Formation of heavy adducts in esterification of acrylic acid: A DFT study

Esterification of acrylic acid with alcohols and the side reactions were studied at the B3LYP/6-31G** level. The main model reaction is predicted to be endoergic which is in line with experimental findings. It appeared that the activation barriers for the esterification reactions in vacuum are equal to ca. 68 ± 3 kcal/mol, they decrease in presence of the H+ ion to ca. 47.5 ± 3.5 kcal/mol, and are relatively insensitive to change of environment simulated by PCM method (a matter of 0.5 kcal/mol). Out of four side reactions studied, the lowest activation barrier (36.5 ± 1.5 kcal/mol) is for addition of the acrylic acid molecule to double bond in acrylates. Next, relatively easily occurring side reactions are the additions of water and alcohols to acrylates (barriers of ca. 48 ± 1 kcal/mol in presence of the H+ ion). Activation barrier for dimerisation of acrylic acid, i.e., addition of one molecule to the double bond of the other, in catalytic reaction is equal to 56.5 kcal/mol. Finally, the addition of alcohol to the acid dimer (leading to the same product as addition of acid to an acrylate) needs to overcome the 61 kcal/mol barrier. Based on the above results we discuss qualitatively our experimental findings of technology using heterogeneous acid catalysts.