Quantum-chemical research of mechanism of the reaction dehydration of 2,3-dimethylbutan-2,3-diol and it’s hexahydrate
DOI:
https://doi.org/10.15587/1729-4061.2013.18864Keywords:
2, 3-Dimethylbutan-2, 3-diol, 3-Dimethylbuta-1, 3-diene, quantum-chemical calculations, MOPAC2009, Winmostar, RM1Abstract
It is established that dehydration of 2,3-dimethylbutane-2,3-diol passes two successive stages. During the first stage, the atom of hydrogen H (11) (state DO) approaches the atom of oxygen O (7) of hydroxogroup with the formation of a tense four-member cycle C (1) H (11) O (7) C (2) (transition state TS1), destruction of which leads to the formation of enol EO and a water molecule (state EO + H2O). At the second stage, in the molecule of enol EO the hydrogen atom H (20) approaches the oxygen atom O (8) of another hydroxogroup with the formation of the tense four-member cycle C (6) H (20) O (8) S (3) (transition state TS2), destruction of which leads to the detachment of the second water molecule and the formation of diene DE (state DE+2H2O).
The difference between the energies of the formation of molecular structures of enol EO and 2,3-dimethylbuta-1,3-diene DE is positive that indicates the endothermicity of reaction. At the same time, the energy barriers of both stages are proportionate (65.6 and 61.8 kcal/mol, respectively), and the value of the released energy is about 54 kcal/mol, which corresponds to the heat of the water molecule formation. Moreover, the formation of the transition state for these reactions is displaced towards the formation of products and is of late nature. This indicates that the destruction of the bond C-H and formation of the bond O-H (transition of the hydrogen atom of the methyl group to the oxygen atom of hydroxogroup) passes faster than destruction of the bond O-C of enol whith simultaneous formation of the bond C=C 2,3-dimethylbuta-1,3-diene.
Reaction of dehydration of hexahydrate 2,3-dimettylbutan-2,3-diol (DO) passes in the presence of the water molecule, the oxygen atom of which acts as an element of the transmission of hydrogen atom of the methyl group DO to the oxygen atom of the hydroxyl DO, resulting in significantly reduced energy barriers of the stages of dehydration reaction of hydrated 2,3-dimethylbutan-2,3-diol in comparison with anhydrous.
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