**Abstract**

In the Conventional laboratory or industry heating technique involve Bunsen burner, heating mental/hot plates and electric heating ovens. To produce a variety of useful compounds for betterment of mankind, the Microwave Chemistry was introduced in year 1955 and finds a place in one of the Green chemistry method. In **Microwave chemistry** is the science of applying microwave radiation to chemical reactions. Microwaves act as high frequency electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid. Polar solvents are heated as their component molecules are forced to rotate with the field and lose energy in collisions i.e. the dipole moments of molecules are important in order to proceed with the chemical reactions in this method. It can be termed as microwave-assisted organic synthesis (*MAOS*), Microwave-Enhanced Chemistry **(***MEC***)** or Microwave-organic Reaction Enhancement synthesis **(***MORE***).** Microwave-Assisted Syntheses is a promising area of modern Green Chemistry could be adopted to save the earth.

**Keywords:** Green synthesis, Microwave synthesis, Organic reactions, Nanomaterials

### **1. Introduction**

Chemistry play major part in our daily life in the form of medicines, food colours, soaps, detergents, sunscreen lotions, toothpaste, pharmaceuticals, etc. what we use in our daily life literally everything. Scientist from various disciplines had used their knowledge and skill to produce these materials. To produce these chemicals in industries and laboratory several techniques are used like stirring, heating, refluxing, protection of specific functional group and deprotection of them. Several harmful materials were also generated during the above processes causing environment pollution. To combat these problems, Green Chemistry has emerged as a challenge for scientist from industries and academia to develop the synthetic process for sustainable development of society in last few decades. In this context, several green synthetic techniques were developed to generate a vast library of organic compounds.

In the Conventional laboratory or industry heating technique involve Bunsen burner, heating mental/hot plates/furnace/oil bath and electric heating ovens. These process comprise the heating of walls of vessels first by convection or conduction of energy then the reaction content, leading to much longer time to achieve the target temperature. These techniques not only give useful materials but also give so many by products in the form of solid, liquid and gases, resulting to

lot of chemical pollution to mankind as well as for other living creatures on Earth. Furthermore, to produce these useful materials, several resources are also going to be consumed. To combat these problems the various arena of Scientist are trying to develop new Green techniques.

Conventional heating usually involves the use of a furnace or oil bath, which heats the walls of the reactor by convection or conduction method. The core of the sample takes much longer to achieve the target temperature, e.g. when heating a large sample of ceramic bricks (**Figure 1**). The microwave portion of electromagnetic radiation is emerged a significant area in the acceleration of chemical reactions in 1980s. A number of reviews were reported by various scientists [1–10]. There are three main reasons for its special place of microwave (MW) assisted synthesis. The first point is high heating efficiency caused by MW energy, which reaches and absorbed directly by substance. The second is selectivity i.e. having dielectric polar molecule will absorb this energy. And thirdly enhancement of chemical syntheses by the MW effect or non-thermal effect. In other sense, the microwave irradiation energy acts as internal heat source, which is able to heat the target compounds without heating the entire furnace or oil bath, consequently, saves time and energy. It produces more uniform heating. This unconventional energy source (MW) has been used to heat food stuff since more than 5 decades and now the concept is also utilized to accelerate a wide range of chemical reactions. The organic synthesis has very special and significant role in the improvement of day to day life of human being from morning toothpaste to all useful medications/drugs, plastic/polymer materials etc. To produce these useful substance in reasonable time and environment friendly manner is the prime responsibility of the scientist of several discipline. In this regard Microwave heating technique has blossomed in a variety of applications in Organic Synthetic methods. Although the initial synthetic work was performed in domestic Microwave ovens but recently most organic reactions are carried out in dedicated high speed specialized Microwave equipment with appropriate pressure and temperature control.

Microwaves (MW) are the electromagnetic waves, having shortest wavelength of radio spectrum. The word microwave means "a very short wave". Microwave irradiation is a low energy phenomena as compared to other radiations. The energy of MW is about 0.0016 eV at 2450 MHz, which is very low to break any chemical bond. However, a microwave energy can effect a number of chemical transformation. When a substance is exposed to microwave energy, then three process can occur. Firstly, if it is a conductor metal, then MW will be reflected and it cannot be heated. Secondly, if insulator material is exposed to MW it will be not heated since it is MW transparent. Thirdly if material is dielectric in nature then MW energy will be absorbed and gets heated. This case is very interesting for a Chemist.

A microwave works by directly coupling with polar molecule in a reacting species. Microwaves radiation are a form of non-ionizing electromagnetic radiation

**179**

**2. Microwave equipment**

wave and prevent such damage.

