Acetaldehyde, Ethanal, CH3CHO.
Acetaldehyde is the most important member of the aldehyde family.
Acetaldehyde is prepared in the laboratory by oxidation of ethanol with acidified sodium dichromate solution.
The apparatus used is shown in Fig 19.2. A mixture of ethanol and sodium dichromate solution is run into boiling dilute H2
. As the liquid comes in contact with the acid, a vigorous reaction takes place and the orange dichromate is reduced to green chromic sulphate. Acetaldehyde containing impurities such as ethanol, acetic acid, and water distils over.
The impure acetaldehyde is purified by converting it into acetaldehyde-ammonia by treatment with ammonia. The crystal of acetaldehyde-ammonia are dried distilled with dilute sulphuric acid and the regenerated acetaldehyde is collected in an ice-cooled receiver.
Acetaldehyde is manufactured:
- By air-oxidation of ethanol. Ethanol vapours and limited amount of air are passed over heated silver catalyst at 500ºC.
- By dehydrogenation of ethanol. Ethanol vapours are passed over heating copper catalyst at 500ºC.
- By hydration acetylene.
- By wacker process. This involves the treatment of ethylene with an acidified aqueous solution of palladium chloride and cupric chloride.
Acetaldehyde is a colourless volatile liquid, bp 21ºC. It has a characteristic pungent smell. Acetaldehyde is a soluble in water, chloroform, ethanol, and diethyl ether.
The chemical properties of acetaldehyde have already been discussed under the ‘General Reaction’ of aldehydes and ketones. Only a few polymerization reactions will be described here.
- When acetaldehyde is treated with a small amount of cone. H2SO4at room temperature, a cyclic trimer Paraldehyde is formed.
Paraldehyde is a liquid, bp 128ºC. It regenerates acetaldehyde on distilling with conc. H2SO4.
- When acetaldehyde is treated with a small amount of conc. H2SO4at 0ºC, a cyclic tetramer Metaldehyde is formed.
Metaldehyde is a solid, mp 246ºC. This tetramer is used for killing slugs and snails.
Acetaldehyde is used in the manufacture of :
- Acetic acid
- Acetic anhydride
- 1.3-Butadiene (used in rubbers)