskolcz and Milán Szőri
The most commonly applied industrial synthesis of 4,40-methylene diphenyl diamine (4,4’-MDA), an important polyurethane intermediate, is the reaction of aniline and formaldehyde. Molecular understanding of the 4,4’-MDA formation can provide a strategy to prevent side reactions. In this work, a molecular mechanism consisted of eight consecutive, elementary reaction steps from anilines and formaldehyde to the formation of 4,40-MDA in acidic media is proposed using an accurate G3MP2B3 composite quantum chemical method. Then G3MP2B3-SMD results in aqueous and aniline solutions were compared to the gas phase mechanism. 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+). Competing reactions of the 4,4’-MDA formation are also thermodynamically analyzed such as the formation of 2,4-MDAH+, 3,4-MDAH+. AMH+ can be formed through a loose transition state, but it becomes a kinetic dead-end, while the formation of a significant amount of 2,4-MDA is plausible through low-lying transition state. 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 pKa calculation.