CH₃CN — Acetonitrile

Acetonitrile (CH₃CN), also known as methyl cyanide, is the simplest organic nitrile. It is a colorless, volatile liquid with a mild ether-like odor and excellent solvent properties. Acetonitrile is produced as a by-product in the manufacture of acrylonitrile and is one of the most widely used polar aprotic solvents in chemical laboratories and industries.

Due to its ability to dissolve a wide range of organic and inorganic compounds, acetonitrile is extensively used in high-performance liquid chromatography (HPLC), chemical synthesis, and pharmaceutical formulations. It is also valued for its stability and low reactivity compared to other organic solvents.

Acetonitrile’s polar nature and relatively high dielectric constant make it particularly effective for reactions involving ions or polar molecules. It is also used in the preparation of amides, amines, and heterocyclic compounds. Its chemical structure enables strong dipole-dipole interactions without forming hydrogen bonds, making it suitable for various organic transformations.

Acetonitrile has the molecular formula:

It consists of a methyl group (–CH₃) attached to a cyano group (–C≡N). The carbon atom in the cyano group forms a triple bond with nitrogen, which gives the molecule its linear geometry along the C≡N axis.

The structure can be represented as:

The carbon-nitrogen triple bond (C≡N) is highly polar due to the electronegativity difference between carbon and nitrogen atoms. This results in a strong dipole moment (3.92 D), making acetonitrile an effective polar aprotic solvent. The hybridization of the atoms in the molecule is as follows:

• The methyl carbon is sp³ hybridized.
• The nitrile carbon is sp hybridized.
• The nitrogen atom is also sp hybridized.

This combination gives the molecule a compact and stable linear structure with strong covalent bonding and excellent chemical resistance.

Acetonitrile can be obtained through several industrial and laboratory methods. The main processes include:

• 1. By-Product of Acrylonitrile Production: The most common industrial route involves the production of acetonitrile as a by-product of the propylene ammoxidation process used to manufacture acrylonitrile.

During this process, small quantities of acetonitrile are formed and later recovered through fractional distillation.

• 2. Dehydration of Acetamide: Acetonitrile can be synthesized in the laboratory by dehydrating acetamide using phosphorus pentoxide (P₂O₅) or sulfuric acid.

• 3. Oxidation of Ethylamine: Acetonitrile is also formed by the oxidation of ethylamine under controlled conditions.

• 4. From Methane and Ammonia: In some industrial setups, methane and ammonia react in the presence of catalysts to yield hydrogen cyanide and acetonitrile as intermediates.

These methods ensure a steady supply of acetonitrile for laboratory and industrial applications.

Physical Properties:

• Acetonitrile is a colorless, volatile, flammable liquid.
• It has a mild ether-like odor.
• It is completely miscible with water and many organic solvents.
• Boiling point: 81.6°C; Melting point: –45.7°C.
• Density: 0.786 g/cm³ at 25°C.
• Dielectric constant: 36.6 (indicating high polarity).
• Flash point: 2°C, making it highly flammable.

Chemical Properties:

• 1. Hydrolysis: Acetonitrile undergoes hydrolysis in acidic or basic medium to form acetamide and, upon further hydrolysis, acetic acid.

\( CH_3CN + H_2O \xrightarrow{H^+} CH_3CONH_2 \xrightarrow{H_2O} CH_3COOH + NH_3 \)

• 2. Reduction: On catalytic hydrogenation, acetonitrile is reduced to ethylamine.

\( CH_3CN + 2H_2 \xrightarrow{Ni} CH_3CH_2NH_2 \)

• 3. Oxidation: Acetonitrile can be oxidized to acetic acid using oxidizing agents such as KMnO₄ or nitric acid.

\( CH_3CN + 2H_2O + 2[O] \rightarrow CH_3COOH + NH_3 \)

• 4. Reaction with Grignard Reagents: Acetonitrile reacts with Grignard reagents to produce ketones after hydrolysis.

\( CH_3CN + RMgX \xrightarrow{H_2O} RC(=O)CH_3 + NH_3 \)

• 5. Nucleophilic Substitution: The cyano group acts as a strong electron-withdrawing group, enabling nucleophilic substitution reactions at the α-carbon.

Acetonitrile has extensive industrial and analytical applications due to its excellent solvent properties and chemical stability:

• 1. Chromatography Solvent: It is the most widely used solvent in HPLC (High-Performance Liquid Chromatography) and UV spectroscopy due to its low UV absorbance and compatibility with detectors.
• 2. Organic Synthesis: Used as a polar aprotic solvent for nucleophilic substitution and condensation reactions.
• 3. Pharmaceutical Industry: Serves as a solvent and intermediate in the manufacture of antibiotics, vitamins, and active pharmaceutical ingredients (APIs).
• 4. Extraction and Purification: Used for extracting fatty acids, steroids, and natural products from biological materials.
• 5. Electrochemistry: Acetonitrile is used as a solvent in electrochemical studies due to its wide electrochemical window and low viscosity.
• 6. Laboratory Use: Commonly employed as a drying agent and reagent for organic analysis.
• 7. Polymer Industry: Utilized in the production of acrylic fibers and plastic precursors.

Because of its versatility, acetonitrile is considered one of the most valuable organic solvents in both academic and industrial chemistry.

Acetonitrile is moderately toxic and highly flammable. Exposure to vapors or liquid can pose health risks, and proper handling measures are essential.

Health Hazards:

• Inhalation causes dizziness, headache, and irritation of the respiratory tract.
• Skin contact can result in dryness, redness, and irritation.
• Eye contact leads to severe irritation and tearing.
• Ingestion may cause nausea, vomiting, and cyanosis due to the release of cyanide in metabolism.

Safety Precautions:

• Use in a well-ventilated area or under a fume hood.
• Wear protective clothing, gloves, and safety goggles when handling acetonitrile.
• Keep away from flames, sparks, and strong oxidizing agents.
• Store in tightly sealed containers in cool, dry conditions.
• Dispose of waste solvent according to hazardous waste regulations.

In case of accidental exposure, rinse affected areas with water and seek immediate medical attention.