JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy III Year GREEN CHEMISTRY Previous Question Paper (September-2021 - R17)

Microwave and ultrasonic irradiation are non-conventional energy sources used in green chemistry to accelerate chemical reactions, improve yields, and reduce solvent consumption by providing efficient energy transfer.

Microwave-Assisted Synthesis: This technique utilizes electromagnetic radiation in the frequency range of 0.3 to 300 GHz. The principle relies on the ability of polar molecules and ions to absorb microwave energy and convert it into heat through two primary mechanisms: dipolar polarization and ionic conduction. Dipolar molecules attempt to align with the rapidly oscillating electric field, causing molecular rotation and internal friction, while ionic conduction involves the migration of dissolved ions under the influence of the field. This results in rapid, uniform internal heating throughout the reaction mixture, often leading to shorter reaction times and cleaner products.

Example: The synthesis of aspirin (acetylsalicylic acid) from salicylic acid and acetic anhydride can be performed in minutes using microwave irradiation, significantly faster than conventional heating methods.

Ultrasonic-Assisted Synthesis (Sonochemistry): This technique uses sound waves with frequencies typically between 20 kHz and 10 MHz. The principle is based on the phenomenon of acoustic cavitation, which involves the formation, growth, and implosive collapse of microscopic bubbles in a liquid medium. When these bubbles collapse, they generate localized 'hot spots' with extreme temperatures (thousands of Kelvin) and high pressures (hundreds of atmospheres) for a very short duration. These conditions facilitate the breaking of chemical bonds and promote the formation of reactive intermediates, effectively accelerating reaction rates.

Example: The Reformatsky reaction, which involves the coupling of an aldehyde or ketone with an alpha-halo ester in the presence of zinc, is significantly enhanced by ultrasound, which cleans the metal surface and promotes the formation of the organozinc reagent.

Green chemistry, often referred to as sustainable chemistry, focuses on designing chemical products and processes that reduce or eliminate the use and generation of hazardous substances. Its applications span across various industrial, pharmaceutical, and environmental sectors, aiming to improve efficiency and safety.

Key applications include:

1. Pharmaceutical Industry: Green chemistry is applied to synthesize drugs with higher atom economy and fewer toxic byproducts. For example, the synthesis of Ibuprofen was redesigned to reduce the number of steps from six to three, significantly increasing atom economy and eliminating the need for auxiliary reagents.
2. Solvent Replacement: Replacing hazardous organic solvents with safer alternatives like water, supercritical carbon dioxide, or bio-based solvents is a major application. Supercritical CO2 is widely used in decaffeination processes and dry cleaning, replacing toxic chlorinated solvents like perchloroethylene.
3. Catalysis: Utilizing catalysts allows reactions to proceed at lower temperatures and pressures, reducing energy consumption. Enzyme-catalyzed reactions (biocatalysis) are increasingly used in the production of fine chemicals and chiral intermediates, offering high selectivity and reducing waste.
4. Polymer Science: Development of biodegradable plastics, such as polylactic acid (PLA) derived from renewable biomass like corn starch, reduces reliance on petroleum-based feedstocks and mitigates environmental persistence of plastic waste.
5. Energy Sector: Green chemistry contributes to the development of more efficient solar cells, fuel cells, and batteries. For instance, the use of non-toxic, earth-abundant materials in photovoltaic cells reduces the environmental impact of energy storage and conversion technologies.
6. Pesticide and Agriculture: Designing targeted, biodegradable pesticides that degrade into harmless components minimizes the accumulation of toxic residues in the food chain and groundwater.

By integrating these principles, industries can achieve significant reductions in waste production, energy usage, and hazardous emissions, leading to more sustainable manufacturing and environmental preservation.