4-Ethylbenzoic acid is an organic compound featuring a benzoic acid core substituted with an ethyl group at the para position. It is a white to off-white crystalline solid with a molecular formula of C9H10O2 and a molecular weight of 150.17 g/mol. It is soluble in common organic solvents like ethanol and ether but has limited solubility in water. This compound serves as a versatile organic synthesis building block. Its carboxylic acid group allows for easy derivatization into esters, amides, or acid chlorides, while the aromatic ring can undergo various electrophilic substitution reactions. A significant application of 4-ethylbenzoic acid and its derivatives is in the field of materials science, particularly as precursors or intermediates for organic light-emitting diodes (OLEDs) and other organic electronic materials. The ethyl-substituted benzoic acid structure can be incorporated into larger conjugated systems or used to modify the properties of charge-transporting or emissive materials. As with many organic acids, it may cause irritation to the skin, eyes, and respiratory system. Proper personal protective equipment should be used when handling. It is primarily used in research and development settings for synthetic chemistry and advanced material fabrication.