C5H9NO

C5H9NO is the molecular formula for a family of organic compounds that contain five carbon atoms, one nitrogen atom, and one oxygen atom, with nine hydrogen atoms. The formula corresponds to two degrees of unsaturation, indicating that any isomers involve rings, double bonds, or both.

In practice, C5H9NO most commonly arises from five‑membered lactam structures (pyrrolidinones). The simplest parent is a gamma-lactam, and the formula can be satisfied by lactams with various substituents. A well-known example is N-methyl-2-pyrrolidinone (NMP), a widely used polar aprotic solvent. NMP is a colorless liquid with a high boiling point, miscible with water and many organic solvents, and it is employed in polymer processing, electronics, and battery electrolytes.

Other isomers with the same formula include methyl-substituted pyrrolidinones such as 3-methyl-2-pyrrolidinone, as well as other regioisomeric forms on the lactam ring. These compounds are generally less common in commercial use but can appear as intermediates in organic synthesis or in specialized applications.

Because C5H9NO describes multiple possible structures, the formula alone does not identify a single compound. Structural information is required to determine the exact substance, its properties, and its applications.

See also: pyrrolidinones, gamma-lactams, N-methyl-2-pyrrolidinone.