Acetaldehyde

ACETALDEHYDE, CH₃ . CH : O, acetic aldehyde, is found in crude wood alcohol and wine spirit, in wine, in many essential oils (camphor, anise, peppermint, and others), as well as in young pea pods. Acetaldehyde is one of the products formed intermediately in the process of alcoholic fermentation of carbohydrates (see Fermentation). Its formation is linked to the action of carboxylase, a special enzyme found in yeast zymase, which decomposes pyruvic acid (see) into acetaldehyde and carbon dioxide: CH₃ . CO . COOH = CH₃ . CHO + CO₂. As a result of further reduction-oxidation interactions, acetaldehyde is converted into ethyl alcohol. Acetaldehyde is also an intermediate product of glucose breakdown in the animal organism. Acetaldehyde is obtained by: 1) the oxidation of ethyl alcohol with chromic acid, manganese peroxide, and sulfuric acid, or with the help of catalysts (copper, silver, platinum): CH₃ . CH₂OH + O = CH₃ . CHO + H₂O; 2) hydration. Acetaldehyde is a volatile, colorless liquid possessing a pleasant odor in weak dilutions; boiling point 21°, specific gravity 0.7951; it is miscible with water, alcohol, and ether in any proportions; it is separated from aqueous solutions by calcium chloride. Among the chemical properties of acetaldehyde, the following are important. 1. The addition of a small amount of concentrated sulfuric acid causes the formation of paraldehyde, a liquid boiling at 124°, which does not exhibit typical aldehyde reactions. Polymerization proceeds with significant heat release according to the equation: 3CH₃ . CHO = C₆H₁₂O₃. Upon heating paraldehyde with acids, depolymerization occurs, i.e., acetaldehyde is obtained back. 2. In the presence of certain substances (HCl, zinc chloride, and especially weak alkalis), acetaldehyde is converted into aldols (see): 2CH₃ . CHO = CH₃ . CH(OH) . CH₂ . CHO. Upon the action of strong alkalis on acetaldehyde, aldehyde resin is formed. 3. Upon oxidation, acetaldehyde yields acetic acid; CH₃ . CHO + O = CH₃ . COOH. 4. Upon reduction, ethyl alcohol is formed: CH₃ . CHO + H₂ = CH₃ . CH₂OH. 5. Hydrocyanic acid adds to acetaldehyde, forming lactic acid nitrile: CH₃ . CHO + HCN = CH₃ . CH(OH) . CN, from which lactic acid (see) can be obtained by saponification. 6. With ammonium cyanide, the amino-nitrile CH₃ . CH(NH₂) . CN is obtained, upon the saponification of which alanine (see) is formed.