Organic compounds that have two carbonyl (>C=O) groups (see ketones). There are three kinds, depending on the location of the carbonyl groups. 1,2-Diketones (also called α-diketones), R.CO.CO.R′, have the carbonyl groups on adjacent carbon atoms. The aliphatic 1,2-diketones are pungent-smelling yellow oils, whereas the aromatic compounds are crystalline solids. 1,3-Diketones (or β-diketones), R.CO.CH2.CO.R′, are more acidic and exist in both keto and enol forms (see keto-enol tautomerism); they form stable compounds with metals. 1,4-Diketones (or γ-diketones), R.CO.CH2.CH2.CO.R′, also exist and readily rearrange to form cyclic compounds.