Guide Microwave Assisted Organic Synthesis

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Ameta obtained his masters degree from the University of Udaipur and was awarded a Gold Medal in He also served as dean of postgraduate studies. Ameta has occupied the coveted position of president, Indian Chemical Society, Kotkata, and is now life-long vice president, beginning in He was awarded a number of prizes during his career, such as national prizes twice for writing chemistry books in Hindi.

He received the Prof. Desai Award, the Prof. He has successfully guided 71 students in the PhD program. Ameta has more than research publications to his credit in journals of national and international repute. He has also contributed in the form of books. He is the author of around 40 undergraduate- and postgraduate-level books.

Microwave Assisted Synthesis

He has also written chapters in books published by several international publishers. Ameta has delivered lectures and chaired sessions at national conferences throughout India. He is also reviewer of number of international journals. Pinki B. Her research interests include photocatalysis and photo-Fenton reactions for the degradation of organic pollutants, microwave-assisted organic synthesis, and polymer synthesis.

She has supervised 22 PhD students, and 7 are currently working under her supervision. She has more than research papers in journals of national and international repute and has attended various conferences and delivered invited lectures.

Molecules | Special Issue : Microwave-Assisted Organic Synthesis

She has also contributed to several undergraduate- and postgraduate-level books. She has published several chapters in books published by international publishers. He has 8 years of experience in teaching and research. He has published more than 50 research papers and at present is guiding 7 research students for their PhD theses. Ameta has received various awards and recognitions in his career, such as securing the first position in for MSc, for which he was awarded a Gold Medal.

He has served at M. He has supervised two PhD students, and seven more are performing research under his supervision on various aspects of green chemistry.

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He has around 50 research publications to his credit in journals of national and international repute. He holds one patent, and two more are under way. He has written five degree-level books and has contributed chapters to books published by several international publishers. Chetna Ameta was awarded a PhD from M. Sukhadia University, Udaipur, India.

She is presently working as assistant professor of chemistry at Sukhadia University, Udaipur, and has 16 research papers to her credit. Her research interests are in microwave-assisted organic synthesis and green chemistry. We provide complimentary e-inspection copies of primary textbooks to instructors considering our books for course adoption.

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Stay on CRCPress. Hence, it is necessary to have a proper appreciation of microwave engineering e. Failure to do so can result in erroneous reporting of microwave effects originating from incorrect temperature measurements. Consequently, it is necessary to implement the following points in order to clarify the microwave effect: i elucidate the meaning of the microwave effect, ii establish an experimental system from which we can observe the microwave effect on the sample using electromagnetic waves and heat, and iii build an experimental system microwave engineering such that measurement errors of temperatures are avoided.

To achieve some of the above points, we have examined photocatalyzed reactions enhanced by microwaves for nearly two decades during which we demonstrated the microwave effect by simultaneously irradiating the metal-oxide photocatalyst TiO 2 with UV light and microwave radiation in reactions taking place during wastewater treatments, something that could not and cannot be achieved by conventional heating 7.

As a non-thermal effect, the microwave effect in photocatalyzed reactions was demonstrated by establishing that the lifetime and utilization efficiency of electrons excited by UV light in the photocatalyst are enhanced by the microwave radiation 8. Other researchers have also investigated such phenomena. For instance, Kishimoto and coworkers reported that an applied microwave field can enhance the photocatalytic reduction of bipyridinium ion using CdS quantum dots QDs via an acceleration of the electron transfer process 9.

Among our series of experimental investigations, we noticed that changing the microwave generator from a magnetron type to a semiconductor type changed the efficiency of the photocatalyzed reaction even though other microwave and reactor components remained the same see below. We hypothesized that this difference was due to the difference in the quality of the microwaves generated from each generator and we further deduced that the microwave effect in chemical reactions can be investigated using different types of microwave generators. In the present study, we examined the microwave effect in chemical reactions using three different types of microwave generators and making certain considerations.

First, we hypothesized that the microwave effect results from the influence of microwaves as electromagnetic waves when the sample is irradiated and the microwave energy is changed into heat. The microwave effect is defined as an electromagnetic wave effect s on the chemical reactions.


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Next, the experiments involved some microwave engineering: for example, i carrying out measurements of microwave oscillation for each generator, ii examining the electromagnetic wave effect in a heterogeneous reaction involving a metal-oxide photocatalyst, iii examining the electromagnetic wave effect in a heterogeneous organic synthesis involving metal catalysts, and iv carrying out intramolecular reactions or organic syntheses in homogeneous media using solvents of different polarities.

