QUANTUM-CHEMICAL RESEARCH OF MECHANISM OF THE REACTION DEHYDRATION OF 2,3-DIMETHYLBUTAN- 2,3-DIOL AND IT’S HEXAHYDRATE

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Both in laboratory practice and in industry the technologically perspective method of DMB receiving during the catalytic dehydration of 2,3-dimethylbutane-2,3-diol is considered to be the one, to which the major attention is paid.[1 -3].Industrially available acetone is consideered to be the original stuff in this process, which turns into 2,3-dymetylbuta-2,3-diol hexahydrate by reduction.The 2,3-dymetylbuta-2,3-diol hexahydrate is dehydrates by azeotropic rectification with benzene [4].

Purpose of work
In this work the quantum-chemical investigation of mechanism of 2,3-dimethylbutane-2,3-diol dehydration was conducted for the sake of the opportunity to identify the possibility of hexahydrate 2,3-dimethylbutane-2,3-diol usage without stage of prior dehydration.

Method of quantum-chemical calculations
The dehydration process of 2,3-dimethylbutane-2,3-diol (DO), or its industrially available analogue -hexahydrate 2,3-dimethylbutane-2,3-diol is endothermic.It is conducted in the result of heating and leads to creating of one water molecule and intermediate compound of 2,3-dymetylbut-3-en-2-ol (enol, EO) and is accompanied by subsequent splitting of another water molecule from EO and by 2,3-dymetylbuta-1, 3-diene (diene, DE) creating [5] (Fig. 1): (1) The quantum-chemical modelling of 2,3-dimethylbutane-2,3-diol (DO) and its hexahydrate dehydration was carried out by semiempirical program MOPAC2009 [6] and the graphic interface Winmostar [7].Geometry optimizaetion, at the same time as calculation of heats of molecular structures formation and activation systems in transition states of the reaction system were carried out by semiempirical method RM1 using unrestricted Hartree-Fokka (UHF) with the coefficient for energy rationing (GNORM) within 0.01-0.5 kcal/mol.The dependence of system potential energy determination from coordinate of the reaction were conducted with the step of 0.02 Å (STEP = -0.02).
The 2,3-dimethylbutane-2,3-diol dehydration is conducted through two consecutive stages.Fig. 1 gives energy profiles of two stages of dehydration.Figure 2 shows the reaction conditions model of it.
At the first stage the hydrogen atom H(11) comes to the hydroxy group atom of oxygen O(7) creating at the same time the tense of four membered rings C(1) N( 11) O( 7   As it has been shown at the graphic of diol DO dehydration energy profiles (Fig. 1) and in the data table 2 the difference between the power of formation of enol EO and diene DE molecular structure is positive what indicates on endothermic reaction.At the same time, the energy barriers of both stages are proportionate 65.6 and 61.8 kcal/mol, respectively.
Energy released value is about 54 kcal / mol, which is equal to the power of water molecule formation (Table 1).
In addition, the formation of the transition state for these reactions goes closer to of the products formation and has the late character [8,9].This indicates that the destruction of C-H bond and the formation of O-H bond (transition the hydrogen atom from methyl group to the hydroxy group atom of oxygen) is faster than the destruction of O-C bond of enol with the formation of C=C bond of diene at once.
The water molecule H(11) O( 7) H(21) are placed accordingly to enol molecule in such a way that it leads to the formation of the intermolecular complex EO:H 2 O due to weak (C(6)-H(20) ... O (7)) and medium (O(7)-H(21) … O( 8)) hydrogen bonds (Fig. 3) [10].7) H(21) in six membered ring transition state TS2 (H 2 O) (Fig. 3) reduces ring tension in comparison with the four membered transition state TS2 (Fig. 1) for 6.2 kcal / mol.∆ f H 298 =63.9-57.7=6.2 kcal/mol In the transition state TS2 (H 2 O) it acts as an element of transmission.So, the atom of hydrogen H(20) from atom C(6) methyl group becomes the atom of oxygen O (7) and the atom of hydrogen H(21) transmits to atom of oxygen O(8) of enol molecule hydroxy group.
The dehydration of diol DO hexahydrate also runs in two stages, but, unlike dehydration of anhydrous diol DO, it is performed in the presence of water (DO:H 2 O).Water molecule of anhydrous diol DO:H 2 O, similar to the dehydration of enol EO: H 2 O water complex (Fig. 3), in both cases acts as an atom of hydrogen transmission element from the methyl group of DO through water hydroxy group atom of oxygen to the DO hydroxyl atom of oxygen.As a result less tense six membered ring (Fig. 5) was created comparing to much more tense four membered ring (Fig. 2 (TS1)).

Fig. 5. Model of hydrated diol (DO: H 2 O)
The creating of TS1 (H 2 O) and TS2 (2H 2 O) transition states (Fig. 6) of two stages hydrated diol DO:H 2 O dehydration also runs by the concerted mechanism, as for complex EO:H 2 O (Fig. 4), and potential energy surfaces for both stages also have the saddle points, energy barriers of which correspond to some transitional states (Table 3).
Energy barriers, during both stages of transition creating (first stage -TS1 (H 2 O) from the initial complex DO :

Conclusions
In this work it has been established that in the result of quantum-chemical research the anhydrous mechanism of dehydration of 2,3-dimethylbutane-2,3-diol and it`s hexahydrate is passed two consecutive stages.The reaction is proceeded in the presence of water, which makes the transmission of methyl group atom of hydrogen to the DO hydroxyl atom of oxygen in the case of using hexahydrate DO for dehydration.This effect of transmission significantly reduces energy barriers stages of hydrated DO dehydration, in comparison with the anhydrous.

Fig. 1 .
Fig. 1.Energy profiles of two stages of anhydrous diol DO dehydration

Fig. 2 .
Fig. 2. Models of reaction states of anhydrous diol DO dehydration

Table 2
Heat of formation (ΔfH298, kcal / mol) and bonds order (n) for reactions states of anhydrous diol dehydration

Table 3 Heats
of molecular structures and transition states (Δ f H 298 , kcal / mol) formation and bonds order (n)of two stages hydrated diol DO: H 2 O dehydration