Ketens
Ketens
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.
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