Acetonitrile
Properties
| State | Liquid |
| Color | Colorless |
| Solubility | Miscible with water and most organic solvents |
| Melting Point | -45°C |
| Boiling Point | 82°C |
About Acetonitrile
Acetonitrile is the solvent that runs almost every modern HPLC and LC-MS column in the world, and the reason for its dominance comes down to a particular combination of properties no other common solvent matches. It's polar enough (dielectric constant ~37) to dissolve organic analytes that water won't touch, transparent in the UV down to about 190 nm so it doesn't interfere with diode-array detection, miscible with water in any ratio so gradient elution is straightforward, and low-viscosity enough that columns don't run at extreme back-pressures. Run a reversed-phase separation with a water/acetonitrile gradient and you can resolve almost any small-molecule mixture you care about. The molecule itself is a methyl group attached to a cyanide via a triple bond, which is why it acts as both a polar aprotic solvent and a weak ligand for soft transition metals — Cu(I), Pd(II), and Rh(I) complexes routinely carry MeCN as a labile placeholder that comes off when a real ligand is introduced. Worldwide consumption runs into the hundreds of thousands of tonnes annually, almost all of it produced as a byproduct of acrylonitrile manufacture for plastics. That dependency caused a serious supply crunch in 2008–2009 when acrylonitrile demand collapsed and acetonitrile prices spiked tenfold, leaving analytical labs scrambling for alternatives.
Where you'll encounter it
If you work in pharma analytics, environmental analysis, or proteomics, acetonitrile is the bottle you go through fastest — typical reversed-phase methods run a 5–95% acetonitrile-in-water gradient and a single LC system can burn a liter a day. In synthesis, it's the solvent of choice for SN2 reactions on alkyl halides (because it doesn't have an acidic O–H to compete with the nucleophile), for the Ritter reaction (which generates amides directly from a carbocation, water, and a nitrile), and for organometallic chemistry where you need polarity without protic interference. The other place it shows up is as the residual solvent in pharmaceutical APIs — ICH guidelines list it as a Class 2 solvent with a 410 ppm limit, which means QC labs spend serious effort tracking down trace acetonitrile in finished products.
Common Uses
- Mobile-phase solvent for reversed-phase HPLC and LC-MS
- Polar aprotic solvent for SN2 and Ritter reactions
- Labile ligand for soft transition-metal coordination chemistry
- Extraction solvent for fatty acids and pharmaceuticals
- Solvent for non-aqueous electrochemistry and battery research
Safety Information
Flammable (flash point 6 °C). Toxicity is moderate but delayed: the body slowly metabolizes acetonitrile to cyanide via cytochrome P450, so symptoms of exposure can appear hours after the event. Inhalation, skin absorption, and ingestion are all viable routes; the standard first-line antidote (hydroxocobalamin) treats the cyanide that gets liberated. Use in a fume hood with appropriate gloves; treat exposure as you would any cyanide-releasing chemical even though the kinetics are gentler.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.