Chemistry Reference
In-Depth Information
nature of these effects [4,19]. Despite the basic understanding of high-frequency electromagnetic irradiation and
microwave-matter interactions, the exact reasons why and how microwaves enhance chemical processes are still not
fully understood. Thus, the investigation of microwave effects is an extremely complex subject [3].
A chemical reaction is a process that results in the interconversion of reactants into one or more products. Basically,
chemical reactions encompass changes that strictly involve the motion of electrons in the breaking and forming of
chemical bonds. Energy is necessary to break chemical bonds in the starting substances and it is released once new
bonds are formed [20]. However, it is not necessary to absorb sufficient energy to break chemical bonds in order to
have a significant influence on reaction kinetics. Stuerga and Gaillard have correctly pointed out that microwave
energy is not large enough to break chemical bonds [21].
In 1889, the Swedish scientist Svante Arrhenius introduced an important equation that describes the rate of a
chemical reaction. According to Arrhenius equation (K = A
e
-Ea /RT
), the reaction rate constant (K) is dependent on
two factors: the frequency of collisions between molecules that have the correct geometry for a reaction to occur (A)
and fraction of those molecules that have the minimum energy required to overcome the activation energy barrier (
e
-Ea /RT
) [8].
Considering this equation, there are basically two ways to increase the rate of a chemical reaction. Some authors
have proposed that the pre-exponential factor (A) can be affected by microwave irradiation, once the microwave
energy induces an increase in molecular vibrations. However, other authors believe that microwave irradiation
produces an alteration in the exponential factor (
e
-Ea /RT
) by affecting the free energy of activation [4].
Based on these aspects, there are essentially three different possibilities for rationalizing rate enhancements observed
in microwave-assisted chemical reactions: (i) thermal effects (kinetics), (ii) specific microwave effects and (iii) non-
thermal (athermal) microwave effects [3].
Today most scientists agree that in most cases the reason for the observed rate enhancements is a purely thermal
effect (kinetic) [5]. The high reaction temperature causes greater movement of molecules, which leads to a greater
number of energetic collisions. This occurs much faster with microwave energy due to high instantaneous heating of
the substance(s), above the normal bulk temperature, and it is the primary factor for observed rate enhancements. In
these cases, microwave energy affects the temperature parameter in the Arrhenius equation [8].
Taking into account a general reaction coorditate (Fig.
8
), the reaction begins with reactants (A and B), which have a
certain level of energy (E
R
). These reactants must collide in the correct geometrical orientation to become activated
to a higher-level transition state (E
TS
) in order to complete the transformation.
Figure 8:
A general reaction coordinate. Figure adapted from Hayes, 2002.