The monitored microwave power transmitted at each oscillation cycle is displayed in Fig. Thus, the MG-A instantaneously irradiates the sample with microwave power at a maximum average power about 4. Most of the commercial devices used in the fields of microwave chemistry and materials science emit microwave pulses of several milliseconds in width similar to those from the MG-A generator.

Note that the pulse width and amplitude depend on the power supplied and the performance of the magnetron. No doubt, the oscillation behavior of the microwaves caused by the difference in microwave generators likely influences a chemical reaction.

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Question is then why did chemists not notice such characteristics of the generators before? Many power meters cannot display the average value of the microwave irradiation power because the measurement intervals and the measurement accuracy are not high. In our experiments, we could observe it because we used a highly accurate power sensor used in communications applications and in a synchroscope system. Such a difference in the characteristics of the generators may be the driving cause that influences a microwave-assisted chemical reaction.

That is, the characteristics of the three generators are reminiscent of the following hypothesis. Since the period of oscillation of the molecule is from ca. Question then is what is the timescale of the conversion of electromagnetic energy into heat? When dipolar polarization is the main phenomenon, dielectric heating involves unorganized movements at the microscale owing to the inability of molecular clusters to move in unison with the electric field.

This hysteresis phenomenon explains how the organized energy of the electromagnetic field is transferred as a sort of Brownian movement in matter.

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The characteristic timescale of this conversion is some picoseconds 11 , i. On the other hand, many reactions of molecules in organic syntheses proceed in a timescale of milliseconds. The accumulated amount of microwave energy of the three generators is the same because it is averaged over minutes or hours. However, the microwave power from the MG-A system could be up to 4.

If the microwave effect proceeded directly from the microwave energy, we would anticipate the synthetic yields obtained from the chemical synthesis with the three generators to be different. In order to verify this hypothesis, a heterogeneous photocatalyzed reaction, a metallic solid catalyzed reaction, a model organic synthesis, and the heating behavior of solvents were examined next. The heating rates and the reaction temperatures were otherwise the same ca.

Moreover, no reaction occurred within experimental error when the metal-oxide was irradiated solely with the microwaves from the MG-A generator. Note that a quantum energy of 3. Evidently, microwaves contribute to the electronic events within the photocatalyst. No effect was observed when visible light and microwave radiation were used simultaneously In summary, irradiating the metal oxide TiO 2 with both UV light and microwaves enhanced the photocatalyzed reaction. However, this efficiency varied depending on the kinds of generators used.

Despite the reaction temperature being in all cases ca.


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Since the promoting effect of microwaves in the photocatalyzed reaction contributes to the behavior of electrons within the metal oxide, the instantaneous microwave power electric field has a more positive effect on electron transfer prior to the microwave energy being converted into heat in the metal oxide. Clearly, the electromagnetic wave effect has proven to be a non-insignificant actor in the photocatalyzed reaction.

In the case of MG-B and SG generators, the synthesis yield of 4-methylbiphenyl increased in proportion to the synthesis time: yields of ca.

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On the other hand, the synthesis yield with the MG-A system was only 2. The yield of toluene in the discharged solution was monitored continuously.

On the other hand, when the MG-A generator was used, the rate of toluene formation remained at 1. Clearly, even though two kinds of syntheses were carried out using metallic solid catalysts, the behaviors of the generators displayed an otherwise identical trend, in that the yield was lowest when using the microwaves emitted by the MG-A generator. In addition, even though the electromagnetic wave effect did clearly manifest itself, the effect was not a positive one.

We hypothesized that the catalytic activity during the process was affected because the metal catalyst particles either melted or formed agglomerates because of heat from the hot spots The decrease of product yields when using the MG-A generator was deduced to be caused by the formation of hot spots on the catalyst surface, which we confirmed through observation with a high-speed camera. Indeed, formation of intense hot spots occurred within the reaction vessel during the synthesis of 4-methylbiphenyl and toluene see, for example, Fig. On the other hand, no occurrence of hot spots was observed when these reactions were carried out using the MG-B and SG generators.

An important aspect of the mechanism of formation of hot spots is the relationship between the size of the catalyst particulates and the space between the catalyst particles. Therefore, the movement of electrons on the activated carbon surface irradiated with microwaves is accelerated, which triggers the formation of hot spots.