C5H8O

C5H8O is a chemical formula that represents organic molecules with five carbon atoms, eight hydrogen atoms and one oxygen atom. The degree of unsaturation for this formula is two, indicating that a compound with this composition must contain either two double bonds, one ring and one double bond, or other equivalent unsaturation patterns. As a result, the formula corresponds to a range of structural isomers that include cyclic ketones, conjugated aldehydes, and unsaturated dicarbonyls.

The most common representative of the formula is cyclopentanone, a cyclic ketone that is a colorless liquid with a faint, fruity odor. Cyclopentanone has a molecular weight of 84.10 g·mol–1, a density of 0.82 g·cm–3 at 20 °C, and boils at about 98–100 °C. It is soluble in most organic solvents and miscible with water in limited quantities. Cyclopentanone serves as a versatile intermediate in the synthesis of fragrances, flavors, and pharmaceuticals. Typical reactions include condensation with amines to form imines, addition of nucleophiles across the carbonyl group, and reduction to cyclopentanol.

Other isomers of C5H8O exist, such as 2-methyl-1,3-butadione (a conjugated dicarbonyl) and 2-methyl-2-buten-1-one (an α,β‑unsaturated ketone). These unsaturated forms also appear as intermediates in chemical syntheses and can undergo Diels–Alder reactions, Michael additions, or cross‑coupling reactions. In each case, the presence of the oxygen atom in the form of a carbonyl group allows the compound to act as a good electrophile toward nucleophilic species.

In industrial settings, C5H8O compounds are employed as solvents, co‑solvents, or reactants in the production of polymers, plasticizers, and fine chemicals. Due to its reactive carbonyl group, cyclopentanone and related isomers find use in laboratory-scale organic synthesis, especially for constructing heterocyclic frameworks and in the manufacture of medicinally active molecules.