A ketone is an organic compound containing a carbonyl group in which the carbonyl carbon is bonded to two carbon containing groups. Its general structure can be represented as R C equals O R prime, where R and R prime are carbon containing substituents. The simplest ketone is propanone, commonly called acetone, with the formula CH3COCH3. Unlike an aldehyde, a ketone does not have a hydrogen directly attached to the carbonyl carbon.
Ketones are widespread in organic chemistry, biochemistry, pharmaceuticals, solvents and industrial materials. The carbonyl group makes the central carbon electrophilic and gives ketones characteristic reactions involving nucleophilic addition, reduction and condensation. The two carbon substituents also influence steric and electronic properties, meaning that different ketones can vary considerably in volatility, solubility, reactivity and biological behaviour.
The defining carbonyl group of a ketone consists of a carbon atom double bonded to oxygen. The carbonyl carbon is approximately sp2 hybridised and has trigonal planar geometry, while the carbonyl oxygen contains lone electron pairs and carries partial negative character. The carbonyl carbon carries partial positive character because oxygen attracts electron density more strongly. This polarisation makes the carbonyl carbon susceptible to nucleophilic attack.
In ketones, the two organic groups attached to the carbonyl carbon can be identical or different. When they are identical, the ketone may be described as symmetrical with respect to those substituents, whereas different groups produce an unsymmetrical ketone. The carbonyl group is therefore both a structural feature and a major determinant of chemical reactivity, molecular polarity and intermolecular interactions.
Ketones are named systematically using the suffix one. The longest chain containing the carbonyl group is selected and numbered so that the carbonyl carbon receives the lowest possible locant. Propanone has three carbon atoms, while butan 2 one has four carbon atoms with the carbonyl group at carbon two. Cyclic ketones can be named using forms such as cyclohexanone. Ketones containing additional functional groups may use other naming conventions depending on which functional group has priority under systematic nomenclature.
Several ketones also have familiar common names, particularly acetone for propanone. The structural requirement is more fundamental than the name: identifying a carbonyl carbon bonded to two carbon groups distinguishes a ketone from an aldehyde, ester, amide, carboxylic acid or other carbonyl containing functional group.
Ketones undergo numerous important reactions. Reduction converts a ketone into a secondary alcohol, commonly using reducing agents such as sodium borohydride or lithium aluminium hydride under appropriate conditions. Nucleophilic addition can occur at the carbonyl carbon because of its electrophilic character. Ketones can react with alcohols under suitable acidic conditions to form ketals through appropriate intermediate steps, and they can react with nitrogen nucleophiles to form imines, oximes or related derivatives.
Ketones containing alpha hydrogens can form enols or enolate ions, which participate in aldol reactions, alkylation and other carbon carbon bond forming processes. Some ketones can undergo oxidation under strong conditions involving carbon carbon bond cleavage, although ordinary ketones are substantially more resistant to oxidation than aldehydes. Their controlled reactivity makes ketones useful intermediates in organic synthesis.
Ketones have important biological roles. Acetone is produced in the human body as one of the ketone bodies during conditions in which fatty acid metabolism is increased and carbohydrate availability or insulin action is insufficient for normal metabolic balance. Other ketone bodies include acetoacetate and beta hydroxybutyrate, although beta hydroxybutyrate is chemically a hydroxy acid rather than a ketone in the strict functional group sense. Ketone containing molecules occur throughout metabolism, including intermediates in carbohydrate, lipid and amino acid pathways.
Steroid hormones and numerous natural products can contain ketone groups that influence their shape and reactivity. The presence of a ketone can change a molecule's polarity, hydrogen bonding behaviour and ability to participate in enzyme catalysis or molecular recognition. Thus ketones are central to both synthetic organic chemistry and biochemistry.
Ketones differ from aldehydes in a structurally simple but chemically important way. In an aldehyde, the carbonyl carbon is bonded to hydrogen and an organic group, whereas in a ketone it is bonded to two organic groups. This distinction generally makes aldehydes more susceptible to oxidation and often makes ketones somewhat less reactive toward nucleophilic addition because the two carbon groups provide greater steric and electronic effects.
Ketones can still be highly reactive under suitable conditions, and reactivity depends strongly on substituents and molecular environment. Ketones should also be distinguished from esters, which contain a carbonyl carbon bonded to an oxygen atom that is itself connected to another carbon group. Amides contain a carbonyl carbon bonded to nitrogen, while carboxylic acids contain a carbonyl carbon bonded to hydroxyl. Identifying the atoms directly attached to the carbonyl carbon is a reliable way to distinguish these functional groups.
Ketones have extensive industrial applications. Acetone is a widely used organic solvent and is important in coatings, adhesives, cleaning formulations and chemical manufacturing. Other ketones are used in polymer production, pharmaceuticals, fragrances, coatings and speciality synthesis. Their usefulness as solvents arises partly from their polarity and ability to dissolve many organic substances while remaining compatible with selected aqueous systems depending on the individual ketone.
Ketones are often volatile and flammable, so industrial use requires appropriate ventilation, ignition control and exposure management. Their environmental behaviour depends on molecular size, volatility, atmospheric lifetime and biodegradability. The practical importance of ketones therefore comes from a combination of useful solvent properties, predictable organic reactivity and the ability to transform carbon skeletons through selective synthetic reactions.
Ketone chemistry also illustrates how molecular structure controls reaction pathways. Electron withdrawing substituents can alter carbonyl electrophilicity, while electron donating groups can reduce it. Steric hindrance around the carbonyl carbon can affect the accessibility of nucleophiles. Conjugation with an alkene or aromatic system can delocalise electron density and modify absorption and reactivity, producing conjugated ketones with distinctive properties. Tautomerism between keto and enol forms can be significant for ketones containing suitable alpha hydrogens, particularly in acidic or basic environments.
Some ketones exist predominantly in the keto form but have measurable enol populations, and the balance can influence stereochemistry and reaction outcomes. A complete understanding of ketones therefore involves carbonyl bonding, nomenclature, nucleophilic chemistry, enolate formation, redox behaviour, biological metabolism and the effects of molecular structure and reaction conditions.
