Ketone

Ketones constitute a major class of organic compounds defined by the presence of a carbonyl group in which the carbonyl carbon is bonded to two carbon-containing substituents. This structural feature distinguishes ketones from related functional groups such as aldehydes, carboxylic acids, esters and amides. The simplest member of the class is acetone, widely used as a solvent and a metabolic intermediate. Ketones are central to organic chemistry, biochemistry and industrial manufacturing owing to their ubiquity, structural diversity and wide-ranging reactivity.

Nomenclature and Etymology

The term ketone derives from the old German word Aketon, historically referring to acetone. The modern name was introduced in 1848 by Leopold Gmelin in his treatise on organic chemistry. Earlier, Auguste Laurent had proposed nomenclatural categories such as syndesmides—compound groups formed from simpler organic molecules—under which acetone had been classified.
According to IUPAC rules, ketones are named by replacing the -ane suffix of the corresponding alkane with -anone. Locants indicate the position of the carbonyl group, as in propan-2-one. However, many traditional names remain widely used, notably acetone and benzophenone, both recognised as retained IUPAC names. In derived naming, the alkyl substituents attached to the carbonyl are listed alphabetically followed by the word ketone. If both substituents are identical, the prefix di- is used. The prefix oxo- designates a carbonyl substituent when the ketone function does not have highest naming priority, while the term keto appears frequently in biochemical contexts.

Structure and Bonding

The carbonyl carbon in ketones is sp²-hybridised, producing a trigonal planar geometry with approximate 120° bond angles. A strongly polar C=O bond arises from the higher electronegativity of oxygen, rendering the carbonyl carbon electrophilic and the carbonyl oxygen nucleophilic. This polarity governs the chemical behaviour of ketones, influencing their solubility, reactivity and physical properties.
Ketones differ from aldehydes in lacking a hydrogen atom directly bonded to the carbonyl carbon. This structural distinction makes aldehydes more readily oxidised, whereas ketones generally require strong oxidising agents capable of cleaving carbon–carbon bonds. The carbonyl group participates in hydrogen bonding only as an acceptor, not as a donor. As a result, ketones do not self-associate and tend to be more volatile and less viscous than alcohols or carboxylic acids of similar molecular mass. Their ability to hydrogen bond with water enhances their solubility relative to non-polar hydrocarbons.

Classes of Ketones

Ketones can be classified according to the nature and arrangement of their substituents:

• Symmetrical ketones possess identical substituents on either side of the carbonyl group, as in acetone and benzophenone.
• Unsymmetrical ketones contain two different substituents, as in acetophenone.
• Diketones incorporate two carbonyl groups; examples include diacetyl and acetylacetone. The latter predominantly exists in its mono-enol form and its enolate is widely used in coordination chemistry.
• Unsaturated ketones include carbonyl compounds conjugated with alkenes or alkynes, such as methyl vinyl ketone.
• Cyclic ketones range from small rings such as cyclopropanone to large macrocyclic structures. Cyclohexanone is a key industrial precursor in nylon manufacture, whereas muscone, a macrocyclic ketone, is an important natural pheromone.

Characterisation

Spectroscopic techniques provide reliable methods for identifying ketones. In infrared spectroscopy, the carbonyl stretch of ketones typically appears near 1,750 cm⁻¹, though substitution patterns can shift this frequency to lower values. In carbon-13 NMR spectroscopy, the carbonyl carbon usually resonates downfield near 200 ppm. Proton NMR spectroscopy is generally not diagnostic for ketones, as they lack characteristic hydrogen atoms directly bonded to the carbonyl carbon.
Qualitative organic tests historically distinguished ketones from aldehydes. In Brady’s test, both aldehydes and ketones form hydrazones with 2,4-dinitrophenylhydrazine. Ketones normally give negative results with reagents such as Tollens’ reagent or Fehling’s solution, which readily oxidise aldehydes. Methyl ketones yield positive results in the iodoform test, while reactions with m-dinitrobenzene and dilute alkali produce a violet coloration.

Synthesis of Ketones

A wide range of synthetic methods exist for preparing ketones, both on industrial and laboratory scales.
Industrial production often relies on oxidation processes. Cyclohexanone, produced on a million-tonne scale annually, is formed by aerobic oxidation of cyclohexane. Acetone is chiefly manufactured via the cumene process as a co-product with phenol.
Laboratory synthesis includes diverse strategies:

• Oxidation of secondary alcohols, using agents such as potassium permanganate, chromium(VI) reagents, Dess–Martin periodinane or Swern oxidation conditions.
• Hydration of alkynes in the presence of acid and mercury(II) salts, yielding ketones via enol intermediates.
• Geminal dihalide hydrolysis and related halogenated precursor transformations.
• Friedel–Crafts acylation, the Houben–Hoesch reaction and the Fries rearrangement for aromatic ketone formation.
• Weinreb ketone synthesis, enabling controlled reaction of organometallic reagents with amides.
• Oxidative cleavage of alkenes, which may afford aldehydes or ketones depending on substitution patterns.
• Rearrangements and cyclisations, such as the Kornblum–DeLaMare rearrangement, the Nef reaction and Ruzicka cyclisation, provide routes to structurally complex ketones.
• Organometallic coupling, including alkylation of thioesters via organozinc reagents in Fukuyama coupling and reactions of acid chlorides with organocopper or organocadmium reagents.

Biological and Industrial Importance

Ketones play crucial roles in biological systems. Several monosaccharides, termed ketoses, contain ketone functional groups. Steroid hormones such as testosterone incorporate ketone units essential for biological activity. In human metabolism, ketone bodies such as acetoacetate and β-hydroxybutyrate provide alternative energy sources during prolonged fasting or carbohydrate deprivation.
Industrial applications extend beyond synthesis and materials production to solvents, fragrances and polymer precursors. The volatility and distinctive odours of many ketones make them valuable in perfumery and in formulations where rapid evaporation is desired.