Melting Point and Key Physical Properties
Acetonitrile (ACN, CAS 75-05-8) has a melting/freezing point of approximately −45°C (−49°F), with SDS and reference sources reporting values in the −45 to −48°C range depending on purity and measurement method. Its boiling point is 81–82°C (179°F) at standard atmospheric pressure. These figures are consistent across NIOSH's Pocket Guide to Chemical Hazards and multiple supplier safety data sheets.
| Property | Value |
|---|---|
| Melting/freezing point | −45 to −48°C (−49°F) |
| Boiling point | 81–82°C (179°F) |
| Flash point (closed cup) | ~2–6°C (42°F) |
| Specific gravity | 0.78 |
| Molecular formula | CH₃CN (C₂H₃N) |
| Molecular weight | 41.05 g/mol |
| Vapor pressure | 73 mmHg |
| Explosive limits | 3–16% (v/v) in air |
| Water solubility | Miscible |
Acetonitrile is a colorless, transparent liquid with a distinctive ether-like odor, and is highly volatile - its vapor pressure and low flash point mean it evaporates readily and forms flammable vapor-air mixtures well below room temperature.
Grades and Purity
Not all acetonitrile is interchangeable - grade matters for both performance and cost:
| Grade | Typical purity | Main use |
|---|---|---|
| Technical grade | >99.0% | Industrial synthesis, pesticide manufacturing, battery electrolytes |
| ACS Reagent grade | >99.5% | General lab synthesis, extractions, routine analytical work |
| HPLC grade | >99.9%, low UV absorbance (≤190–210 nm) | Mobile phase for HPLC and general chromatography |
| LC-MS grade | >99.9%, tested for low metal/background noise | Mass spectrometry, proteomics, drug discovery |
| Spectrophotometric grade | High optical transparency | UV-Vis and fluorescence spectroscopy |
Supply and Storage Notes
Acetonitrile is produced almost entirely as a by-product of acrylonitrile manufacturing (used to make ABS plastics and acrylic fiber). Because of this, acetonitrile supply and pricing track the acrylonitrile/ABS market rather than moving independently - when acrylonitrile plants run at reduced rates or go down for maintenance, acetonitrile availability tightens. This dynamic caused a well-documented global shortage in 2008–2009, and it's part of why buyers with steady, high-volume needs (e.g., pharmaceutical QC labs) often prefer long-term supply contracts over spot purchases.
Acetonitrile is also hygroscopic, meaning it absorbs moisture from air over time. This matters for both storage and use: water content can degrade chromatography baseline performance or shift reaction outcomes in synthesis. It's typically stored in tightly sealed containers - amber glass for lab-grade material, to also limit UV-driven degradation.
Solubility and Stability
Acetonitrile is fully miscible with water, alcohols, and a wide range of organic solvents, which is part of why it's so widely used as a chromatography and extraction solvent.
Chemically, it's fairly resistant to oxidation and reduction under normal conditions. However, the carbon–nitrogen triple bond in its structure is reactive toward addition reactions - it reacts with hydrogen halides, hydrogen sulfide, alcohols, acids, and acid anhydrides. A few reaction pathways worth noting:
- Hydrolysis - Under acidic or basic catalysis, acetonitrile hydrolyzes to an amide (acetamide), and further hydrolysis yields acetic acid.
- Reduction - Acetonitrile can be reduced to ethylamine.
- Reaction with metals - It reacts with sodium metal, sodium alkoxides, and sodium amide.
- Incompatibilities - NIOSH flags strong oxidizers as a key incompatibility; contact can generate hazardous byproducts.
Acetonitrile (ACN) Applications
- Solvent - Dissolves a broad range of organic, inorganic, and gaseous substances, making it a workhorse solvent across chemical processing.
- Pharmaceutical industry - Used in synthesizing vitamin A, cortisone, carbamates, and related intermediates, and as a reaction medium for vitamin B1 and amino acid chemistry.
- Chemical analysis - A standard mobile-phase solvent and organic modifier in HPLC, thin-layer chromatography, paper chromatography, spectroscopy, and polarography.
- Hydrocarbon extraction and separation - Used in extractive distillation to separate components like butadiene from C4 hydrocarbon streams.
- Synthesis intermediate - A raw material for pharmaceutical and pesticide intermediates, including vitamin B1 and metronidazole.
- Semiconductor cleaning - Its solvency profile makes it effective at removing oils, inorganic salts, and polymeric residues in electronics manufacturing.
- Organic synthesis and catalysis - Frequently used as a ligand or component in transition-metal catalyst systems, and in textile dyeing and coating formulations.
- Stabilizer - Used to help stabilize chlorinated solvents.
Frequently Asked Questions
What is the melting point of acetonitrile?
Acetonitrile's melting (freezing) point is approximately −45°C, though some references cite values as low as −48°C depending on sample purity.
What is the boiling point of acetonitrile?
Its boiling point is 81–82°C at standard atmospheric pressure - notably lower than water, which is why it's so volatile at room temperature.
Is acetonitrile flammable?
Yes. It's classified as a Class IB flammable liquid, with a flash point around 2–6°C - meaning it can form flammable vapor even at temperatures well below typical room temperature. Its vapor-air explosive range is roughly 3–16% by volume.
Is acetonitrile toxic?
Acetonitrile is metabolized in the body to release cyanide, and acute exposure can cause symptoms including nausea, headache, and respiratory irritation, sometimes with a delayed onset of several hours. It should be handled with appropriate ventilation and personal protective equipment, per NIOSH guidance.
Why is acetonitrile used in HPLC?
Its high polarity, UV transparency at low wavelengths, low viscosity, and full miscibility with water make it an ideal mobile-phase solvent for reversed-phase HPLC, giving good peak resolution and low background interference.
What happens when acetonitrile hydrolyzes?
Under acidic or basic conditions, the nitrile group hydrolyzes stepwise: first to an amide (acetamide), then further to acetic acid, releasing ammonia in the process.
What's the difference between technical grade and HPLC grade acetonitrile?
The main differences are purity and UV transparency. Technical grade (>99.0%) is fine for industrial synthesis or general processing, but it isn't purified for low UV absorbance, so it can interfere with chromatography baselines. HPLC grade (>99.9%) is filtered and tested to keep UV absorbance low at critical wavelengths (around 190–210 nm), which is necessary for clean peak resolution. LC-MS grade goes a step further, with additional testing for trace metals and background noise relevant to mass spectrometry.
