Fifth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-5), http://www.mdpi.org/ecsoc-5.htm, 1-30 September 2001
[E0034]
 

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

Szczepan Bednarz, Dariusz Bogdal*

*e-mail: [email protected]

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


Received: 15 August 2001 / Uploaded 22 August 2001

 

       In the recent few years there has been a growing interest in the use of microwaves in chemistry: organic synthesis, polymer technology, material processing, analytical and environmental chemistry [1-8]. On these fields, processes run faster than by conventional heating and often chemical reactions could be more selective. In conventional thermal processing, energy is transferred to the material through convection, conduction, and radiation from surfaces of the material, whereas microwave energy is delivered directly to the volume of the material (in situ) through molecular interaction whith the electromagnetic field. This effect originates from the microwave electric field which forces dipoles to rotate and ions to migrate and from a slower response of dipoles and ions to follow the rapid reversal of the electric field.

       Acceleration of the reaction rates, compared to the normal conditions could be due to different mechanism of transferring heat, other suggest that specific nonthermal microwave effect exist. In our investigations, we observed specific microwave effect.


       In our work, we studied kinetics of Knoevenagel condensation [8] of salicylaldehyde and diethyl malonate, in the presence of piperidine as a catalyst and toluene as a solvent (Scheme 1). In earlier work [9] the rate equation was determined empirically (Scheme 2). We have measured the reaction rate constant at various temperature under microwave and conventional conditions (Figure 2, Table 1). The reaction mixture was analyzed by GCMS and naphtalene was added to the reaction as an internal standard.  

Reaction mechanism
Scheme 1. Salicylaldehyde (A), diethyl malonate (M), piperidine (P), 3-ethoxycarbonylcoumarine (C)

 

 
Second-order kinetics equation.
Scheme 2. Second-order kinetics equation. [P] = 0.048 M = const. [A]0 = 0.247 M, [M]0 = 0.181 M.

       Knoevenagel condensation reaction rate has been reported to be more than three times higher during microwave irradiation than conventional heating [9]. In our work, we studied the influence of microwave power energy on described chemical systems. We used specific system (Figure 1): a monomode mirowave reactor (Synthewave 402 - Prolabo), operating at various microwave powers. It was equipped with an infrared pyrometer to measure reaction temperatures. Since the reacting mixture strongly absorbs microwave radiation, we used cyclohexane (minimal microwave absorption) flow in glass cooler to refrigerate.  

equipment.gif
Figure 1. Experiment equipment. Microwave reactor (1), quartz tube (2), cooler (3), magnetron (4), IR pyrometr (5). Cool cyclohexane flow (C).

We observed incomprehensible behaviour of the reaction system (Figure 2). When we applied 150 W microwave power, reaction rate was higher than with 225 W, despite of a temperature at the same level. Whereas at 95°C, under microwave irradiation at 225 W, we observed small rate enhancements. The influence of the microwave power and the temperature on the kinetics of the chemical reactions is complex, so if an optimal range of these parameters exists we can lead chemical processes in maximal rate. 

Table 1. Rate constant for the Knoevenagel condensation of salicylaldehyde, diethyl malonate in toluene, in the presence of piperidine, under different conditions - k [l/mol·s]. 
 

Conventional heating
Microwave
150 W
225 W
70 °C 3.4 ´ 10-3
76 °C 1.6 ´ 10-2
86 °C 2.3 ´ 10-2
80 °C 4.5 ´ 10-3
81 °C 4.1 ´ 10-2
90 °C 2.7 ´ 10-2
90 °C 6.4 ´ 10-3
85 °C 7.0 ´ 10-2
95 °C 1.6 ´ 10-2
88 °C 4.4 ´ 10-2

 

 
chart.gif
Figure 2. Comparison of rate constant in reactions under microwave irradiation and by conventional heating.


       Further study of a similar reaction system and more accurate method of the determination of the temperature (fiberoptic thermometer) are needed before a definitive conclusion can be reached.



References

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[2] Fini A., Breccia A., Pure Appl. Chem., 1999, 71, 573.
[3] Thostenson E. T., Chou T. W., Composities, 1999, 30, 1055.
[4] Zlotorzynski A., Critic. Rev. Anal. Chem., 1995, 25, 43.
[5] Abramovitch R.A., Huang Z., Chemosphere, 1994, 29,2517.
[6] Varma R.S., Saini K., Tetrahedron Lett., 1998, 39, 1481.
[7] Loupy A., Pigeon P., Ramdani M., Tetrahedron, 1996, 52, 6705.
[8] Bogdal D., J. Chem. Res., (S) 1998, 468.
[9] Bogdal D., monografia PK nr 248, Kraków 1999.