Organic compounds are molecules that contain carbon and hydrogen atoms, and can also include other elements such as oxygen, nitrogen, and sulfur. Esters are organic compounds composed of an acid and an alcohol, and are formed when the acid reacts with the alcohol. Enols are organic compounds that contain a hydroxyl group and a double bond between two carbon atoms. Ethers are organic compounds that contain two hydrocarbon groups bonded to an oxygen atom. Ketones are organic compounds that contain a carbonyl group, which is a carbon atom double-bonded to an oxygen atom.