Mechanisms of formation of hemiacetals: intrinsic reactivity analysis.
The reaction mechanism of the hemiacetal formation from formaldehyde and methanol has been studied theoretically at the B3LYP/6-311++G(d,p) level. In addition to the study of the reaction between the isolated reactants, three different kinds of catalysis have been explored. The first one examines the use of assistants, especially bridging water molecules, in the proton transfer process. The second one attempts to increase the local electrophilicity of the carbon atom in formaldehyde with the presence of a Brønsted acid (H(+) or H(3)O(+)). The last one considers the combined effect of both catalytic strategies. The reaction force, the electronic chemical potential, and the reaction electronic flux have been characterized for the reaction path in each case. In general, it has been found that structural rearrangements represent an important energetic penalty during the activation process. The barriers for the reactions catalyzed by Brønsted acids show a high percentage of electronic reorganization contribution. The catalytic effects for the reactions assisted by water molecules are due to a reduction of the strain in the transition state structures. The reaction that includes both acid catalysis and proton assistance transfer shows the lowest energy barrier (25.0 kJ mol(-1)).
Instituto de Química Médica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain.
Luis Miguel
LM
Azofra
Ibon
I
Alkorta
José
J
Elguero
Alejandro
A
Toro-Labbé
D000080:Acetals / Q000138:chemical synthesis* / Q000737:chemistry; D005557:Formaldehyde / Q000737:chemistry; D000432:Methanol / Q000737:chemistry; D015394:Molecular Structure; D011789:Quantum Theory
D000080:Acetals; D005557:Formaldehyde; D000432:Methanol