Ketone

A ketone is an organic compound featuring a carbonyl group—a carbon-oxygen double bond—bonded to two carbon-containing groups, often denoted as R and R'. The simplest ketone is acetone, where both groups are methyl. Many ketones play significant roles in biology and industry, including certain sugars (ketoses), steroids like testosterone, and the solvent acetone.

The term "ketone" comes from *Aketon*, an old German word for acetone. In IUPAC nomenclature, ketone names replace the suffix *-ane* of the parent alkane with *-anone*, with a number often indicating the carbonyl group's position. However, common nonsystematic names like acetone and benzophenone are retained as IUPAC names. Alternatively, ketones can be named by listing the two alkyl groups attached to the carbonyl, followed by "ketone"—traditionally in order of increasing complexity, but IUPAC rules now require alphabetical order. If the alkyl groups are identical, the prefix "di-" is used. Substituent positions are indicated by Greek letters, with the α-carbon adjacent to the carbonyl. The prefix "oxo" is used when the ketone group is not the highest priority, while "keto" appears in biochemical contexts.

The ketone carbon is typically sp² hybridized, giving a trigonal planar geometry around it, with bond angles near 120°. Ketones differ from aldehydes, where the carbonyl is bonded to one carbon and one hydrogen at the end of a chain, and from other carbonyl groups like those in carboxylic acids, esters, and amides. The carbonyl group is polar due to oxygen's higher electronegativity, making ketones nucleophilic at oxygen and electrophilic at carbon. They can accept hydrogen bonds from water, increasing solubility compared to related methylene compounds, but cannot donate hydrogen bonds to themselves. This lack of self-association makes ketones more volatile than alcohols or carboxylic acids of similar molecular weight, contributing to their use in perfumery and as solvents.

Ketones are classified by substituents: symmetrical (e.g., acetone, benzophenone) or unsymmetrical (e.g., acetophenone). Diketones include diacetyl, once used as butter flavoring, and acetylacetone, which mainly exists as an enol and whose enolate is a common ligand in coordination chemistry. Unsaturated ketones contain alkene or alkyne units, such as methyl vinyl ketone, an α,β-unsaturated carbonyl compound. Cyclic ketones range from cyclopropanone to larger rings; cyclohexanone is key in nylon production, isophorone is a polymer precursor, and muscone is an animal pheromone.

Unlike aldehydes, ketones lack a hydrogen atom on the carbonyl carbon, making them resistant to oxidation unless powerful agents cleave carbon–carbon bonds. In spectroscopy, ketones show strong infrared absorption near 1750 cm⁻¹ due to the carbonyl stretch, with lower energy for aryl or unsaturated ketones. While ¹H NMR is not useful for identifying ketones, ¹³C NMR shows signals near 200 ppm, though these are weak and similar to aldehyde signals, requiring multiple resonance experiments for distinction.