Green Sustainable Process for Chemical and Environmental Engineering and Science

Microwaves in Organic Synthesis

Rajender Boddula, Abdullah M. Asiri, Dr. Inamuddin

• Publisher: Elsevier
• Year: 2020
• Print ISBN:
  9780128198483
• eBook ISBN:
  9780128226704
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería química y bioquímica

Microwaves in Organic Synthesis provides an in-depth overview in the area of organic and pharmaceutical chemistry of the microwave technology in separation, purification and extraction of medicinal, biological, and organic compounds.This book methodically explores the application of microwaves in all types of organic synthesis. It includes stereoselectivity, regioselectivity, oxidation, reduction, protection, deprotection, addition, condensation, coupling, C-X bond formation, named reactions, heterocyclic, biological drugs, fluoro-organics and polymers. After a brief introduction discusses the main parameters which influence the process, and the limitations and advantages of the practical use of microwave in organic synthesis. This book is a vital resource on green chemistry technologies for students and academic researchers, R& D professionals, students and university professors working in the field of organic chemistry, medicinal chemistry and chemical engineering. - Outlines microwave technology for green organic synthesis - Includes a description of the significant factors and challenges of the microwave-assisted green organic synthesis - Outlines the eco-friendly microwave based chemical processes and their applications in organic reactions, polymer synthesis, biofuel production, etc. - Gives detail account of the numerous real industrial applications such as polymers, pharmaceutical, fluoroorganics, biofuel, carbon, etc. - Discusses recent advances in microwave technology in organic chemistry

Chapter 1: Microwave catalysis in organic synthesis

Kavita J. Lanjekar; Virendra K. Rathod Department of Chemical Engineering, Institute of Chemical Technology, Mumbai, India

Abstract

Microwave radiation is a proven technology to enhance the efficiency of organic synthesis. The selective absorption of radiation by polar compounds and conversion of electromagnetic energy into heat by dielectric losses forms the basis of the nonconventional microwave dielectric heating. This technology facilitates the acceleration of the reaction rate and enhancement in the product yields through uniform and selective heating thereby; it aids in the development of cleaner and greener synthetic routes. The initial part of the chapter is more comprehensive and discusses the theoretical background, microwave mechanisms, and role of microwaves in different organic reactions. The latter part of the chapter discusses briefly the types of microwave reactors, current challenges in microwave-assisted synthesis regarding energy efficiency, and scale-up of organic transformations.

Keywords

Microwave; Catalysis; Organic reactions; Dielectric heating; Synthesis

1: Introduction

Microwave technology is a transformational technique for organic synthesis. Microwave irradiation has gained popularity as a nonconventional energy source because it offers an enhanced rate of conversions and improved yields within a very short span of time for the production of diverse organic molecules. In 1855, Robert Bunsen invented the Bunsen burner which supplied heat energy in a focused manner from this source to a reaction vessel for organic synthesis. Later, the advent of oil baths or hot plates replaced the Bunsen burner, whereas in the 21st century the use of microwave energy for organic reactions has been a bonanza for the scientific community.

1.1: History

Microwave energy was originally utilized to heat up food. Application of microwaves for cooking food was accidentally discovered by Percy Spencer in the 1940s when he was constructing magnetrons for radar sets with the company Raytheon. He observed that a peanut chocolate bar he had in his pocket started to melt with microwaves. Eventually, Spencer's microwave was purposefully used to cook the first food which was popcorn, and the second was an egg. In 1946, the Raytheon Company patented the microwave dielectric heating for food and a prototype oven was set up in a Boston restaurant. During the 1970s and 1980s, domestic microwave ovens became popular and were in use widely.

1.2: Early development in utilization of microwave heating for organic synthesis

In 1986, Gedye and Giguere pioneered the application of microwaves for organic synthesis using domestic microwave ovens [1, 2]. They proved that microwave heating could cause significant acceleration of the rate of reactions by independently reporting several successful microwave-assisted reactions. In initial days, experiments were usually performed in sealed glass or Teflon vessels without any temperature or pressure measurement in a domestic microwave oven. Under closed-vessel conditions, intensive explosions were frequent owing to the quick heating of organic solvents in an uncontrolled manner. For organic synthesis, there was a slow uptake of microwave technology till the mid-1990s because of inadequate measures of reaction parameters, lack of safety in domestic microwave ovens, and lack of reproducibility. However, due to sheer interest in shorter reaction time, there was an advent of commercial microwave equipment developed for organic chemistry with improved safety aspects, which resulted in increased use of microwaves in organic synthesis.

2: Factors influencing microwave heating in organic reactions

Microwaves are electromagnetic radiations with frequencies between 0.3 and 300 GHz and have wavelengths of 1 mm–1 m (Fig. 1). Of these, many of the frequencies are used by telecommunication and microwave radar equipment. To ensure no intervention, the frequency of industrial and domestic microwave apparatus manoeuvres is regulated to 2.450 (± 0.050) GHz and a wavelength of 12.2 cm. Therefore, electromagnetic radiation of a specific frequency is used in microwave instruments for organic reactions to bring about change in temperature.

Fig. 1 The spectrum of electromagnetic waves. Reproduced from K. K. Rana, S. Rana, Microwave reactors: a brief review on its fundamental aspects and applications, Open Access Libr. J. 01(686) (2014) 1–20.

2.1: Microwave heating mechanism

Microwave radiation has two components—electric field and magnetic field. The heating is due to the electric field component through two main mechanisms: Dipolar polarization and ionic conduction (Fig. 2).

Fig. 2 Schematic illustration of the two main mechanisms responsible for dielectric heating by microwave radiation. Reproduced from V. Gude, P. Patil, E. Martinez-Guerra, S. Deng, N. Nirmalakhandan, Microwave energy potential for biodiesel production, Sustain. Chem. Process. 1(5) (2013) 1–31.

2.1.1: Dipolar polarization mechanism

A substance must possess a dipole moment (i.e., molecular structure with a partly positive and partly negative charge) so as to generate heat under microwave irradiation. When such molecules are positioned in an oscillating electric field, they attempt to align with the field by rotation. The dipole reorients itself to align with the changing electric field, resulting in phase difference between the field orientation and that of the dipole. This phase difference causes molecular frictions and collisions, giving rise to heat energy.

For example, when water and benzene are irradiated respectively, in a microwave instrument at constant parameters of r...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Chapter 1: Microwave catalysis in organic synthesis
• Chapter 2: Microwave-assisted CN formation reactions
• Chapter 3: Microwave-assisted multicomponent reactions
• Chapter 4: Catalytic, ultrasonic, and microwave-assisted synthesis of naphthoquinone derivatives by intermolecular and intramolecular N-arylation reactions
• Chapter 5: Microwave-assisted condensation reactions
• Chapter 6: Microwave-assisted oxidation reactions
• Chapter 7: Microwave-assisted reduction reactions
• Chapter 8: Microwave-assisted stereoselective organic synthesis
• Chapter 9: Microwave-assisted heterocyclics
• Chapter 10: Microwave-assisted synthesis of fluoroorganics
• Chapter 11: Microwave-assisted Grignard reactions
• Chapter 12: Microwave-assisted for solvent-free organic synthesis
• Chapter 13: Microwave-assisted catalyst-free organic synthesis
• Chapter 14: Application of microwave synthesis in biodiesel production
• Chapter 15: High-performance microwave reactors for small-scale microwave synthesis
• Chapter 16: Microwave reflux extraction—An alternative approach for phenolic-rich oleoresins extraction from functional plants
• Chapter 17: Microwave-assisted synthesis of chiral polymeric materials: Properties and applications
• Index