Study On The Thermal Effect In Classical Organic Reaction Under Microwave Irradiation

Microwave assisted technique has been widely used in organic chemical synthesis,like drug chemistry,inorganic chemistry and polymer chemistry.Microwave-assisted chemistry also has the advantages of green,environmental benign,high efficiency,and energy saving.With the invention of high efficiency microwave reactors,the application scope of microwave has also expanded.At present,the essential reason for the microwave acceleration of organic reaction is still in the discussion stage.With the deepening of the research,people basically believe that"microwave effect" can be explained by "thermal effect" and "non-thermal effect",but it has not yet obtained a unified theoretical basis.This dissertation compares the reactions between the traditional heating and microwave heating under similar conditions,because the temperature measuring device of the microwave reactors can only measure the average temperature of the system,but can not measure the actual temperature of the reaction molecule,so the temperature measurement system has a great limitation in the microwave chemistry.The mechanism of microwave accelerated organic reactions was explained by the method of kinetic calculation and the actual temperature of the reactant under microwave radiation was calculated.The following studies were conducted:(1)The classical Diels-Alder[4+2]cycloaddition reaction was used to explore the selective heating and the "hot spot" problem under microwave conditions.Through the screening of reaction conditions,we have identified maleimide and furan as reaction molecular models,and explored the selective heating of microwaves in different solvents.The corresponding kinetic data were measured by the conventional heating method,the appropriate reaction temperature and reaction time were selected.Finally,the kinetic data of the reactions were fitted.The thermodynamic parameters-reaction apparent activation energy was obtained and the reaction kinetics data under the microwave condition were replaced to find the true "hot spot" temperature of the reactant.The reaction temperature of the reactants is higher than the temperature of the reaction system,and the "hot spot" effect in the microwave reaction is verified by the classical Diels-Alder reaction.(2)The "cold spot" phenomenon in microwave reaction was explored by using Claisen rearrangement reaction.By selecting appropriate reaction system,we verified the phenomenon of "cold point" in the microwave reaction system.That is,in the self rearrangement reaction of 4-methyl-phenyl allyl ether in N-methyl-pyrrolidone,through the heating method of the traditional oil bath,the kinetic data of reaction at the reaction rate at different temperatures,that is,the Arrhenius equation,lnk=lnA-Ea/RT,obtained the linear relationship between Ink and 1/T,that is,reaction.The linear relation between the rate constant k and the temperature T.By means of microwave heating,a fixed amount of naphthalene is added as an internal standard to determine the reaction rate of the different reactant concentrations.The actual temperature of the reactant during the microwave heating process was calculated by the kinetic calculation of the reaction.The actual temperature of the reaction is compared with the heating temperature of the reaction system.It is concluded that the"cold point" phenomenon exists in the microwave reaction.This experiment makes full use of the kinetic calculation of the reaction,and selects a good reaction system,and then proves the "cold point" problem in the microwave reaction,which proves the existence of the microwave "thermal effect",rather than the "non-thermal effect",and further reveals the mechanism of the microwave reaction in the promotion of the mechanical chemical reaction.(3)The Hammett linear equation was calculated by the "one pot" experiments of the aminolysis reaction,and the Hammett equation was used as the "temperature probe" to explore the "hot spots”and microwave selective heating of the microwave reaction.Through continuous exploration,we got a suitable reaction molecular model,and calculated the Hammett equation under two different conditions of ordinary heating and microwave heating.The experimental results show that under the same temperature conditions,the linear Hammett equation of the reaction does not change under the condition of the oil bath,but the slope of the linear equation of Hammett decreases much under the microwave conditions than that under the conditions of the oil bath,that is,their reaction rate is increased and the reaction temperatures are higher.The reactants are in the "hot spot" of the reaction system.The phenomenon of microwave selective heating appears under microwave condition,and it can also be proved that this phenomenon is thermal effect rather than non thermal effect.