In ethers, oxygen forms two covalent single bonds with two carbon atoms, C–O–C, whereas in alcohols oxygen forms one single bond with carbon and one with hydrogen, C–O–H.
A carbon atom forms one single bond to oxygen in alcohols, ethers, and peroxides, two in acetals,[3]: 524 [5]: 35, 340–348 three in ortho esters,[5]: 345 and four in orthocarbonates.
[6] Carbon forms a double bond to oxygen in aldehydes, ketones and acyl halides.
[7] Carbon and oxygen form terminal double bonds in functional groups collectively known as carbonyl compounds to which belong such compounds as ketones, esters, carboxylic acids and many more.
Internal C=O bonds are found in positively charged oxonium ions.
Compounds with formal C≡O triple bonds do not exist except for carbon monoxide, which has a very short, strong bond (112.8 pm), and acylium ions, R–C≡O+ (typically 110-112 pm).