Microwave Assisted Reactions in Green Chemistry

Definition of microwave

A microwave (MW) is a form of electromagnetic energy. It is defined as a measurement of the frequency of 300 to 3000000 MHz which comes from the lower end of the electromagnetic spectrum, corresponding to wavelengths of 1 cm to 1 m. 

The main difference between microwave energy and other forms of radiation such as X-rays and Y-rays is that microwave energy is non-ionizing and thus does not change the molecular structure of the compounds, it only provides thermal activation.

Mechanism of microwave heating

Microwave heating works upon the interaction of molecules in a reaction mixture with electromagnetic waves generated by a "microwave dielectric effect".

The heating effect of microwaves can be understood by two different mechanisms:

1. Dielectric polarization (Dipole interaction)

The heating effect generated in microwave-assisted organic transformations is mainly due to dielectric polarization.

This mechanism states that when a molecule is irradiated with microwaves, it aligns itself with the applied field. The rapidly changing electric field affects the orientation of the dipoles and consequently, the dipole continuously attempts to align itself with the oscillating electric field of the microwaves, and energy is absorbed.

These collisions and friction between the moving molecules result in heating.

2. Ionic conduction

This mechanism states that if there are free ions or ionic species present in the substance being heated. The electric field generates ionic motion as the molecules try to orient themselves to the fast-changing field. This causes quick superheating.

Advantages of microwave heating over conventional heating

1. Microwave heating does not depend on the thermal conductivity of the vessel whereas conventional heating depends on the thermal conductivity of the vessel.

2. Microwave heating enhanced product purities as unwanted side reactions are less compared to conventional heating.

3. In microwave heating, the reaction mixture is homogeneously heated whereas, in conventional heating, heat is transferred from surface to reaction solvent and then to species, making it less efficient.

4. In microwave heating, reaction time is dramatically reduced as compared to conventional heating.

5. Microwave heating gives us better reaction kinetics compared to conventional heating.

6. Microwave heating gives us more product yield with high purity as compared to conventional heating.

7. Microwave heating has high energy efficiency as the surrounding heat loss can be reduced as compared to conventional heating.

8. Microwave heating minimizes the chances of unwanted side chemical reactions compared to conventional heating.

9. Microwave heating is modern and more reliable as compared to conventional heating.

10. Microwave heating gives high reproducibility in results as compared to conventional heating.

Solvents used in the microwave-assisted synthesis

Solvents having higher dielectric constant are preferred like water, methanol, DMF, ethyl acetate, acetone, acetic acid, etc.

Solvents having a low dielectric constant like hexane, toluene, carbon tetrachloride, etc. are not preferred because they do not couple and do not get heated rapidly under microwave irradiation.

Examples of microwave-assisted reactions

1. Microwave-assisted reactions in water

1. Hofmann Elimination

In this method, thermally unstable Hofmann elimination product is synthesized using microwave irradiation. It is done in a water-chloroform system to get a high yield.

2. Hydrolysis of methyl benzoate to benzoic acid (Saponification)

Saponification of methyl benzoate in aqueous sodium hydroxide under microwave conditions (2.5 min) gives 84% yield of the benzoic acid.

3. Oxidation of Toluene

Oxidation of toluene with KMnO4 under normal conditions of refluxing takes 10-12 hr compared to the reaction in microwave conditions, which takes only 5 min and the yield is 40%.

4. Oxidation of Alcohols

(a) Oxidation of Primary alcohols

Primary alcohols can be oxidized to carboxylic acid using sodium tungstate as a catalyst in 30% aqueous hydrogen peroxide.

(b) Oxidation of Secondary Alcohols

R, R1 = Various aromatic, aliphatic, and heterocyclic groups

Secondary alcohols can be oxidized under microwave irradiation by using doped supports like clayfen, silica manganese dioxide, clay cap, CrO3- wet alumina, iodobenzene diacetate-alumina, CuSO4-alumina, and oxone-wet alumina.

Oxidation of linear and cyclic secondary alcohols and benzylic alcohols to carbonyl compounds can be done under microwave irradiation conditions.

2. Microwave-assisted reactions in organic solvents

1. Diels-Alder reaction

This reaction involves 1,4-addition of an alkene (say maleic anhydride) to a conjugated diene (say anthracene) to form an adduct of a six-membered ring. Under usual conditions, the reaction requires a reflux period of 90 min. However, under microwave conditions, diglyme is used as a solvent and 80% yield of the adduct is obtained in 90 seconds.

2. Decarboxylation reaction

Conventional decarboxylation of carboxylic acids involves refluxing in quinoline in presence of copper chromite and the yields are low. However, in the presence of microwaves, decarboxylation takes place in a much shorter time.