acetaldehyde(redirected from Acetic aldehyde)
Also found in: Dictionary, Thesaurus, Medical.
acetic aldehyde, CH3CHO, an organic compound; a colorless liquid with a sharp odor. Its boiling point is 20.8°C; its melting point, - 124°C. Its density is 783 kg/m3, and it is miscible in all proportions with water, alcohol, and ether. Acetaldehyde possesses all the properties characteristic of the aldehydes. In the presence of mineral acids it polymerizes into liquid trimeric paraldehyde (CH3CHO)3 and the crystalline tetrameric metaldehyde (CH3CHO)4. When both polymers are heated in the presence of sulfuric acid, acetaldehyde is liberated.
One of the long-known, basic methods for the preparation of acetaldehyde consists in combining water with acetylene in the presence of mercuric salts at a temperature of about 95°C (Kucherov’s reaction):
Because of the high cost and toxicity of mercury, another method for the direct catalytic hydration of acetylene has been worked out. A variant of indirect hydration is also possible; it consists in combining alcohol with acetylene in the presence of solid KOH, followed by saponification of the vinyl ether:
Acetaldehyde is also produced from ethyl alcohol by catalytic dehydrogenation at approximately 400°C. When acted upon by aluminum alkoxides, the acetaldehyde is transformed into ethylacetate (Tishchenko’s reaction):
This method has industrial significance. The oxidation of acetaldehyde by atmospheric oxygen over a catalyst is an industrial method for obtaining acetic anhydride. Acetaldehyde participates easily in aldol condensation:
2CH3CHO → CH3—CH(OH)—CH2—CHO
Acetaldehyde is used in huge quantities in the production of acetic acid and acetic anhydride, in the preparation of various pharmaceuticals, and so on.