What Acetonitrile (ACN) Is
Acetonitrile, abbreviated ACN and also called methyl cyanide or cyanomethane, is the simplest organic nitrile, with the formula CH3CN and CAS number 75-05-8. It is a colourless, volatile liquid with a mild ether-like odour, a molecular weight of 41.05 g/mol, a boiling point of about 82 °C, a melting point near -45 °C and a density of about 0.786 g/cm3 at 20 °C. It mixes freely with water and most organic solvents, which is the property that makes it one of the most widely used solvents in industry and analytical science.
Nearly all commercial acetonitrile is recovered as a by-product of acrylonitrile manufacture, where propylene, ammonia and oxygen react in the ammoxidation process and acetonitrile is separated by distillation. Supply therefore follows acrylonitrile output rather than acetonitrile demand, so availability and price can be volatile and buyers often qualify more than one source.
Solvent Use in Synthesis and Extraction
Dissolves a wide range of polar and non-polar compounds, including resins, dyes, pigments and pharmaceutical intermediates.
Acts as an organic extractant in separation and purification trains, recovering valuable components from complex mixtures.
Works in both aqueous and non-aqueous systems because it is fully miscible with water.
Provides an inert, low-boiling medium for reactions needing a polar aprotic solvent with good chemical stability.
Assists crystallisation and recrystallisation of active ingredients where a controlled polarity window is required.
Chromatography and Analytical Chemistry
Acetonitrile is the mobile phase of choice for reversed-phase high-performance liquid chromatography and is also used in gas chromatography, thin-layer chromatography and mass spectrometry. Its low ultraviolet absorbance gives clean baselines, its low viscosity keeps column back-pressure manageable, and its elution strength is easily adjusted by blending with water or buffer. Analytical grades are therefore controlled far more tightly than technical grades.
| Grade | Assay (GC) | Water content | Typical application |
|---|---|---|---|
| Technical | 99.5 % min | Low hundredths of a percent | Synthesis, extraction, general solvent duty |
| HPLC gradient grade | 99.9 % min | Tightly controlled, low | Chromatography mobile phase with UV detection |
| Preparative UV grade | 99.9 % min | Tightly controlled, low | Spectroscopy and preparative chromatography |
| Property | Value |
|---|---|
| Molecular formula | CH3CN |
| Molecular weight | 41.05 g/mol |
| Boiling point | About 82 °C |
| Melting point | About -45 °C |
| Density at 20 °C | About 0.786 g/cm3 |
| Flash point | About 2 °C in a closed cup |
| UN number and class | UN 1648, Class 3 flammable liquid |
Reagent and Specialty Industrial Uses
Acetonitrile acts as a reagent and reaction medium in the manufacture of pharmaceuticals, agrochemicals, dyes and pigments. In electronics it is a high-purity cleaning agent that removes flux residues, waxes, greases and fingerprints from silicon wafers and precision components. Other duties include degreasing of industrial surfaces and equipment, and use as a carrier and extraction solvent in speciality separations.
Safety, Storage and Regulatory Notes
Acetonitrile is a Class 3 flammable liquid with a flash point near 2 °C, so it must be handled away from ignition sources and bonded and earthed during transfer. It is also toxic: the body converts it into cyanide, and symptoms of overexposure can be delayed by hours, which makes prompt action essential after any suspected exposure. Store it cool, dry and well ventilated below 30 °C, away from oxidisers, reducing agents, acids and alkalis. Use a fume hood with butyl rubber gloves, safety goggles and protective clothing, and follow the safety data sheet supplied with each lot. Because acetonitrile is regulated as a hazardous air pollutant in many jurisdictions, plan for recovery or licensed disposal rather than sewer discharge.
Frequently Asked Questions
Q: What is acetonitrile used for in industry?
It is used as a solvent and reagent in chemical synthesis, as a mobile phase in chromatography, as a cleaner in electronics manufacturing, and in the production of pharmaceuticals, agrochemicals and plastics.
Q: What is the CAS number of acetonitrile?
The CAS number of acetonitrile is 75-05-8, and its molecular formula is CH3CN.
Q: Is acetonitrile miscible with water?
Yes, acetonitrile mixes freely with water and with most common organic solvents.
Q: Why is acetonitrile preferred as an HPLC mobile phase?
Its low ultraviolet absorbance, low viscosity and consistent purity deliver stable baselines and reliable separations.
Q: How is acetonitrile produced?
It is recovered mainly as a by-product of the ammoxidation route to acrylonitrile and then purified by distillation.
Q: Is acetonitrile flammable or toxic?
It is both: a Class 3 flammable liquid with a flash point near 2 °C, and a toxic material that metabolises to cyanide, so engineering controls and personal protective equipment are essential.