Compounds with the aldehyde group, -\mathrm{CHO}, are called aldehydes, and they form when primary alcohols are oxidized.
When a coiled copper wire is heated in a flame, its surface turns into black copper(II) oxide. If the hot wire is then put into a test tube containing methanol vapour, it turns red-brown again, and methanal (formaldehyde), which has a sharp smell, forms.
\mathrm{CH_3OH} + \mathrm{CuO} \longrightarrow \mathrm{HCHO} + \mathrm{Cu} + \mathrm{H_2O}
A solution of methanal in water is called formalin and is used to preserve biological specimens. Methanal is also a raw material for plastics such as phenolic resins.
Aldehydes are easily oxidized, so they are reducing agents. When an aldehyde is added to ammoniacal silver nitrate solution and the mixture is warmed, silver is deposited on the inside of the test tube, which becomes a mirror.
This is called the silver mirror reaction. The silver ions are reduced to silver, while the aldehyde is oxidized. The reaction is used to test for aldehydes.
Fehling's solution is a deep blue solution that contains copper(II) ions. When an aldehyde is added to it and the mixture is heated, the copper(II) ions are reduced, and a brick-red precipitate of copper(I) oxide, \mathrm{Cu_2O}, forms.
Glucose also reduces Fehling's solution, because in solution some of its molecules take an open-chain form that has an aldehyde group.
In ketones, the carbonyl group is bonded to two hydrocarbon groups. The simplest ketone is propanone (acetone), \mathrm{CH_3COCH_3}, which forms when propan-2-ol, a secondary alcohol, is oxidized.
Ketones are not easily oxidized, so they do not give a silver mirror or reduce Fehling's solution. These reactions are used to tell aldehydes and ketones apart.
Acetone is a colourless liquid that mixes with water and with many organic compounds, so it is widely used as a solvent, for example in nail polish remover.
When iodine and sodium hydroxide solution are added to acetone and the mixture is warmed, a yellow precipitate of iodoform, \mathrm{CHI_3}, which has a characteristic smell, forms. This is called the iodoform reaction.
The reaction is given by compounds with the structure \mathrm{CH_3COR}, such as acetone and ethanal, and by compounds with the structure \mathrm{CH_3CH(OH)R}, such as ethanol and propan-2-ol, where R is a hydrogen atom or a hydrocarbon group.
Compounds with the structure \mathrm{CH_3CH(OH)R} are first oxidized to \mathrm{CH_3COR} by the iodine and then react. Iodoform was once used as an antiseptic.