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).
Acetaldehyde , ethanal

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.