Reaction temperature enhancement using

In this case, the reaction medium is heterogeneous because terephthalic acid is hard to dissolve in ethylene glycol. However, the use of small monomer feed ratio and high reaction temperature are useful to enhance the solubility. Water formed during the reaction was collected to estimate the reaction conversion. Addition of a catalyst is not necessary since the acid group of terephthalic acid self-catalyze the reaction. But, when concentration of acid groups decrease, metal catalysts could be employed to maintain the reaction rate (2). 

The formation of a-dithiopyrone by-products during the reaction of an enamine with elemental sulfur and carbon disulfide is enhanced by one or a combination of the following the carbon disulfide is allowed to stand for a long period of time with the enamine in the absence of sulfur, a high reaction temperature, and the use of a relatively nonpolar solvent (135). 

This reaction was also used to deposit epitaxial silicon at the temperature range of 1000-1040°C, but the deposit was generally unsatisfactory and the reaction is no longer used for that purpose. However, if the reaction is enhanced with a plasma using electron cyclotron resonance (ECR), fluxes may be independently controlled and high-quality epitaxial silicon deposits are obtained at temperatures below 500°C.P 1... 

The issue of parallel versus sequential synthesis using multimode or monomode cavities, respectively, deserves special comment. While the parallel set-up allows for a considerably higher throughput achievable in the relatively short timeframe of a microwave-enhanced chemical reaction, the individual control over each reaction vessel in terms of reaction temperature/pressure is limited. In the parallel mode, all reaction vessels are exposed to the same irradiation conditions. In order to ensure similar temperatures in each vessel, the same volume of the identical solvent should be used in each reaction vessel because of the dielectric properties involved [86]. As an alternative to parallel processing, the automated sequential synthesis of libraries can be a viable strategy if small focused libraries (20-200 compounds) need to be prepared. Irradiating each individual reaction vessel separately gives better control over the reaction parameters and allows for the rapid optimization of reaction conditions. For the preparation of relatively small libraries, where delicate chemistries are to be performed, the sequential format may be preferable. This is discussed in more detail in Chapter 5. 

Most importantly, microwave processing frequently leads to dramatically reduced reaction times, higher yields, and cleaner reaction profiles. In many cases, the observed rate enhancements may be simply a consequence of the high reaction temperatures that can rapidly be obtained using this non-classical heating method, or may result from the involvement of so-called specific or non-thermal microwave effects (see Section 2.5). 

The rate enhancement for the esterification of benzoic acid with methanol was close to 100, when compared with the classical heating under reflux. On the other hand, the rate enhancement for the esterification with n-pentanol, using the same power level (560 W) was only 1.3. The approximate reaction temperature was almost the same for the two alcohols (134 °C and 137 °C respectively). It should be noted, however, that the rate enhancement for the esterification in pentanol increased to 6 times when a higher power level (630 W) was used, the reaction temperature being higher (162 °C). 

It is interesting to note that when the same reaction was performed using a variable frequency MW system [49] with temperature control at 80 °C in the absence of a solvent, it occurred at the same rate as a similar reaction heated conventionally at the same temperature. The use of variable frequency provides very uniform heating, minimizing the possibility of hot spots. Thus it can be concluded that the modest rate enhancement observed in ethanol under reflux was because of hot spots or to a general superheating of the solvent. Again, it should be emphasized that these modest MW rate enhancements should not be taken as hard evidence for nonthermal MW effects. 

The issue of parallel versus sequential synthesis using multimode or monomode cavities, respectively deserves special comment. While the parallel setup allows for considerable throughput that can be achieved in the relatively short timeframe of a microwave-enhanced chemical reaction, the individual control over each reaction vessel in terms of reaction temperature and/or pressure is limited. In the parallel... 

This undesirable effect largely compromises the advantage that increased temperature has in accelerating a coupling reaction. Rather than risking decomposition of the diazonium compound by elevating the reaction temperature, it is therefore much more useful to increase the pH or the concentrations of the reactants in order to enhance the rate of a coupling reaction. 

---