Acetaldehyde
Acetaldehyde
Acetaldehyde is also known as ethanol. Acetaldehyde is a
colorless, pungent-smelling liquid, b.p. 21oC, miscible with water,
ethanol and ether in all proportions.
Preparation of acetaldehyde
Acetaldehyde is prepared industrially:
1. By the dehydrogenation or air oxidation of ethanol in the
presence of a silver catalyst at 300oC (cf. formaldehyde):
CH3CHO
+ H2 (Ag) ⟵ CH3CH2OH (air+Ag) ⟶ CH3CHO + H2O
2. By the hydration of acetylene. This method is decreasing
in importance.
3. By passing a mixture of ethylene and oxygen, under
pressure, into an aqueous solution of palladium and cupric chlorides at 50oC.
CH2=CH2
+ PdCl2 + H2O ⟶ CH3CHO + Pd
+ 2HCl
2CuCl2 + Pd ⟶ 2CuCl +
PdCl2
2CuCl + 2HCl
+ 1/2O2 ⟶ 2CuCl2 + H2O
Polymers of acetaldehyde
1. When acetaldehyde is treated with a few drops of
concentrated sulphuric acid, a vigorous reaction takes place and the trimer paraldehyde,
(CH3CHO)3, is formed. This ispleasant-smelling liquid,
b.p. 128oC, and is used in medicine as an hypnotic. When paraldehyde is
distilled with dilute sulphuric acid, acetaldehyde is regenerated. Paraldehyde has
no reducing properties, and its structure is believed to be (I).
2. When acetaldehyde is treated with a few drops f concentrated
sulphuric acid at 0oC, the tetramer metaldehyde (CH3CHO)4,
is formed. This is a white solid, m.p. 246oC, and regenerates
acetaldehyde when distilled with dilute sulphuric acid. Its structure may be
(II).
Use of Acetaldehyde
Acetaldehyde is used in the preparation of acetic acid, ethanol,
paraldehyde, rubber accelerators, phenolic resins, synthetic drugs, etc.
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