Cyclohexanone is the organic compound with the formula (CH₂)₅CO. The molecule consists of six-carbon cyclic molecule with a ketone functional group. This colorless oily liquid has a sweet odor reminiscent of benzaldehyde. Over time, samples of cyclohexanone assume a pale yellow color.

Cyclohexanone is slightly soluble in water and miscible with common organic solvents. Millions of tonnes are produced annually, mainly as a precursor to nylon.

The compound was discovered by Edmund Drechsel [de] in 1888 among the products of AC electrolysis of slightly acidified water solutions of phenol. He named it hydrophenoketone and correctly suggested that phenol was first hydrogenated by electrolytic hydrogen to cyclohexanol, which he wasn't able to isolate, and then oxidized by electrolytic oxygen.

Cyclohexanone can be prepared from cyclohexanol by oxidation with chromium trioxide (Jones oxidation). An alternative method utilizes the safer and more readily available oxidant sodium hypochlorite.

Cyclohexanone is produced by the oxidation of cyclohexane in air, typically using cobalt catalysts:

This process forms cyclohexanol as a by-product, and this mixture, called "KA Oil" for ketone-alcohol oil, is the main feedstock for the production of adipic acid. The oxidation involves radicals and the hydroperoxide C₆H₁₁O₂H as an intermediate. In some cases, purified cyclohexanol, obtained by hydration of cyclohexene, is the precursor. Alternatively, cyclohexanone can be produced by the partial hydrogenation of phenol:

This process can also be adjusted to favor the formation of cyclohexanol.

ExxonMobil developed a process in which benzene is hydroalkylated to cyclohexylbenzene. This latter product is oxidized to a hydroperoxide and then cleaved into phenol and cyclohexanone. Therefore, this newer process without producing the acetone by-product appears attractive and is similar to the cumene process as a hydroperoxide is formed and then decomposed to yield two key products.