Cyclohexanone is one of the small group of solvents that is both a high-volume industrial intermediate and a workhorse of formulation chemistry. It is produced by the oxidation of cyclohexane or by the hydrogenation of phenol, and its molecule combines an alicyclic hydrocarbon ring with a single ketone group. That structure explains the behaviour that puzzles so many users: the compound dissolves resins, oils and polymers aggressively, yet it mixes with water only to a limited extent and separates into layers when the two are combined. This article sets out the real solubility picture, the properties behind it, and the practical consequences for extraction, formulation and storage.
Solubility of Cyclohexanone in Water and Organic Solvents
Cyclohexanone is only slightly soluble in water. At room temperature the dissolved amount is of the order of a few grams per 100 millilitres, which corresponds to a few percent by weight: measurable, but far short of true miscibility. In practice, adding a small quantity of cyclohexanone to water produces a cloudy mixture, and larger additions separate into two layers, with the organic phase floating on the water because the density of cyclohexanone is below 1.0 g/cm3. A uniform single-phase system normally requires vigorous stirring, a controlled temperature, or the addition of a co-solvent such as ethanol or acetone.
Against that limited water solubility, the organic solubility of cyclohexanone is very broad. It is miscible in all proportions with the common alcohols, ketones, esters, aromatic and chlorinated solvents listed below, and this is the property that makes it commercially valuable.
| Solvent class | Typical examples | Behaviour with cyclohexanone |
|---|---|---|
| Alcohols | Ethanol, isopropanol, butanol | Miscible in all proportions |
| Ketones and esters | Acetone, methyl ethyl ketone, ethyl acetate | Miscible in all proportions |
| Ethers | Diethyl ether, glycol ethers | Miscible in all proportions |
| Aromatic and chlorinated solvents | Toluene, xylene, chloroform | Miscible in all proportions |
| Water | Water | Slightly soluble only; forms two layers |
| Saturated hydrocarbons | Hexane, white spirit | Limited; hydrocarbon diluents often separate |
The contrast between water and hydrocarbons is useful in process design. Because cyclohexanone prefers polar organic media but is not fully hydrocarbon-compatible, washing a cyclohexanone stream with water removes water-soluble salts and polar by-products while leaving the ketone in the organic phase, and adding a hydrocarbon diluent can be used to force separation where a two-phase system is wanted.
Physical and Chemical Properties
The properties below are the ones that drive equipment selection, drying strategy and safety classification.
| Property | Typical value |
|---|---|
| CAS number | 108-94-1 |
| Molecular formula | C6H10O |
| Boiling point | Approximately 155.6 C |
| Melting point | Approximately minus 31 C |
| Density at 20 C | Approximately 0.947 g/cm3 |
| Flash point | Approximately 44 C, closed cup |
| Appearance and odour | Colourless liquid with a peppermint-like ketone odour |
| Water solubility | Slight; a few percent at room temperature |
Because the vapour of cyclohexanone can form an explosive mixture with air and the flash point places it in the medium-flash flammable category, tanks and reactors should be inerted or vented correctly and all transfer operations earthed. Chemically, the ketone group undergoes addition, reduction and condensation reactions, and this reactivity is why cyclohexanone is a preferred building block for downstream synthesis. It also oxidises slowly in air, and prolonged storage in contact with air and light can generate peroxides, so inventory should be rotated and kept in closed, shaded vessels.
Cyclohexanone as a Nylon Intermediate
The largest single use of cyclohexanone is not as a solvent at all. It is the entry point to the nylon chain: oximation converts cyclohexanone into cyclohexanone oxime, and under acidic conditions the oxime undergoes a Beckmann rearrangement to caprolactam. Ring-opening polymerisation of caprolactam produces nylon 6, the polymer used in textile filaments, engineering plastics and film. Cyclohexanone can also be oxidised further to adipic acid, which after condensation with hexamethylenediamine gives nylon 66.
This position in the chain means that cyclohexanone quality and availability propagate directly into downstream nylon economics. Purity specifications for caprolactam-grade material are correspondingly tight, with limits on water, cyclic by-products and colour bodies, because impurities carried into oximation affect conversion and final polymer colour.
Industrial Solvent, Leather and Fine Chemical Uses
As a solvent, cyclohexanone dissolves a wide range of polymer resins and is classed among the strong solvents used by the coatings and printing ink industries. In paint it dissolves nitrocellulose, acrylic and epoxy resins and is used to adjust viscosity, leveling and gloss; in ink it dissolves pigments and binders and improves printability and drying behaviour. It is also used as a diluent for synthetic resins and rubbers, and in adhesive manufacture it dissolves the base resin to give stable, high-strength bonding products, including footwear and decorative adhesives that have to bond difficult substrates.
In cleaning formulations, cyclohexanone-based products remove heavy oil contamination from mechanical parts and are used in equipment maintenance and remanufacturing. In leather processing it acts as a degreasing agent that removes oil and dirt from raw hides before tanning. In agrochemical and pharmaceutical production it serves as a solvent for synthesis and crystallisation, and in fine chemistry it remains an important route to antioxidants, light stabilisers and fragrance compounds.
Safety, Storage and Handling
Cyclohexanone is harmful if swallowed or inhaled in quantity and is irritating to eyes, skin and the respiratory tract. Work in ventilated areas or under local extraction, use chemical-resistant gloves, goggles and protective clothing, and avoid breathing vapour or mist. Eliminate ignition sources of every kind, use explosion-proof electrical equipment, and bond and earth containers during transfers so that static cannot accumulate.
Store the material in tightly closed, labelled steel or high-density polyethylene containers in a cool, dry, well-ventilated place away from heat, oxidising agents and strong bases. Keep inventory levels modest so that stock is used within its shelf life, and screen long-stored material for peroxide formation before it is returned to a process. In the event of a spill, remove ignition sources, contain the liquid with inert absorbent, ventilate the area and dispose of the waste through approved channels in accordance with local regulations.
Frequently Asked Questions
Q: Does cyclohexanone dissolve in water?
Only slightly. It dissolves to a few percent at room temperature; larger additions form a separate organic layer above the water, so it is not water-miscible.
Q: What are the CAS number and molecular formula of cyclohexanone?
Cyclohexanone has the CAS number 108-94-1 and the molecular formula C6H10O, and it is an alicyclic ketone with a six-membered carbon ring.
Q: Which solvents is cyclohexanone miscible with?
It is miscible in all proportions with ethanol, diethyl ether, acetone, benzene, toluene and chloroform, though its compatibility with saturated hydrocarbon diluents is limited.
Q: What is cyclohexanone mainly used to produce?
Its largest use is as an intermediate for caprolactam and adipic acid, the monomers of nylon 6 and nylon 66, alongside its role as an industrial solvent.
Q: Is cyclohexanone flammable?
Yes. Its flash point is approximately 44 C, it is classified in the medium-flash flammable liquid category, and its vapour can form explosive mixtures with air.
Q: Why does the cyclohexanone layer sit on top of water?
Its density is about 0.947 g/cm3, slightly below that of water, so in a two-phase mixture the cyclohexanone-rich phase separates as the upper layer.