Aldehydes and ketones are classes of organic compounds that contain carbonyl groups (groups in which a carbon atom is doubly bonded to an oxygen atom). In aldehydes, the carbonyl group is linked to two hydrogen atoms, or to one hydrogen atom and an alkyl or aryl group. Alkyl and aryl groups are hydrocarbon fragments.
In methanal (formaldehyde), the simplest and most familiar aldehyde, two hydrogen atoms are attached to the carbonyl group. In all other aldehydes, the carbonyl group bears just one hydrogen atom, the other portion of the molecule being an alkyl or aryl group. Ethanal (acetaldehyde), for example, is identical to methanal, except that one of the hydrogen atoms has been replaced by a methyl group.
Ketones are different structurally in that the carbonyl group bears no hydrogen atoms. Instead, it is linked to two alkvl or aryl groups. Propanone (also called dimethyl ketone or acetone), the most widely used and familiar ketone, consists of a carbonyl group bonded to two methyl groups.
Preparation and uses
Aldehydes and ketones are usually made by oxidizing an alcohol. These compounds have a variety of uses, mostly as solvents or chemical reagents. Aldehydes and ketones can both be converted into alcohols and alkanes by chemical means. In addition, aldehydes can be oxidized to form carboxylic acids, which are important organic acids.
Low-molecular-weight aldehydes such as methanal and ethanal have an unpleasant smell. In contrast, the odors of some aldehydes and many ketones are sometimes very agreeable and used in flavoring or perfume. Camphor, a ketone, has long been valued for its medicinal properties. Testosterone, also a ketone, is the sex hormone that promotes development of male characteristics in humans and other mammals.
Methanal and other simple aldehydes
With a boiling point of 2° F. (21° C), methanal is the only member of the aldehyde family that is a gas at room temperature. Ethanal has a boiling point of just 70° F. (21° C). All other low-molecular-weight aldehydes are liquids at room temperature.
Methanal has long been used as a fluid for preserving biological specimens. It is first dissolved in water to form a 37 per cent solution called formalin. Formalin is also important in the manufacture of resins and plastics. Ethanal can also form more complex compounds. One such compound, paraldehyde, was once used medicinally as a sedative and hypnotic, but it had dangerous side effects.
Propanone and simple ketones
Like all other simple ketones, propanone is a liquid at room temperature and it has a boiling point of 133° F. (56° C). It mixes well with water and, like butanone (methyl ethyl ketone)—another simple ketone—propanone is an excellent solvent for dissolving organic compounds. Although the carbonyl group in ketones is less reactive than that in aldehydes, it is still prone to react with other chemicals.
Like aldehydes, ketones can be converted into alcohols by the addition of hydrogen under suitable conditions.
When mixed with enough alcohol, aldehydes react to form acetals and hemiacetals, which are common bio-organic molecules of great use in chemical synthesis. During the reaction, the double bond in the carbonyl group is destroyed by an attacking alcohol molecule. This alcohol molecule replaces the carbonyl group on the aldehyde molecule, forming a hemiac-etal. Acetals are formed when hemiacetals are subjected to further attack by alcohol.
Imines are formed by reacting aldehydes or ketones with amines. Amines are organic compounds obtained from ammonia, and some of them are important in biology. Their formation is a key step in the metabolism of amino acids, the building blocks of proteins. Chemists use cyanohydrins (made by reactions of aldehydes and ketones with hydrogen cyanide) in chemical synthesis.