Aldehyde
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An aldehyde or alkanal, is an organic compound containing a functional group with the structure −CHO (or -CH=O), consisting of a carbonyl center (a carbon double-bonded to oxygen) with the carbon atom also bonded to hydrogen, and to an R group, which is any generic alkyl or side chain, or some inorganic group (H, halogen, CF3, etc.).
Properties
Aldehydes are volatile compounds, with a strong smell. They are flammable and react with bases, both metal hydroxides and amines, reaction which releases lots of heat. Aldehydes degrade in air via the process of autoxidation, forming various polymers.
Aldehydes polymerize in contact with a strong base, and react exothermically with amines, producing a yellowish insoluble precipitate. Under controlled conditions, it forms the corresponding primary alcohol, and a salt of the corresponding carboxylic acid (Cannizzaro reaction). Aldehydes also react with amines. The reaction of formaldehyde with ammonia for example gives hexamine.
Aldehydes characteristically form "addition compounds" with sodium bisulfite.
Many high weight aldehydes (benzaldehyde, cinnamaldehyde, etc) have strong and pleasant smells.
Preparation
Aldehydes can be easily prepared through the Jones oxidation of primary alcohols, which consists of chromium trioxide or potassium dichromate dissolved in a mixture of acetone and dilute sulfuric acid.
Chromium-free oxidation routes involve 2-iodoxybenzoic acid (IBX acid), Dess–Martin periodinane, Swern oxidation, TEMPO, or the Oppenauer oxidation.
Aldehydes can also be produced by oxidizing primary alcohols through near red hot copper. This method is simple enough that it can be used to produce large amounts of aldehyde in a lab setting.
Industrially aldehydes are prepared via hydroformylation of alkenes.
Some aldehydes, like benzaldehyde, can be produced by oxidizing toluene with chromyl chloride. Others, like glyoxal are made by oxidizing ethanol or acetaldehyde with nitric acid.
Pyrolysis of calcium formate will yield formaldehyde (dubious):
- Ca(HCOO)2 → CH2O + CaCO3
Benzaldehyde and other aromatic aldehydes can be prepared from a benzyl halide and hexamine via the Sommelet reaction.
Safety
Low weight aldehydes are toxic and carcinogenic. Some may form peroxides upon prolonged contact with air.