*Microwave Assisted Synthesis of Organic Compounds and Nanomaterials*

food. As noted microwave ovens can operate at many frequencies.

with 300 MHz to 300 GHz frequency with the corresponding wavelength 1 mm to 1 m, which places MW in between Infrared radiations and radio waves. The molecules get heated in microwave irradiation process by a dielectric heating phenomenon. The electric dipoles, i.e. a partial positive charge at one end and a partial negative charge at the other, rotate as they try to align themselves with the alternating electric field of the microwaves. Additionally, molecular rotation occurs in a material/food (containing polar molecules) causing electric dipole moment which can partially/completely aligned themselves with an applied electromagnetic field. Since electromagnetic field alternates, the rotating molecule resulting they change their directions by pushing/pulling/colliding. Rotating molecules hit other molecules and put them into motion, thus dispersion of energy happens. This energy, dispersed as molecular rotations, vibrations and/or translations in solids and liquids, raises the temperature of the food, in a process similar to heat transfer by contact with a hotter body. It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the

Additionally, nanomaterials or nanoparticles (NPs) are the materials having a single unit small sized between 1 and 100 nm. During the past few decades nanoparticles research have become a subject of intensive interest because of its potential applications in various fields i.e. biosensing, drug delivery, bioimaging, catalysis, lubrication, electronics, textile manufacturing, water treatment systems. Nanoparticles (NPs) are synthesized from several types of materials i.e. inorganic, organic, hybrid and biological by adopting various methods. The main methods are ball milling, vapor deposition, electro-spraying, reduction of metal salts, sol–gel, coprecipitation and thermal decomposition for inorganic NPs. While organic NPs are synthesized by microemulsion, nanoprecipitation, dialysis and rapid expansion of supercritical solutions. Hybrid NPs are synthesized from both organic and inorganic materials. However, the present trend for the synthesis of nanomaterials endures a challenge to redesign the synthetic strategies offering the use of less hazardous chemicals and reduction in the reaction time and required energy. In this context, microwave (MW)-assisted methods can be considered as a promising green approach for synthesis of nanomaterials and nanocomposites. Besides, MW-assisted strategies offer a homogenous heating with reliable nucleation and growth environment leading to the formation of NPs with uniform size. Several reviews were reported on microwave assisted synthesis of NPs [11, 12].

Though the range of MW frequencies is 0.3 to 300 GHz, out of this most of the frequencies are used in Radar and Telecommunication equipment. However very limited frequency range can be used in microwave heating equipment i.e. 2.45 GHz corresponding to 12.2 cm wave length. So almost all domestic and commercial microwave heating equipment either for domestic or scientific purpose has a fixed frequency 2.45 GHz. In general MW accelerated reactions are carried out either domestic microwave oven or especially designed microwave equipment. These equipment works in between 500 to 1500 W power. In these ovens Microwaves are generated by magnetron, and the temperature is maintained by turning this on and off. The generated microwaves are travelled into vessel (cavity) and reflected back by the walls of cavity. If the generated microwaves are not absorbed, it may reflect back down and damage the magnetron. Thus it is essential to have microwave active dummy load should be used during the process, which will absorb excess micro-

*DOI: http://dx.doi.org/10.5772/intechopen.98224*

**Figure 1.** *Process of heating.*

#### *Microwave Assisted Synthesis of Organic Compounds and Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.98224*

with 300 MHz to 300 GHz frequency with the corresponding wavelength 1 mm to 1 m, which places MW in between Infrared radiations and radio waves. The molecules get heated in microwave irradiation process by a dielectric heating phenomenon. The electric dipoles, i.e. a partial positive charge at one end and a partial negative charge at the other, rotate as they try to align themselves with the alternating electric field of the microwaves. Additionally, molecular rotation occurs in a material/food (containing polar molecules) causing electric dipole moment which can partially/completely aligned themselves with an applied electromagnetic field. Since electromagnetic field alternates, the rotating molecule resulting they change their directions by pushing/pulling/colliding. Rotating molecules hit other molecules and put them into motion, thus dispersion of energy happens. This energy, dispersed as molecular rotations, vibrations and/or translations in solids and liquids, raises the temperature of the food, in a process similar to heat transfer by contact with a hotter body. It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food. As noted microwave ovens can operate at many frequencies.

Additionally, nanomaterials or nanoparticles (NPs) are the materials having a single unit small sized between 1 and 100 nm. During the past few decades nanoparticles research have become a subject of intensive interest because of its potential applications in various fields i.e. biosensing, drug delivery, bioimaging, catalysis, lubrication, electronics, textile manufacturing, water treatment systems. Nanoparticles (NPs) are synthesized from several types of materials i.e. inorganic, organic, hybrid and biological by adopting various methods. The main methods are ball milling, vapor deposition, electro-spraying, reduction of metal salts, sol–gel, coprecipitation and thermal decomposition for inorganic NPs. While organic NPs are synthesized by microemulsion, nanoprecipitation, dialysis and rapid expansion of supercritical solutions. Hybrid NPs are synthesized from both organic and inorganic materials. However, the present trend for the synthesis of nanomaterials endures a challenge to redesign the synthetic strategies offering the use of less hazardous chemicals and reduction in the reaction time and required energy. In this context, microwave (MW)-assisted methods can be considered as a promising green approach for synthesis of nanomaterials and nanocomposites. Besides, MW-assisted strategies offer a homogenous heating with reliable nucleation and growth environment leading to the formation of NPs with uniform size. Several reviews were reported on microwave assisted synthesis of NPs [11, 12].
