Mechanistic insights into the self-esterification of lactic acid under neutral and acidic conditions

Lactic acid (LA) is frequently used as the starting material for the production of biodegradable and/or bio-compatible polymers. We study several mechanisms which have been envisaged as important for the formation of lactide (LT, the cyclic diester of LA) in toluene solutions, both under neutral and acidic con-ditions. Quantum mechanical calculations of molecules and aggregates have been performed at the DFT level. We determine the free energies and molecular geometries of the reactants, products, the transition states, and intermediates. Among many proposed mechanisms, we confirm that, under neutral conditions, there is no promising reaction at the prevailing experimental conditions. This is to a large extent due to entropic effects. Under acidic conditions, on the other hand, both initial conversion of LA to dimers and the subsequent conversion to lactide will proceed, in particular if additional LA molecules are available (asynchronous concerted trimolecular reaction). However, to be selective towards LT, the systems should not contain an excess of LA, a condition that could be met in suitable porous media.

Automated summary

In this study, quantum mechanical calculations were performed to investigate the formation mechanisms of lactide from lactic acid in toluene solutions under neutral and acidic conditions. The results showed that there is no promising reaction under neutral conditions, while under acidic conditions, lactic acid can be converted into dimers and then further converted into lactide, especially when additional lactic acid molecules are available. However, to selectively produce lactide, the system should not contain an excess of lactic acid, which can be achieved in suitable porous media.