The aliphatic aldehydes are a homologous series of organic compounds of carbon, hydrogen and oxygen, where the aldehyde functional group, -C=O.H , is contained within the molecule. The general formula is CnH2n+2.CHO. The aliphatic aldehydes are named by replacing "-e" at the end of the alkane from which they are derived with "-al".
Aromatic aldehydes (e.g. benzaldehyde, C6H5.CHO), are also known which undergo a number of chemical reaction which do nor occur for aliphatic aldehydes and which are unique to aromatic aldehydes.
Aliphatic aldehydes are readily prepared by the oxidation of alcohols. For example, ethanal, (i.e. Acetaldehyde) is prepared by the slow addition of ethanol, to a boiling solution of chromic acid, which oxidises the ethanol. This oxidisation is effectively a removal of two hydrogen atoms from the ethanol, rather than the addition of an oxygen atom.
[O] CH3CH2OH ==> CH3.C=O.H Ethanol Ethanal
It is important that the aldehyde thus formed is removed from the oxidising agent by distillation as soon as it is formed to prevent further oxidation, as aldehydes are easily further oxidised to carboxylic acids.
For example, ethanal is readily oxidised to ethanoic acid.
[O] CH3.C=O.H ==> CH3.C=O.OH Ethanal Ethanoic Acid
Aldehydes are reducing agents, and undergo characteristic addition and condensation reactions. They readily polymerise.