2-Methylcyclopentanone
[CAS 1120-72-5]

Identification

• Classification: Chemical reagent >> Organic reagent >> Fatty ketone (including enol)
• Name: 2-Methylcyclopentanone
• Molecular Formula: C₆H₁₀O
• Molecular Weight: 98.14
• CAS Registry Number: 1120-72-5
• EC Number: 214-318-4
• SMILES: CC1CCCC1=O

Properties

• Density: 0.9±0.1 g/cm³ Calc.^*
• Melting point: -75 °C (Expl.)
• Boiling point: 141.6±8.0 °C 760 mmHg (Calc.)^*, 139 °C (Expl.)
• Flash point: 26.1 °C (Calc.)^*, 26 °C (Expl.)
• Solubility: water: soluble (Expl.)
• Index of refraction: 1.443 (Calc.)^*, 1.435 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS02 WarningGHS02
• Risk Statements: H226
• Safety Statements: P210-P233-P240-P241-P242-P243-P280-P303+P361+P353-P370+P378-P403+P235-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Flammable liquids	Flam. Liq.	3	H226

• Transport Information: UN 1224

Discovery and Applications

2-Methylcyclopentanone is a substituted alicyclic ketone consisting of a five-membered cyclopentane ring bearing a ketone functional group and a methyl substituent at the carbon adjacent to the carbonyl carbon. It belongs to the family of cycloalkanones and is an important intermediate in organic synthesis due to the reactivity of its carbonyl group and the presence of activated α-hydrogen atoms.

Structurally, the molecule is based on a cyclopentanone framework, in which one carbon atom of the five-membered ring is incorporated into a carbonyl group (C=O). The carbonyl carbon is designated as position 1, while the methyl substituent is attached to the adjacent carbon (position 2). This arrangement places the methyl group at the α-position relative to the carbonyl functionality.

The carbonyl carbon is sp²-hybridized and adopts a trigonal planar geometry. The carbon–oxygen double bond consists of one σ bond and one π bond, with the oxygen atom carrying two lone pairs of electrons. The strong electronegativity of oxygen polarizes the carbonyl group, producing a partial positive charge on the carbonyl carbon and a partial negative charge on the oxygen atom. This polarization makes the carbonyl carbon the principal electrophilic center of the molecule.

The remaining ring carbon atoms are predominantly sp³-hybridized and adopt a nonplanar conformation. Unlike six-membered cyclohexanones, five-membered cyclopentanone rings cannot adopt a chair conformation. Instead, the ring undergoes rapid interconversion among envelope and half-chair conformations, which reduce torsional strain while maintaining ring flexibility.

The methyl substituent contributes a weak electron-donating inductive effect and increases the steric environment around the α-carbon. Because the methyl group is directly adjacent to the carbonyl group, it slightly influences the electronic properties and reactivity of the ketone while introducing a stereogenic center if the remaining substituents on the ring are distinguishable.

The α-carbon bearing the methyl group contains at least one hydrogen atom that is activated by the neighboring carbonyl group. These α-hydrogen atoms are relatively acidic compared with those of simple hydrocarbons because removal of a proton generates an enolate ion. The resulting negative charge is delocalized between the α-carbon and the carbonyl oxygen through resonance, providing significant stabilization.

From an electronic perspective, conjugation is limited to the localized carbonyl group because the molecule contains no additional π-bonded systems. Consequently, its chemical behavior is dominated by the carbonyl functionality rather than by extended electron delocalization.

Physicochemically, 2-methylcyclopentanone possesses moderate polarity due to the carbonyl group. The oxygen atom serves as a hydrogen bond acceptor, although the molecule lacks hydrogen bond donor groups because it contains neither hydroxyl nor amino functionalities. The hydrocarbon ring and methyl substituent contribute nonpolar character, making the molecule amphiphilic but predominantly organic in its solvent preferences.

Chemically, the carbonyl carbon readily undergoes nucleophilic addition reactions characteristic of ketones. The compound also participates in reactions involving enolate formation, including alkylation, aldol condensation, Michael addition, and related carbon–carbon bond-forming processes. Reduction of the ketone yields the corresponding secondary alcohol, while oxidation under ordinary conditions is generally less favorable because ketones are relatively resistant to further oxidation without carbon–carbon bond cleavage.

The conformational flexibility of the cyclopentane ring influences the stereochemical outcome of many reactions by altering the accessibility of the carbonyl group and the α-carbon. The methyl substituent also affects the preferred conformations by minimizing steric interactions within the five-membered ring.

Overall, 2-methylcyclopentanone is a substituted cycloalkanone composed of a flexible five-membered ring containing a polarized ketone functional group and an α-methyl substituent. Its electrophilic carbonyl carbon, resonance-stabilized enolate-forming capability, and conformationally dynamic ring structure define its chemical and physical properties, making it a versatile intermediate in synthetic organic chemistry.

References

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