The Tishchenko reaction, which is based on a series of methods to obtain esters from the respective aldehydes were developed, raises much interest due to its numerous useful advantages. After discovering its unique features, enabling the effect of stereoselectivity of its course to be obtained, the Tishchenko reaction was rediscovered and appreciated in the 1990’s, specifically in connection with its features and applications in the synthesis of pure enantiomers, that had been demonstrated not long before.

A number of modified Tishchenko [1] reactions leading to various products, depending on the carbonyl substrate or type of catalysts used, are known.

In connection with the high reactivity of the aldehydes as their specific feature, it is necessary to select the suitable reaction conditions in planning the course of the synthesis in order to reduce the impact of side processes [2] - [4] . The most widely known side reactions, accompanying typical conditions of the Tishchenko reactions include: aldol condensation [5] , Tollens reaction [6] , Canizzaro reaction [7] , oxidation by means of atmospheric air, transesterification [8] , hydrolysis of reaction products [9] , the Meerwein-Ponndorf-Verley reduction (MPV reduction) [10] [11] , the Oppenauer oxidation reaction [12] . The equilibrium between the Tishchenko reaction and side reactions may be controlled by suitably selecting the catalyst, temperature, and type of solvent.

The reaction which is the subject of this characterization is also known as the Claisen-Tishchenko reaction in connection with the series of experiments with benzaldehyde and sodium alcoholates, carried out by Claisen in 1887. As mentioned earlier, the Tishchenko aldol condensation runs in the alkaline catalyst environment and concerns aldehydes having a hydrogen atom α, determining the course of the aldol condensation. The reaction leads to the formation of monoesters of 1,3-diols.

Owing to its commercial importance, the above mentioned modification of the Tishchenko reaction has been investigated rather extensively [13] . The major reaction product is a mixture of isomers of 1,3-glycols, widely used as coalescents in the coating industry and as lubricants [14] .

The system of two isomeric esters of isobutyric acid is formed in the Tishchenko aldol condensation reaction, during which trimerisation of three molecules of isobutyric aldehyde occurs in accordance with the following general stoichiometric equation:

The process of trimerisation starts with a reaction of two molecules of 2-methylbu- tanal, running according to the aldol condensation mechanism, leading to the formation of the aldol: 3-hydroxy-2,2,4-trimethylpentanal [15] - [18] .

Based on the analysis of the possible reaction paths during the process of isobutanal condensation in the presence of an alkaline catalyst as well as experimental findings signalled in patent literature [19] and own research [20] [21] , a series of reactions were selected as ones representing the most probable mechanism of isobutanal condensation, forecasting the occurrence of the following steps:

1. Aldol condensation of 2 molecules of isobutanal.

1)

2. Condensation and disproportioning of isobutanal and aldol obtained in Reaction 1).

2)

3. Transesterification with diester formation.

3)

4. Partial hydrolysis of diester.

4)

5. Hydrolysis of monoester.

5)

6. The Canizzaro reaction of the aldol obtained in step 3).

6)

Acceptance of the proposed reaction mechanism for further studies is empirically justified by the findings of GC/MS analyses of samples of the reaction mixtures, collected from various steps of the process [14] .

Isobutanal (2-methylpropanal)-pure, from GA ZA Kędzierzyn.

3-Hydroxy-2,2,4-trimethylpentyl isobutyrate-pure, from Sigma-Aldrich.

Sodium hydroxide, 40% solution-from POCH Gliwice S.A.

Acetic acid 80% pure, from POCH Gliwice S.A.

A thermostated (60˚C) reaction vessel with a weighed amount of post-reaction mixture was filled with a weighed amount of isobutyric aldehyde, and 2.7 g of 40% NaOH solution was added to the obtained mixture. The resulting reaction mixture was stirred vigorously until the time t, was then placed in an ice bath whereafter 2.5 g of 80% acetic acid solution was added. The mixture was cooled down to a temperature in the range 15˚C - 20˚C and analyzed by GC to determine the concentrations of the respective reacting substances.

The experiment was carried out for the times t: 2, 4, 8, 15, 30, 60 min, for reaction systems obtained by mixing 3-hydroxy-2,2,4-trimethylpentyl isobutyrate with isobutanal in quantitative ratios shown in Figure 4.

Condensation of isobutyric aldehyde is performed in continuous-flow tank reactor equipped with efficient propeller or turbine type stirrer, providing fulfillment of kinetic criterion:

where t_mix denotes mixing time and t_1/2 half-life time of reaction. In such conditions the isobutyric aldehyde condensation reactor may be approximated by an ideal continuous- flow tank reactor model (Figure 1) described by continuity equation of its general differential form:

where ρ_i is density of i―component, w―velocity field, j_i―dispersion flow field of i, M_i―molecular weight of i, ν_ri―stoichimetric factor of i in reaction r, r―reaction rate, R―total number of reaction involved in process.

Integration of (3) over volume V of reactor results Equation (4):

Considering Gauss-Ostrogradsky theorem, volume integral of divergence appearing in (4) may be converted to the surface integral over the closed oriented boundary F:

where and n is surface normal vector.

Since the mass flows coming from the surface surrounding inside of reactor flow only through the surfaces f₀ and f₁, the surface integral (11) may be expressed by an algebraic sum of simple integrals as follows:

Assuming the field of i-compound velocity on surfaces f₀ and f₁ equals the barycentric velocity and then in consequence negligibility of dispersion flow of i, what could be expressed by criteria:

then

In view of (10) and (14), assuming the process runs in condition of non-compressible fluid, the mass balance of continuous-flow tank reactor may be expressed by model equation, applicable for engineering purposes:

where and are inlet and outlet mass flows respectively.

Considering the presence of the kinetic member in model Equation (9), in order to determine its form, an analysis of the isobutanal condensation reaction kinetics was performed as it follows below.

Acceptance of the proposed reaction mechanism for further studies, step 1)-6), is empirically justified by the findings of GC/MS analyses of samples of the reaction mixtures, collected from various steps of the process.

Figure 2 shows the GC/MS chromatogram of the reaction mixture “frozen” sample

collected after 1 min of reaction.

Significant feature of the shape of this chromatographic curve occurs to be an appearance of peaks characteristic for the presence of mentioned above system hemiacetale/dioxanole and transitional aldole reaction product of 2 moles of isobutanal: 2-hy- droxy-2,2,4-trimethylpentanal. As it has been already stated, both undergo consequent Tishchenko reaction, what finds reflection in the shape of the chromatogram of the sample of post reaction mixture (Figure 3).

The shape of post reaction mixture sample proves the decay of transitional by products of Tishchenko reaction appearing at initial stages of reaction, hemiacetale/dioxa- nole system and 2-hydroxy-2,2,4-trimethylpentanal, which underwent esterification through the reaction run, according to the Tishchenko reaction mechanism scheme signaled earlier.