Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

128 ACETIC ANHYDKIDE
solution more rapidly if dilute alkali hydroxide solution is used
instead of water.
Acetic anhydride is very often used to introduce the acetyl group
into an alcoholic or phenolic hydroxyl group or into a derivative of
ammonia HNRRj^. The reaction is accelerated to an extraordinary
extent by the presence of a drop of concentrated sulphuric acid.
Experiment.—Acetic anhydride is added to alcohol, aqueous
ammonia, aniline, and phenol. A drop of concentrated sulphuric
acid is added to the phenol mixture.
By thermal decomposition on the surface of a glowing platinum
wire, acetic anhydride loses water and is converted into ketene, the
unimolecular anhydride of acetic acid (Wilsmore) :
H3C.CO
0 -H2O —>• 2H2C:CO.
H3C.CO
In practice ketene is prepared by the thermal decomposition of
acetone (Schmidlin) :
CH3.CO.CH3 —> CH2:CO + CH4.
Ketene can be prepared conveniently and in good yield by means of
E. Ott's " ketene lamp "}
When water is excluded ketene also serves as an acetylating agent.
When the close relationship between the two classes of compounds
is examined more closely, the analogy of the anhydrides to the acid
chlorides becomes more intelligible. In both, the hydroxyl of the carboxyl
group is replaced by the anionic portion of an acid : in the chloride
by Cl, in the anhydride by acetoxyl O.CO.CH3.
The anhydrides of the organic acids can also be regarded as diacyloxides
(acyl=acid radicle, e.g. CH3.CO = acetyl) and can be thus
assimilated to the ethers, or dialkyl oxides. The ethers are amongst
the most indifferent of all the conpounds of organic chemistry. Whence,
then, comes the great reactivity of the similarly constituted anhydrides ?
The weak point in the anhydride molecule is to be found, not at the
oxygen bridge, but at the double linkage >C=O. Additions, e.g.
of water and ammonia, take place here :
H3C.C=O
>
H,C.C=O
•"•3
H 3 C . C = O
>
O H
; w i t h N H 3
/ N H 2 |
H g C . C ^ O H
| H 3 C . C = O
J . p r . C h e m . , 1 9 3 1 , 1 3 0 , 1 7 7 ; cf. a l s o B e r l a n d K u l l m a n n , B e r . , 1 9 3 2 , 6 5 , 1 1 1 4 .