Density functional theory calculations were conducted on the title reactions with water molecules.
Malonaldehyde, acetylacetone, and malonic acid were adopted as reactants. A reaction of malonaldehyde
and (2 + 2) water molecules was found to proceed by a small (ca. 7 kcal/mol) activation energy. Two are
reactant and two are catalyst molecules, respectively. The intramolecular hydrogen bond in the enol form is
disrupted by the intermolecular one. Tautomerization transition-state geometries for acetylacetone and malonic
acid are similar to that of malonaldehyde. Without the enhanced reactivity of water hydrogen-bond networks,
tautomerization reactions undergo large activation energies. Various reaction models of (malonaldehyde)
n
(n
= 2, 3, and 4) have given the energetic result. The keto−enol tautomerization has an analogy with the
bimolecular nucleophilic elimination (E2) mechanism.
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