This is a knowledge base on chemical synthesis using laboratory microwave reactors.

Basics of Microwave Synthesis

How does microwave heating work? Which solvents can be heated with microwave irradiation? What are the advantages? This crash course introduces you to the most important and interesting topics in microwave synthesis. Learn about the basics and the advantages of this field of chemistry, gather helpful hints and read up on scientific microwave equipment.

 

Dipolar polarization

Simplified movement of dipoles in a microwave field

Figure 1: Simplified movement of dipoles in a microwave field
For a substance to be able to generate heat when irradiated with microwaves, it must be a dipole, i.e. one part of its molecular structure must be negatively charged while the other part is positively charged. Since the microwave field oscillates, the dipoles in the field align with the oscillating field (Figure 1). This alignment causes rotation, which results in friction and ultimately generates heat energy.


Ionic conduction

Simplified movement of ions in a microwave field

Figure 2: Simplified movement of ions in a microwave field
During ionic conduction, dissolved and (completely) charged particles – usually ions – oscillate back and forth under the influence of microwave irradiation (Figure 2). This oscillation causes collisions of the charged particles with neighboring molecules or atoms, which are ultimately responsible for creating heat energy.

 

 
In dielectric heating, electric energy is converted into kinetic energy
which is ultimately converted into heat.

 

Table 1: List of common organic solvents
classified by microwave heating
efficiency
 

Depending on the dielectric properties of a compound (taking into account the dielectric constant ε’ as well as the dielectric loss ε’’), solvents can be classified as high, medium or low absorbers, depending on their loss tangents tan δ (tan δ = ε’/ ε’’).

 

The tan δ values for some commonly used organic solvents are summarized in table 1, which shows the classification of solvents into high (tan δ > 0.5), medium (tan δ 0.1-0.5), and low microwave absorbers (tan δ < 0.1). Solvents without a dipole moment, such as benzene and dioxane, are more or less microwave transparent (tan δ < 0.01).

 

A solvent with a high or at least significant tan δ value is required. However, this does not mean that solvents with low tan δ values cannot be used for microwave synthesis. Since both substrates and reagents/catalysts will likely be polar, the overall dielectric properties of a reaction mixture will commonly allow sufficient heating by microwaves, even with non-polar solvents.

 
Polar mixtures are needed for efficient microwave heating.