4-(Chlorocarbonyl)benzenesulfonic acid is primarily used as an intermediate in organic synthesis. Its reactive carboxylic acid chloride group facilitates the formation of amide and ester bonds, enabling the construction of complex molecules for pharmaceuticals, dyes, and other fine chemicals. The sulfonic acid moiety provides enhanced water solubility and polarity, making it suitable for synthesizing hydrophilic derivatives. Additionally, it is employed in the modification of polymers and resins to incorporate sulfonic acid functionalities, improving properties such as solubility, ion-exchange capacity, or catalytic performance. Due to its high reactivity, it must be handled in controlled environments for specialized applications.