Span 60 Assists in Research on Microwave-accelerated Retro-Diels-Alder Reaction

Span 60 is an organic substance with the chemical formula C₂₄H₄₆O₆. It is a white to light yellow powder or waxy or lumpy substance with a slight fatty smell. It can be dispersed in hot water and miscible with oils and general organic solvents. Span 60 is a non-ionic surfactant, mainly used in medicine, cosmetics, food, pesticides, coatings, textiles, emulsifiers, stabilizers, and textile industries as antistatic agents, softeners, etc.

Non-ionic Surfactants Product List

CAS	Product Name
10248-74-5	2-[Bis(2-Hydroxyethyl)Amino]Ethyl Stearate
111-03-5	1-monoolein
11139-88-1	1-Glyceryl Caprate
115-83-3	Pentaerythrityl Tetrastearate
121684-92-2	Acetylated Sucrose Distearate
122-32-7	Triolein
13127-82-7	Poe (2) Oleyl Amine

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Microwave is an electromagnetic wave with short wavelength and high frequency. It does not generate heat by itself, but heats through the interaction between polar molecules in the medium and the microwave electromagnetic field. Under the action of microwaves, polar molecules are polarized and alternate orientations as the polarity of the electromagnetic field changes, resulting in high-frequency friction between molecules, thereby converting electromagnetic energy into thermal energy. In recent years, many chemists have applied this function to organic chemical synthesis, which not only avoids heat conduction between the heat source and the reaction vessel during conventional heating, but also achieves rapid and uniform heating. However, the current research on the mechanism of microwave heating-accelerated reaction is still not very clear, and further research is needed to clarify it.

By designing reactants of different polarities, some researchers have explored the retro-Diels-Alder reaction of anthracene and fumaric acid derivatives at high temperatures, and used this to study the related mechanisms of microwave-accelerated reactions.

Compared with traditional heating methods, microwave heating can significantly increase the rate constant of reactions, especially for reactants with larger molecular polarities. In addition, changing the concentration of reactant molecules and the viscosity of the reaction solvent will also affect the reaction rate constant under microwave heating conditions, but has no effect on traditional heating methods.

At a reaction temperature of 200 °C, the reactants decompose into non-polar anthracene and olefin products. Compared with traditional heating methods, the reaction rate is significantly increased when heated with microwave radiation. At the same temperature, the two heating methods have completely different first-order reaction rate constants. This may be because certain polar molecules can be selectively heated under microwave heating conditions, and the temperature detected macroscopically is not the actual reaction temperature. If the reaction temperature is deduced from the reaction rate constant, the researchers found that the actual reaction temperature under microwave heating conditions should be 204°C.

Subsequently, the researchers explored the effects of surfactant, reactant concentration and solvent viscosity on the microwave heating process. When Span 60 is added to the reaction, the reaction rate of the microwave heating process will change slightly, while there is no change in the traditional heating process; when the reactant concentration decreases, the reaction rate of the microwave heating process will increase, while the traditional heating process will not change. The reaction rate during the heating process remained unchanged; when a more viscous solvent (heptadecane) was used for the reaction, the reaction rate during the microwave heating process was significantly increased (increased by 66%), while the reaction rate during the traditional heating process was The speed is almost unchanged. These results illustrate the existence of microwave selective heating process.

Finally, the researchers explored the conversion rates of different retro-Diels-Alder reaction substrates under microwave heating and traditional heating conditions. After reacting for 3 hours under standard conditions, the conversion rate of dimethyl ester under microwave heating and traditional heating (60% vs 51%) increased as much as that of diamide (81% vs 72%), while the increase in dibenzyl ester was is larger (48% vs 31%).

Reference

1. Microwave-specific acceleration of a retro-Diels-Alder reaction
   Chem. Commun., 2020, 56, 11247-11250