Ionic Liquids: Structural Properties, Synthetic Approaches, and Characterization

ABSTRACT Ionic liquids (ILs), defined as salts with melting points below 100 °C, have emerged as a versatile class of materials with tunable physicochemical properties, low volatility, and thermal stability. This review presents a comprehensive examination of ILs, distinguishing them from traditional electrolytes and deep eutectic solvents (DESs) by their purely ionic composition. The manuscript discusses the structural heterogeneity arising from bulky, asymmetric ions and delocalized charges, which underpin their functional diversity. Generational evolution of ILs is mentioned, emphasizing advances in designing biocompatible and low‐toxicity ILs through the incorporation of benign functional groups or bio‐derived building blocks. Synthetic routes, including acid‐base neutralization, metathesis, alkylation, microwave irradiation, and ultrasound‐assisted techniques, are critically evaluated. Characterization tools such as nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and advanced spectroscopic methods are discussed in the context of structural elucidation and impurity profiling. Finally, the review underscores the future potential of ILs in enabling sustainable technologies and drug delivery systems by aligning their design with principles of green chemistry and toxicological safety.