Study On Microwave Assisted Organic Synthesis And The Nonthermal Microwave Effect In Chemical Reactions

Since the first published reports on the use of microwave irradiation to accelerate organic chemical transformations by the groups of Gedye and Giguere/Majetich in 1986, more and more chemists engage in the in this fast-moving and exciting field, today generally referred to as microwave-assisted organic synthesis (MAOS). In many of the published examples, microwave heating has been shown to dramatically reduce reaction times, increase product yields, and enhance product purities by reducing unwanted side reactions compared to conventional heating methods. The use of microwave irradiation in chemistry has thus become such a popular technique in the scientific community that it might be assumed that most chemists will probably use microwave energy to heat chemical reactions on a laboratory scale.The effect of microwave irradiation in organic synthesis is a combination of thermal effects, arising from the heating rate, superheating or''hot spots''and the nonthermal effect, arising from the directly interaction of substances with electric field. Unfortunately, the subject of"nonthermal microwave effects"is highly controversial and has led to heated debates in the scientific community.In this paper, the microwave assisted synthesis method was applied to promote organic reactions and the effect of microwave in organic reactions was debated.The main research and results are as below:In the first part, an efficient synthesis of 3, 4-dihydropyrimidinones from benzoic aldehyde, ethyl acetoacetate and urea in ethanol with p-toluenesulfonic or NH4HF2 as catalysis under microwave irradiation was described. Microwave assisted technology can dramatically enhance the Biginelli condensation.In part two, esterifications of diverse benzoic acid derivates and alcohols were conducted with careful condition controlling under microwave heating and classical conduction heating. It was found that microwave can enhance reactions more significantly where reactants have larger steric hindrance, the conversion and rate constant k'of the reaction conducted under microwave irradiation heating were larger than that conducted under classical heating.In order to check the effect of microwave irradiation obtained in esterifications of aromatic acids and alcohols, the reactions of cyclopropanecarboxylic acid and ethanol alcohol or isopropyl alcohol were studied at dielectrical heating and convective heating, the result of which supported that the microwave effect changed with the reactant structures, which resulted to different hindrance.Furthermore, the effect of the microwave on the hydrolysis of several esters were also discussed, which supported that microwave affected reaction that the reactant was more substitute much significantly, microwave irradiation results more efficient when more harsh reaction conditions are requiredThe differences in conversion and kinetic shown in this paper may not only due to thermal effect of the microwave.