The Thermal Path Effect Anisothermal Conditions

According to the author, before claiming microwave heating effects in preference to collisional or mechanistic terms, it is necessary to estimate the effect of strong heating rates induced by microwave heating. The energy density used in a domestic oven is sufficient to raise temperature from ambient to 200 °C in less than 1 min, and so cause the total reaction time to be reduced by a factor dose to 10.  

This natural tendency for thermal racing is accentuated by these implicated energy densities which are usually different from those used with a water or oil bath, particularly for sealed vessels and autoclaves as used by organic chemists. The temperature and, especially, the thermal path T(t) seems to be a crucial variable. The authors have shown theoretically and experimentally that strong heating rates (up to 5° s ) can induce selectivity or inversion between two competitive reactions [141]. 

The first case, depicted by Fig. 1.20, is called induced selectivity . Numerical results correspond to a values of kinetic terms which give an optimum selectivity effect exact values could be found elsewhere [141]. Under classical heating conditions, or a very slow heating rate, a mixture of the two products Pi and P2 is obtained. The ratio of the concentrations of Pi and P2 could be controlled by adjusting the rate of heating. Hence, under microwave heating conditions, pure Pi or Pi 

The second case, called Isomeric inversion , is illustrated by Fig. 1.21. Under classical conditions Pi alone is obtained whereas rapid heating or microwave heating leads to P2. This situation is more interesting than the first because it forecasts reactivity changes induced by heating rate despite the same chemical operating conditions (reagent concentrations). 

To observe such kinetic effects of microwave heating, however, it is necessary to have reactions with reaction times dose to heating time. Under most operating conditions reactions times are close to several tenths of minutes and anisothermal 

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