Sixth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-6), 1-30 September

[E009]

THE COMPARATIVE STUDY OF THE KINETICS OF KNOEVENAGEL CONDENSATION UNDER MICROWAVE AND CONVENTIONAL CONDITIONS. Part II.

Szczepan Bednarz, Dariusz Bogdal*

*e-mail: [email protected]

Department of Polymer Science and Technology Politechnika Krakowska ul. Warszawska 24, 31-155 Krakow, Poland

       In the recent article [1] we presented comparative study of the Knoevenagel condensation under microwave radiation and conventional heating. This paper is continuation of that work.
       The most important factor, which influence directly on reaction rate is, of course, the temperature and thermal homogenity of reaction system [2]. In our work we have used fiberoptic thermometer (Nortech), that is designed for temperature measurement in strong electromagnetic fields (radio and microwave frequencies). Additionally we checked this instrument by measure boiling points of typical liquids (water, ethanol) under microwave radiation (to avoid superheating of solvent boiling chips and stirrer was used). Arrrhenius parameters for examinated reaction (Scheme 1) were determinated and concentration of reagents depend on temperature profile in two cases: (1) second-order and (2) pseudo first-order condition (Scheme 2, Scheme 3) were calculated. Since the reacting mixture strongly absorbs microwave radiation, we used water flow in glass cooler to keep appropriate temperature of reaction mixture. [1].

Scheme 1. Synthesis of 3-ethoxycarbonylcumarine (C) over Knoevenagel condensation of salicylaldehyde (A) and diethylmalonate (M), piperydine (P) catalysed in toluene as solvent.

Scheme 2. Second-order reaction kinetics equation. [X]₀ = 0.18 mol/l, [P]=0.047 mol/l. Case (1).

Scheme 3. Pseudo first-order reaction kinetics equation. [A]₀ = 1.22 mol/l, [M]₀ = 0.11mol/l, [P] = 0.043 mol/l. Case (2).

Figure 1. Typical example of reaction course – case (1) 27W.

Figure 2. Typical example of reaction course – case (2) 150W.

Table 1. Deviation (SSE) between calculated and measured concentration. Case (1) N=6 and (2) N=5 data points (Y)

The significant influence (expressed as the sum of squares of the error SSE) of microwave irradiation on reaction rate (Table 1) were observed. In case (1), in each experiments measured concentration of substrats were lower than calculated. Observed microwave effect is significant, but small. It is thought that there is a non-thermal microwave effect [3], because reaction temperature were measured precisely and reaction medium was non-polar (toluene) to avoid solvent superheating [4]. In more polar medium – case (2) (excess of salicylaldehyde) the affirmation of microwave influence were not detected.

[1] ECSOC5
[2] Stuerga D., Gaillard P., Tetrahedron, 1996, 52, 5505
[3] Perreux L., Loupy A., Tetrahedron, 2001, 57, 9199
[4] Gabriel C., Gabriel S., Grant E.H., Halstead S.J., Mingos D.M.P., Chem. Soc. Rev., 1998, 27,213