Ketone
Table of Contents
Ketones are a significant class of organic compounds characterized by the presence of the carbonyl functional group (C=O). In ketones, the carbonyl carbon is bonded to two carbon-containing groups, which may be alkyl or aryl in nature. The general formula is R–CO–R′, where R and R′ can be identical or different. [1-4]
Ketones occur widely in both nature and daily life. For instance, fructose, a simple sugar found in fruits and honey, contains a ketone group. Acetone is a common laboratory solvent and a key ingredient in household products such as nail polish remover.
IUPAC Nomenclature of Ketones
According to the IUPAC system, ketones are named by replacing the –e ending of the parent alkane with –one. The carbon chain is numbered so that the carbonyl carbon receives the lowest possible number. This locant is placed before the name of the ketone or before the suffix –one. [4,5]
- Butanone – also known as methyl ethyl ketone (MEK)
- 3-Pentanone or pentan-3-one – also known as diethyl ketone
When the carbonyl group is attached to an aromatic ring, the compound is referred to as an aromatic ketone. If the carbonyl carbon is directly bonded to a benzene ring, it is called an aryl ketone. The name is formed by stating the aryl group first, followed by the alkyl group, and then the word “ketone”.
- 1-Phenylethanone (acetophenone) – a benzene ring bonded to a carbonyl group with a methyl substituent
- Diphenylmethanone (benzophenone) – two benzene rings attached to a carbonyl group
Common Ketones and Their Applications [2,5]
| Ketone Name | Molecular Formula | Uses and Applications |
|---|---|---|
| Propanone(Acetone) | C3H6O | Industial solvent, nail polish remover, plastics, synthetic fibers |
| Butanone(Methyl Ethyl Ketone (MEK)) | C4H8O | Solvent for resins, lacquers, varnishes, adhesives, and coatings; paint removers, chemical intermediate |
| Cyclohexanone | C6H10O | Solvent for paints, dyes, and pesticides; nylon production |
| 1-Phenylethanone(Acetophenone) | C8H8O | Perfumes, fragrances; flavoring agent in food and beverages |
| Diphenylmethanone(Benzophenone) | C13H10O | Sunscreens, plastics, inks, perfumes; photoinitiator in polymerization |
| 3-Pentanone (Diethyl Ketone) | C5H10O | Solvent for resins, lacquers, dyes; intermediate in organic synthesis. |
Physical Properties [5,6]
1. Structure and Bonding – The carbonyl group (C=O) consists of a sigma (σ) bond and a pi (π) bond. The carbonyl carbon is sp2 hybridized, giving ketones a trigonal planar geometry with bond angles nearly 120°.
2. Polarity – Due to the higher electronegativity of oxygen, the C=O bond is polar, with the oxygen atom carrying a partial negative charge (δ–) and the carbon atom a partial positive charge (δ+).
3. Volatility – Lower molecular weight ketones, such as acetone, are more volatile. Volatility decreases with increasing molecular weight due to stronger intermolecular forces.
4. Boiling Point – Higher than those of hydrocarbons of similar molecular weight because of dipole–dipole interactions.
- Propanone: 56 °C
- Butanone: 79.6 °C
5. Melting Point – Generally higher than those of comparable alkanes, increasing with chain length due to stronger van der Waals forces.
- Propanone: –95 °C
- 2-Pentanone: –60 °C
6. Solubility in Water – Low-molecular-weight ketones are water-soluble due to hydrogen bonding between the carbonyl oxygen and water molecules. Solubility decreases with increasing hydrocarbon chain length.
7. Odor – Lower ketones like acetone have a sharp, sweet smell, while higher ketones like acetophenone possess a more fragrant or less pungent aroma.
Chemical Reactions [4,5]
The reactivity of ketones is primarily due to the polar carbonyl group, making them susceptible to nucleophilic attack.
1. Oxidation – Ketones resist mild oxidizing agents. Strong oxidants cleave C–C bonds to form carboxylic acids.
2. Reduction – Reduced to secondary alcohols using sodium borohydride (NaBH4), lithium aluminium hydride (LiAlH4), or catalytic hydrogenation. Further reduction to alkanes can be achieved via Clemmensen or Wolff–Kishner reduction.
3. Nucleophilic Addition – Reaction with nucleophiles forms products such as cyanohydrins, hemiketals, and ketals.
4. Aldol Reaction – Ketones with α-hydrogens undergo condensation to yield β-hydroxyketones.
5. Haloform Reaction – Methyl ketones react to give a yellow precipitate of iodoform, serving as a qualitative test.
Ketones are versatile organic compounds distinguished by their carbonyl group bonded to two carbon atoms. Their unique structure imparts distinct physical and chemical properties, making them valuable in a wide range of applications.
