2-Methyl-1,3-cyclohexanedione
[CAS 1193-55-1]

Identification

• Classification: Chemical reagent >> Organic reagent >> Fatty ketone (including enol)
• Name: 2-Methyl-1,3-cyclohexanedione
• Molecular Formula: C₇H₁₀O₂
• Molecular Weight: 126.15
• CAS Registry Number: 1193-55-1
• EC Number: 214-773-9
• SMILES: CC1C(=O)CCCC1=O

Properties

• Density: 1.1±0.1 g/cm³ Calc.^*
• Melting point: 206 - 208 °C (Expl.)
• Boiling point: 235.7±23.0 °C 760 mmHg (Calc.)^*
• Flash point: 85.9±19.6 °C (Calc.)^*
• Solubility: water: insoluble (Expl.)
• Index of refraction: 1.459 (Calc.)^*

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

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H315-H319-H335
• Safety Statements: P261-P264-P264+P265-P271-P280-P302+P352-P304+P340-P305+P351+P338-P319-P321-P332+P317-P337+P317-P362+P364-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin irritation	Skin Irrit.	2	H315
Specific target organ toxicity - single exposure	STOT SE	3	H335
Eye irritation	Eye Irrit.	2A	H319
Eye irritation	Eye Irrit.	2	H319

Discovery and Applications

2-Methyl-1,3-cyclohexanedione is a cyclic β-diketone consisting of a six-membered cyclohexane ring containing two carbonyl groups at the 1- and 3-positions and a methyl substituent at the carbon between the two carbonyl groups. It belongs to the class of cyclic 1,3-dicarbonyl compounds, which are characterized by distinctive tautomeric behavior, enol formation, and enhanced acidity of the central methylene or methine hydrogen atoms.

The molecular framework is based on a cyclohexane ring containing two ketone functionalities arranged in a 1,3-dicarbonyl pattern. The carbonyl groups are separated by one carbon atom, creating an active methine center at the 2-position. In 2-methyl-1,3-cyclohexanedione, this central carbon bears a methyl substituent and a hydrogen atom, making it the key reactive site of the molecule.

Each carbonyl carbon is sp²-hybridized and adopts a trigonal planar geometry. The carbon–oxygen double bonds are strongly polarized because oxygen is more electronegative than carbon, resulting in electrophilic carbonyl carbons and electron-rich carbonyl oxygens. The two carbonyl groups exert a combined electron-withdrawing effect on the adjacent carbon, greatly increasing the acidity of the hydrogen attached to the 2-position.

The molecule exists in equilibrium between keto and enol tautomeric forms. In the enol form, one carbonyl group is converted into a hydroxyl group and a carbon–carbon double bond forms between the central carbon and the adjacent carbonyl carbon. The enol tautomer is stabilized by conjugation between the carbon–carbon double bond and the remaining carbonyl group, as well as by possible intramolecular hydrogen bonding between the hydroxyl group and the neighboring carbonyl oxygen.

The methyl substituent at the 2-position modifies the electronic and steric properties of the active methine center. It provides electron donation through inductive effects and increases steric bulk near the dicarbonyl system. Because the 2-position carbon is attached to four different substituents in the enol or certain derivative forms, stereochemical considerations may arise depending on the molecular environment.

The cyclohexane ring is nonplanar and generally adopts conformations such as chair-like or distorted half-chair forms. However, the presence of two carbonyl groups introduces local planar regions and restricts conformational freedom near the 1,3-dicarbonyl segment. The balance between ring flexibility and carbonyl conjugation influences the preferred molecular geometry.

From an electronic perspective, the 1,3-dicarbonyl arrangement creates a highly stabilized enolate system. Removal of the acidic proton from the central carbon produces an enolate anion in which the negative charge is delocalized over both oxygen atoms. This resonance stabilization is the primary reason β-diketones are significantly more acidic than ordinary ketones.

Physicochemically, 2-methyl-1,3-cyclohexanedione is a polar organic compound due to its two carbonyl groups and its ability to form hydrogen bonds. The carbonyl oxygens act as hydrogen bond acceptors, while the enol hydroxyl group in the tautomeric form can act as a hydrogen bond donor. The cyclohexane ring and methyl group contribute hydrophobic character, giving the molecule amphiphilic properties.

Chemically, the compound readily participates in reactions characteristic of β-diketones. The acidic 2-position hydrogen can be removed by bases to form metal enolates, which serve as nucleophilic intermediates in carbon–carbon bond-forming reactions such as alkylation and acylation. The carbonyl groups can also undergo nucleophilic addition reactions and condensation reactions under suitable conditions.

The enolate form of 2-methyl-1,3-cyclohexanedione can coordinate with metal ions through the two oxygen atoms, forming chelate complexes. This behavior is common among β-dicarbonyl compounds because the delocalized enolate provides a stable bidentate binding arrangement.

Overall, 2-methyl-1,3-cyclohexanedione is a cyclic β-diketone featuring two adjacent carbonyl-derived functionalities, an acidic methyl-substituted central carbon, and a flexible cyclohexane framework. Its tautomerism, enolate stabilization, hydrogen-bonding ability, and metal-chelating properties define its characteristic chemical behavior and make it a representative compound of cyclic 1,3-dicarbonyl chemistry.

References

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DOI: 10.1055/a-1809-6545

2022. Biomimetic Diels–Alder Reactions in Natural Product Synthesis: A Personal Retrospect. Synlett.
DOI: 10.1055/a-1748-4744