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1=Organic compound of the form >C=C=O
the generic structure class

In organic chemistry, a ketene is an organic compound of the form |RR'C\dC\dO, where R and R' are two arbitrary monovalent chemical groups (or two separate substitution sites in the same molecule). Although they are highly useful, most ketenes are unstable. When used as reagents in a chemical procedure, they are typically generated when needed, and consumed as soon as (or while) they are produced.

Ketenes were first studied as a class by Hermann Staudinger before 1905. Ketenes were systematically investigated by Hermann Staudinger in 1905 in the form of diphenylketene (conversion of chlorodiphenyl acetyl chloride with zinc). Staudinger was inspired by the first examples of reactive organic intermediates and stable radicals discovered by Moses Gomberg in 1900 (compounds with triphenylmethyl group).

Ketenes are highly electrophilic at the carbon atom bonded with the heteroatom, due to its sp character. Ketene can be formed with different heteroatom bonded to the sp carbon atom, such as O, S or Se, respectively named ketene, thioketene and selenoketene. Ethenone, the simplest ketene, has different experimental lengths for each of the double bonds; the C=O bond is 1,160Å and the C=C bond is 1,314Å. The angle between the two H atoms is 121.5°, similar to the theoretically ideal angle formed in alkenes between sp2 carbon atom and H substituents. Ketenes are unstable and cannot be stored. In the absence of nucleophiles with which to react, ethenone dimerises to give β lactone, a cyclic ester. If the ketene is disubstituted, the dimerisation product is a substituted cyclobutadione. For monosubstituted ketenes, the dimerisation could afford either the ester or the diketone product.

Due to their cumulated double bonds, ketenes are very reactive.

Enol esters are formed from ketenes with enolisable carbonyl compounds. The following example shows the reaction of ethenone with acetone to form a propen 2 yl acetate:

Ketenes can react with alkenes, carbonyl compounds, carbodiimides and imines in a [2+2] cycloaddition. The example shows the synthesis of a β lactam by the reaction of a ketene with an imine (see Staudinger synthesis):

Ketenes are generally very reactive, and participate in various cycloadditions. One important process is the dimerization to give propiolactones. A specific example is the dimerization of the ketene of stearic acid to afford alkyl ketene dimers, which are widely used in the paper industry. AKD's react with the hydroxyl groups on the cellulose via esterification reaction. They will also undergo [2+2] cycloaddition reactions with electron rich alkynes to form cyclobutenones, or carbonyl groups to form beta lactones. With imines, beta lactams are formed. This is the Staudinger synthesis, a facile route to this important class of compounds. With acetone, ketene reacts to give isopropenyl acetate. A variety of hydroxylic compounds can add as nucleophiles, forming either enol or ester products. As examples, a water molecule easily adds to ketene to give 1,1 dihydroxyethene and acetic anhydride is produced by the reaction of acetic acid with ketene. Reactions between diols (|HO\sR\sOH) and bis ketenes (|O\dC\dCH\sR'\sCH\dC\dO) yield polyesters with a repeat unit of (|\sO\sR\sO\sCO\sR'\sCO). Ethyl acetoacetate, an important starting material in organic synthesis, can be prepared using a diketene in reaction with ethanol. They directly form ethyl acetoacetate, and the yield is high when carried out under controlled circumstances; this method is therefore used industrially.