Cyclopentanone: Properties, Uses & Lab Guide

Cyclopentanone is a five-membered cyclic ketone that serves as a versatile and widely used intermediate in organic synthesis, pharmaceutical research, and specialty chemical manufacturing.

CAS Number

120-92-3

Molecular Formula

C₅H₈O

Molecular Weight

84.12 g/mol

Boiling Point

130–131 °C at 760 mmHg

Appearance

Colorless to pale yellow liquid with a peppermint-like odor

Solubility

Slightly soluble in water; miscible with common organic solvents

Flash Point

Approximately 26 °C (closed cup)

Chemical Identity and Physical Properties

Cyclopentanone (CAS 120-92-3) is a saturated five-membered cyclic ketone with the molecular formula C₅H₈O and a molecular weight of 84.12 g/mol. It presents as a colorless to pale yellow, mobile liquid with a characteristic peppermint- or spearmint-like odor. The compound boils at approximately 130–131 °C at atmospheric pressure and has a density of roughly 0.95 g/mL at 20 °C.

The ring strain inherent in the five-membered ring confers enhanced reactivity compared with acyclic analogues of similar molecular weight. The carbonyl group participates readily in nucleophilic addition, aldol condensation, and enamine formation reactions. Cyclopentanone is slightly soluble in water but miscible with most common organic solvents including ethanol, diethyl ether, acetone, and dichloromethane, making it straightforward to incorporate into multi-solvent reaction systems.

Synthesis and Common Synthetic Applications

Cyclopentanone is manufactured industrially through several established routes, including the Dieckmann condensation of glutaric acid derivatives and catalytic rearrangement of 1,2-cyclopentanediol or cyclopentanol. On the laboratory scale, it can also be prepared by the oxidation of cyclopentanol using chromium-based or other oxidants.

In synthetic chemistry, cyclopentanone is valued as a building block for constructing five-membered carbocyclic frameworks. Key applications include:

• Aldol and Robinson annulation chemistry: The active alpha-hydrogens flanking the carbonyl enable straightforward enolate formation under both acidic and basic conditions, facilitating aldol additions and the Robinson annulation to access bicyclic systems.
• Enamine synthesis: Reaction with secondary amines (e.g., pyrrolidine, morpholine) gives stable enamines that are widely used as nucleophilic equivalents in carbon–carbon bond-forming reactions.
• Pharmaceutical intermediates: Cyclopentanone-derived scaffolds appear in the synthesis of prostaglandin analogues, carbocyclic nucleosides, and a range of bioactive natural products that incorporate a cyclopentanone or cyclopentanol core.
• Wittig and Horner–Wadsworth–Emmons reactions: The ketone serves as an electrophilic partner for olefination reactions to install exocyclic alkene functionality.

Safe Handling, Storage, and Regulatory Notes

Cyclopentanone is a flammable liquid (flash point approximately 26 °C, closed cup) and should be treated with standard precautions applicable to Class IB or equivalent flammable solvents, depending on local regulatory classification. Key handling guidelines include:

• Store in a cool, well-ventilated area away from ignition sources, oxidising agents, and strong bases.
• Use appropriate personal protective equipment: chemical-splash goggles, laboratory coat, and solvent-resistant gloves (nitrile or neoprene recommended).
• Work in a fume hood when handling bulk quantities or during operations that may generate vapour.
• Dispose of waste material in accordance with local regulations for flammable organic solvents.

Prolonged or repeated skin or inhalation exposure may cause irritation; the compound has not been classified as a carcinogen by major regulatory bodies, but standard laboratory hygiene should be observed. Always consult the current Safety Data Sheet (SDS) supplied with the product before use.

Analytical Characterization and Purity Assessment

Quality control of cyclopentanone for research applications typically involves the following analytical methods:

• Gas Chromatography (GC): The primary technique for purity determination. High-purity research grades are typically ≥98% or ≥99% by GC area normalization. Common impurities include cyclopentanol and glutaraldehyde-derived byproducts.
• Refractive Index: A rapid identity check; literature value is approximately n²⁰D 1.4366.
• ¹H NMR Spectroscopy: Characteristic signals include the alpha-methylene protons (δ ~2.6 ppm) and beta-methylene protons (δ ~1.9 ppm) in CDCl₃.
• ¹³C NMR: The carbonyl carbon resonates at approximately δ 218 ppm, consistent with a strained cyclic ketone.
• IR Spectroscopy: A strong C=O stretching absorption appears at approximately 1740 cm⁻¹, shifted from the typical acyclic ketone value (~1715 cm⁻¹) due to ring strain.

Researchers requiring high-purity material for sensitive catalytic or biological assays should confirm purity specifications with Aladdin Scientific's certificate of analysis (CoA) prior to use.

Frequently asked questions

What is the CAS number and molecular formula of cyclopentanone?

Cyclopentanone has the CAS number 120-92-3 and the molecular formula C₅H₈O, with a molecular weight of 84.12 g/mol. It is a five-membered saturated cyclic ketone.

What are the primary uses of cyclopentanone in organic synthesis?

Cyclopentanone is widely used as a building block for five-membered carbocyclic compounds. It participates in aldol condensations, Robinson annulations, enamine formations, and Wittig-type olefinations. It is also an important precursor in the synthesis of pharmaceutical intermediates, including prostaglandin analogues and carbocyclic nucleosides.

How should cyclopentanone be stored in the laboratory?

Cyclopentanone should be stored in a tightly sealed container in a cool, dry, and well-ventilated area, away from sources of ignition, heat, and incompatible materials such as strong oxidisers and strong bases. Its flash point of approximately 26 °C means it must be treated as a flammable liquid under standard laboratory safety protocols.

What spectroscopic data can be used to identify cyclopentanone?

Cyclopentanone can be identified by its characteristic IR carbonyl stretch at approximately 1740 cm⁻¹, ¹H NMR signals with alpha-methylene protons near δ 2.6 ppm in CDCl₃, and a ¹³C NMR carbonyl resonance at approximately δ 218 ppm. GC analysis can confirm purity, and the refractive index (n²⁰D ≈ 1.4366) provides a rapid identity check.

Is cyclopentanone water-soluble, and what solvents are compatible with it?

Cyclopentanone has limited water solubility but is miscible with most common organic solvents, including ethanol, diethyl ether, acetone, and dichloromethane. This broad organic solvent compatibility makes it easy to use in a wide variety of reaction and workup conditions.

References

1. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley-Interscience: New York, 1992.
2. National Institute of Standards and Technology (NIST). Cyclopentanone. NIST WebBook, SRD 69. https://webbook.nist.gov/
3. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press: Oxford, 2012.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.