Ketones

Ketones

Ketones have the general formula RCCOCR' where R and R' may be the same or different groups (they can be alkyl or aryl groups but not hydrogen). The central carbon atom has a double bond with the oxygen. Ketones differ from aldehydes in that the latter has a hydrogen atom attached as one of the R groups. The name of a ketone is derived from the longest chain that contains the carbonyl group, with the -e ending of the alkane being replaced by -one. The location of the carbonyl group is given by numbering the chain in the direction that will give the lowest number. It is also correct to name ketones by listing alphabetically the groups attached to the carbonyl group and following this with the word ketone. Some ketones are also known by common names. The ketone CH₃COCH₃ is known as propanone, dimethyl ketone, and acetone.

Ketones may be prepared by the partial oxidation of a secondary alcohol (complete oxidation would give carbon dioxide and water). This can be carried out using chemical oxidation agents such as hydrogen peroxide, ozone, or potassium dichromate. Manufacture of ketones can also be carried out by the dehydrogenation of a secondary alcohol at high temperature.

Typical reactions of ketones are addition reactions although as a group they are less reactive than the aldehydes. Secondary alcohols can be formed by reduction of the ketone by hydrogen gas in the presence of a catalyst. Ketones are extensively used as solvents, with acetone (propanone) being the most well known example. The carbonyl functional group gives polarity to the acetone, and acetone is completely soluble in water and dissolves a wide range of organic substances. Ethyl methyl ketone is also used as an industrial solvent. Aliphatic ketones are volatile liquids with a pleasant smell and aromatic ketones are usually solids. Aromatic ketones can be prepared by the Friedel-Crafts reaction. The solubility of ketones in water decreases as their molecular mass increases. Ketones are readily inflammable and will burn in air with a clear blue flame to produce carbon dioxide and water. Ketones can undergo a process known as autocondensation, where several ketone molecules react together to eliminate water. When this occurs with acetone in the presence of sulfuric acid (to remove the water) the product is 1,3,5- trimethylbenzene (mesitylene). This reaction is of interest because it is one of the few examples of a synthetic reaction where an aliphatic (straight chain) compound is transformed into an aromatic (ring structure) compound. This type of reaction is very common in biosynthesis, where many structures are made (especially by plants) by the "polyketide" pathway that uses large numbers of ketone condensation reactions. The reaction of ketones with strong bases gives enolate anions, which are the intermediates used in these condensation reactions.

Acetone is one of the oldest known ketones. It was known by the alchemists who called it spirit of Saturn. Antoine Bussy (1794-1882) gave acetone its current common name in 1833. Historically acetone was prepared by the destructive distillation of wood, followed by fractional distillation of the product. Ketones can display isomerism, for example methyl n-propyl ketone and isopropyl methyl ketone have the same formula but different arrangements of the atoms leading to different properties.

Ketones are a homologous series of organic chemicals containing the functional group -CCOC-. The first two members of the group are extensively used as organic solvents.