Methyl isobutyl ketone
Methyl isobutyl ketone (MIBK), chemically known as 4-methyl-2-pentanone or hexone, is a colorless, volatile, flammable liquid organic compound with the molecular formula C₆H₁₂O and a molecular weight of 100.16 g/mol.[1] It has a pleasant odor, with key physical properties including a boiling point of 116 °C, a density of 0.80 g/cm³ at 20 °C, a flash point of 23 °C, and low water solubility of approximately 19 g/L at 25 °C.[1] MIBK is primarily utilized as an industrial solvent in paints, varnishes, lacquers, resins, and nitrocellulose coatings, as well as a denaturant for alcohols, an extractant for rare metals, and a synthetic flavoring agent.[2][1] It is produced mainly through aldol condensation of acetone followed by dehydration and hydrogenation; as of 2024, global production capacity exceeds 590,000 tonnes per year, with Asia emerging as a net exporter in 2025.[3][4][5] MIBK has been detected in environmental media, including air (up to 6 µg/m³), industrial effluents (up to 62,000 µg/L), and food (trace levels).[2][1] It poses safety risks including irritation to eyes, skin, and respiratory tract, central nervous system depression, and potential liver/kidney effects; it is classified by IARC as Group 2B (possibly carcinogenic to humans) and by EPA as Group D (not classifiable for human carcinogenicity).[1] Occupational exposure limits include ACGIH TLV of 20 ppm (TWA) and 75 ppm (STEL), and OSHA PEL of 100 ppm (TWA).[1][6] Due to flammability and toxicity, handling requires ventilation, protective equipment, and regulatory compliance (e.g., OSHA, TSCA).[2]Properties
Physical properties
Methyl isobutyl ketone (MIBK), with the chemical formula C₆H₁₂O and structural formula (CH₃)₂CHCH₂C(O)CH₃, is a branched-chain ketone.[7] Its molar mass is 100.16 g/mol.[7] MIBK appears as a clear, colorless liquid with a pleasant odor.[7] It has a density of 0.802 g/mL at 20°C.[7] The melting point is -84.7°C, and the boiling point is 117–118°C at standard pressure.[7] MIBK exhibits limited solubility in water, at 1.91 g/100 mL at 20°C, but is miscible with most organic solvents such as ethanol, ether, and benzene.[7] Its vapor pressure is approximately 20 mmHg at 25°C.[7] The flash point is 23°C (closed cup), the autoignition temperature is 456°C, and the explosive limits are 1.4–7.5 vol% in air.[8][7] The refractive index is 1.396 at 20°C, and the dynamic viscosity is 0.58 mPa·s at 20°C.[9]Chemical properties
Methyl isobutyl ketone (MIBK), systematically named 4-methylpentan-2-one, features a ketone functional group characterized by a carbonyl (C=O) at the 2-position, bonded to a methyl group on one side and an isobutyl chain (with a methyl branch at the 4-position) on the other. This structural arrangement imparts the typical properties of an aliphatic ketone, where the carbonyl carbon serves as an electrophilic site due to the partial positive charge induced by the oxygen atom. MIBK exhibits chemical stability under standard ambient conditions and remains largely unreactive in dilute aqueous acidic or basic environments, though exposure to strong acids, strong bases, oxidizing agents, or reducing agents can promote decomposition or hazardous reactions. Its polarity is moderate, stemming from the polar carbonyl group, with an octanol-water partition coefficient (log P) of 1.3, rendering it less polar than water but more polar than hydrocarbons and facilitating selective solubility in mixed systems.[10] As a ketone, MIBK undergoes basic reactions inherent to the functional group, including nucleophilic addition at the carbonyl carbon by nucleophiles such as hydride or organometallic reagents, forming tetrahedral intermediates that can lead to alcohols or other derivatives. The alpha-hydrogens adjacent to the carbonyl are mildly acidic (pKa ≈ 20), allowing enolization under acid- or base-catalyzed conditions to form the enol tautomer, stabilized by resonance with the carbonyl.[11]/Reactions/Reactivity_of_Alpha_Hydrogens/Acidity_of_Alpha_Hydrogens_and_Keto-enol_Tautomerism)Production
Laboratory synthesis
Methyl isobutyl ketone (MIBK) is commonly synthesized in the laboratory through a three-step process starting from acetone, involving aldol condensation, dehydration, and hydrogenation. This classic route is suitable for small-scale preparations in educational or research settings, providing a practical demonstration of sequential organic transformations. The first step is the base-catalyzed self-condensation of acetone to form diacetone alcohol. In a typical procedure, commercial acetone (1190 g, 20.5 mol) is refluxed in a Soxhlet extractor with barium hydroxide as the catalyst for 95–120 hours on a steam bath. The product is isolated by distillation under reduced pressure, yielding 850 g (71% based on total acetone consumed) of diacetone alcohol as a colorless liquid boiling at 71–74°C/23 mmHg.[12] The second step involves dehydration of diacetone alcohol to mesityl oxide. Crude diacetone alcohol (1100 g, ~9.5 mol) is distilled in the presence of 0.1 g iodine catalyst using a fractionating column. The pure mesityl oxide fraction is collected at 126–131°C, followed by redistillation of the crude portion after drying, affording 650 g (65% theoretical based on initial acetone) of mesityl oxide.[13] The final step is the selective hydrogenation of mesityl oxide to MIBK using a palladium catalyst. In a representative liquid-phase procedure, mesityl oxide is reacted with hydrogen over 0.52 wt% Pd/Al₂O₃ in an acetone solvent within a stirred autoclave at 100°C and hydrogen partial pressures up to 700 kPa (approximately 7 atm), achieving near-complete conversion with MIBK selectivity exceeding 99%.[14] Overall laboratory yields for MIBK through this stepwise process are typically around 45-50%, depending on purification efficiency and starting material purity. The key reactions are summarized as follows: Aldol condensation:$2 \ce{(CH3)2CO} \rightarrow \ce{(CH3)2C(OH)CH2COCH3} Dehydration:
\ce{(CH3)2C(OH)CH2COCH3 ->[(-\ce{H2O})] (CH3)2C=CHCOCH3} Hydrogenation:
\ce{(CH3)2C=CHCOCH3 + H2 -> (CH3)2CHCH2COCH3}