– 4-Nitrophenyl butyrate is composed of a butyric acid chain esterified with a 4-nitrophenol group, giving it a yellow appearance. This compound is commonly used as a substrate in enzyme assays to measure the activity of esterases and lipases. When these enzymes cleave the ester bond, the nitrophenol group is released, changing the color from yellow to orange. The rate of this color change can be used to determine enzyme activity. Additionally, 4-Nitrophenyl butyrate serves as a reagent in organic synthesis and as an intermediate in dye production.