Ketens are compounds which are characterized by the presence of the grouping C=C=O. if the compounds is of the type RCH=C=O, it is known as an aldoketen; and if R2C=C=O, then a ketoketen.

Ketoketens are generally prepared by debrominating an a-bromoacyl bromide with zinc, e.g., dimethylketen from a-bromoisobutyryl bromide:

(CH3)CBrCOBr  +  Zn    ⟶   (CH3)2C=C=O   +   ZnBr2

Aldoketens are usually prepared by refluxing an acid chloride in pyridine solution, e.g.,

CH3CH2COCl  +  C5H5N    ⟶    CH3CH=C=O   +   C5H5NH+Cl-

The simplest member of the keten series is keten (ketene), CH2=C=O,  and may be prepared by debrominating bromoacetyl bromide with zinc:

CH2BrCOBr  +   Zn    ⟶    CH2=C=O   +   ZnBr2

It is, however, usually prepared by the thermal decomposition of acetone (ethyl acetate or acetic anhydride may also be used as the starting material):

CH3COCH3    ⟶   CH2=C=CO   +   CH4

The thermal decomposition of acetone has been shown to be a free-radical chain reaction.

Keten is a colorless, poisonous, pungent gas, b.p. -41oC, which oxidizes in air to the unstable peroxide, the structure of which may be as shown. It rapidly polymerises to diketen, and undergoes photochemical decomposition to give methylene.

The reactions of keten are generally those of an acid anhydride; it acetylates most compounds with an active hydrogen atom.

 The mechanism of acetylations by keten can be generalized in terms of the addition of a nucleophile as the rate-determining step:


1.  When keten is passed into water, acetic acid is formed slowly:

CH2=CO   +   H2O    ⟶    CH3CO2H

2.  When keten is passed into glacial acetic acid, acetic anhydride is formed:

CH2=CO   +   CH3CO2H    ⟶    (CH3CO)2O

By means of this reaction a mixed anhydride may be obtained, one group of which, of course, must be acetyl:

RCO2H   +   CH2=CO    ⟶    RCOOCOCH3

3.  Keten reacts with aliphatic or aromatic hydroxyl-compounds:

CH2=CO   +   ROH    ⟶    CH3CO2R

The reaction, however, is best carried out in the presence of a catalyst, e.g., sulphuric acid.

4.  keten reacts with Grignard regents to form methyl ketones:

RMgX   +   CH2=CI    ⟶   RCOCH3

In addition to behaving as an acetylating regent, keten behaves as an unsaturated compound, e.g.,

1.  Keten adds on bromine to form bromoacetyl bromide:

CH2=CO   +   Br2   ⟶    BrCH2COBr

The halogeno-acetyl halide is also formed by the interaction of keten and phosphorus pentahalides:

CH2=CO   +   PCl5    ⟶   ClCH2COCl  +   PCl3

2.  Keten adds on halogen acid to form the acetyl halide:

CH2=CO   +   HX    ⟶    CH3COX

Alkyl halides also react with keten in the presence of charcoal at 100oC to form acid chlorides:

RX   +   CH2=CO    ⟶    RCH2COX

This offers a means of stepping up a series.