Cyclopentanone
Cyclopentanone is an organic compound with the molecular formula C₅H₈O, featuring a five-membered carbocyclic ring bearing a single ketone functional group at position 1.[1] It appears as a clear, colorless liquid with a petroleum-like odor, exhibiting a boiling point of 130–131 °C, a melting point of −51 °C, and a density of 0.951 g/mL at 25 °C.[1][2][3] Cyclopentanone is practically insoluble in water but miscible with most organic solvents, and it has a flash point of 31 °C (87 °F), indicating moderate flammability.[1][2] In terms of production, cyclopentanone is industrially synthesized via the catalytic decarboxylation of adipic acid, often by heating the calcium salt or using vapor-phase processes with catalysts like ceria.[4] Alternative methods include the vapor-phase cyclization of 1,6-hexanediol or the conversion of bio-based furfural through hydrogenolysis and cyclization steps, enhancing its sustainability as a platform chemical.[5][6] Cyclopentanone serves as a versatile intermediate in organic synthesis, particularly for pharmaceuticals, where it is used to produce biologically active compounds such as analgesics and antivirals.[7] It is also essential in the fragrance industry for synthesizing jasmine-like scents and aroma chemicals, as well as in the production of rubber additives, fungicides, and agrochemicals.[8] Additionally, its solvent properties make it valuable in electronics for photoresist formulations and in specialty polymer manufacturing.[9] Despite its utility, cyclopentanone is classified as a flammable liquid and potential irritant, requiring careful handling in industrial settings.[2]Physical Properties
Appearance and Structure
Cyclopentanone, the preferred IUPAC name for this compound (with the systematic name cyclopentan-1-one), has the molecular formula C5H8O. It features a five-membered saturated carbocyclic ring with a carbonyl (C=O) group attached at position 1, making it a cyclic ketone.[1][10] Cyclopentanone appears as a clear, colorless liquid at room temperature. It has a petroleum-like odor.[1][11] The molecular structure of cyclopentanone exhibits a slightly puckered ring conformation, which alleviates angle strain compared to a hypothetical planar pentagon. The C-C-C bond angles in the ring are approximately 105–108°, close to the ideal tetrahedral angle of 109.5° for sp3-hybridized carbons, resulting in minimal angle strain overall. In contrast to acyclic ketones, where flexible alkyl chains allow free rotation and maintain planarity only at the carbonyl group, the cyclic framework in cyclopentanone constrains the overall geometry while preserving the local planarity of the sp2-hybridized carbonyl carbon with bond angles near 120°. This puckered envelope conformation also reduces torsional strain through pseudo-rotation.[12][13]Spectroscopic Data
Infrared (IR) spectroscopy is a primary method for identifying the carbonyl group in cyclopentanone, with the characteristic C=O stretching vibration appearing as a strong absorption band at 1740–1745 cm⁻¹.[14] The C-H stretching vibrations from the methylene groups occur in the 2950–2850 cm⁻¹ region, typical for aliphatic C-H bonds adjacent to the carbonyl.[14] Nuclear magnetic resonance (NMR) spectroscopy provides detailed structural confirmation. In the ¹H NMR spectrum (recorded in CDCl₃), the alpha protons (CH₂ groups adjacent to the carbonyl) resonate as a multiplet at approximately 2.3 ppm, while the beta protons appear at around 2.0 ppm, reflecting the deshielding effect of the carbonyl on the alpha positions.[1] The ¹³C NMR spectrum shows the carbonyl carbon at ~217 ppm, the alpha carbons at ~38 ppm, and the beta carbons at ~23 ppm, consistent with the symmetric ring structure and carbonyl influence.[1] Mass spectrometry of cyclopentanone exhibits a molecular ion peak at m/z 84, corresponding to its formula C₅H₈O. A common fragmentation pathway involves loss of CO, yielding a prominent peak at m/z 56 (C₄H₈⁺), along with other fragments such as m/z 55 and 42 from ring cleavage.[15] Ultraviolet-visible (UV-Vis) spectroscopy reveals a weak n→π* transition for the carbonyl group, with absorption maximum around 300 nm (ε ≈ 12 M⁻¹ cm⁻¹ in hexane), indicative of the forbidden nature of this electronic transition in saturated ketones.[16]Thermodynamic Properties
Cyclopentanone exhibits characteristic thermodynamic properties that define its behavior as a liquid at standard conditions. Its boiling point is 130.6 °C at 760 mmHg, allowing it to remain in liquid form under ambient pressures but requiring moderate heating for distillation processes.[17] The melting point is -51.3 °C, indicating it is a stable liquid well above typical freezing temperatures.[17] The density of cyclopentanone is 0.947 g/cm³ at 20 °C, which is slightly less than that of water, facilitating its separation in aqueous mixtures.[18] Its refractive index is 1.4366 at 20 °C, a value typical for cyclic ketones and useful in purity assessments via refractometry.[18]| Property | Value | Conditions | Source |
|---|---|---|---|
| Boiling Point | 130.6 °C | 760 mmHg | NIST WebBook[17] |
| Melting Point | -51.3 °C | - | NIST WebBook[17] |
| Density | 0.947 g/cm³ | 20 °C | PubChem[18] |
| Refractive Index | 1.4366 | 20 °C | PubChem[18] |