Superheating of the solvent

Superheating of the solvent was believed to be responsible of the observed rate enhancement under microwave irradiation in the synthesis of 3,5-disubstituted 4-amino-1,2,4-triazoles when conducted in 1,2-ethylene glycol as (polar) solvent (Eq. 4) [32]. 

It is to be noted that the reactions mentioned so far were performed under homogeneous conditions and, in most cases, using polar solvents, which are efficient absorbers of MW energy. Rate enhancements were attributed to the superheating of the solvent due to the elevated pressures generated in the closed vessels. 

In a subsequent paper [32], however, Berlan himself cast doubt on the existence of nonthermal effects, attributing the observed rate increases to localized hot-spots in the reaction mixture or to superheating of the solvent above its boiling point. He also mentioned the difficulty of measuring the temperature accurately in MW cavities. Furthermore, kinetic studies by Raner et al. [33], showed that the Diels-Alder reaction of 3 with 23 (Scheme 4.12) occurred at virtually the same rate under MW and conventional heating at the same temperature. 

The small increase in racemization rate observed when an aqueous solution of L-pro-line was heated under reflux on a MW oven at atmospheric pressure could be attributed to localized superheating or a generalized superheating of the solvent. It is known that water superheats by 4—10 °C when boiled in a MW oven [39, 40]. 

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. 

It is well known that a wide variety of organic reactions are accelerated substantially by microwave irradiation in sealed tubes. These rate enhancements can be attributed to superheating of the solvent, because of the increased pressure generated when the reactions are performed in the a.m. manner. Furthermore several reports have described increased reaction rates for reactions conducted under the action of microwave irradiation at atmospheric pressure, suggesting specific or nonthermal activation by microwaves. Some of these re-studied reactions occur at... 

As described above, however, some rather small differences could be observed, taking into account the superheating effect of the solvent under the action of micro-waves in the absence of any stirring. This probably occurs in the isomerization of sa-frole and eugenol in ethanol under reflux [31] (MW 1 h, A 5 h to obtain equivalent yields). 

We have found it convenient to compare MW and conventional reactions using reflux conditions, since the temperatures are constant at the boiling point of the solvent. To eliminate the problem of the time required to reach the reflux temperature, reaction mixtures without one of the reactants or catalyst are heated to reflux and then the other reactant or catalyst quickly added. The reflux times required to give similar yields for a reaction, taken only partially to completion by MW and classical heating, are then compared. Small rate enhancements might still be expected merely because of superheating by up to 40 °C by the MW [39, 40, 46], and localized heating... 

To elucidate the cause of the microwave-induced enhancement of the rate of this reaction in more detail the transformation of 2-t-butylphenol was performed at low temperatures (up to -176 °C). At temperatures below zero the reaction did not proceed under conventional conditions. When the reaction was performed under micro-wave conditions in this low temperature region, however, product formation was always detected (conversion ranged from 0.5 to 31.4%). It was assumed that the catalyst was superheated or selectively heated by microwaves to a temperature calculated to be more than 105-115 °C above the low bulk temperature. Limited heat transfer in the solidified reaction mixture caused superheating of the catalyst particles and this was responsible for initiation of the reaction even at very low temperatures. If superheating of the catalyst was eliminated by the use of a nonpolar solvent, no reaction products were detected at temperatures below zero (see also Sect. 10.3.3). 

If the pressure over a solution is reduced below the partial pressure of the solvent over the solution, then the solution is said to be superheated. The degree of superheat is represented by the difference between the equilibrium partial pressure of the solvent over the solution and the total pressure (vacuum level) over the solution Pi — Pq- The higher the vacuum level, the higher the superheat at a given concentration and temperature. Increasing the temperature at a fixed pressure level, of course, also increases the superheat. 

Instead of a steam bath or a water bath, an electrically heated oil bath is suitable. This permits easy regulation of the rate of distillation. However, the temperature of the bath should not exceed 95-100° when isopropyl alcohol is the solvent if possible dehydration of a sensitive alcohol is to be avoided. For this same reason, a burner, hot plate, or sand bath is not recommended for heating. Especially at the end of the reduction, superheating of the concentrated alkoxide solution may bring about dehydration or other decomposition of the product. 

Temperature and solvent effects were also examined. When the reaction temperature was gradually reduced from reflux temperature (199 °C), the rate enhancement factor increased from 1.0 at 199 °C to 1.4 at 105 °C, to 2.6 at 75 °C, and to 4.1 at 24 °C. These results may indicate that superheating of the catalyst is more... 

Lead tetraacetate (0.25 mol) and 1 mol of a carboxylic acid were heated together at 60-80°C, and the acetic acid was distilled at 10 mmHg pressure as it formed. The product was pure enough for further reactions without other treatment. When the lead tetra-salt melted > 80-100°C, a solvent, such as 1,1,2,2-tetrachloroethane, o-dichlorobenzene, or mineral oil, was used to prevent caking and local superheating of the product. 

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