Heptane (N-Heptane)

n-Heptane

Other Names: Heptane, n-Heptane, Heptan, Heptyl hydride
Chemical Formula: C₇H₁₆
Molar Mass: 100.20 g/mol
CAS Number: 142-82-5
SMILES: CCCCCCC
Appearance: Clear, colorless liquid
Odor: Characteristic, gasoline-like odor

n-Heptane is a saturated, straight-chain hydrocarbon belonging to the alkane homologous series. Its molecular structure consists of seven carbon atoms connected by single carbon–carbon bonds, with the remaining valences occupied by hydrogen atoms. Because the molecule contains no permanent polar functional groups and has a very low dielectric constant, n-heptane behaves as a strongly nonpolar solvent. Its intermolecular interactions are dominated primarily by London dispersion forces. These characteristics make n-heptane an important model compound for studies of hydrocarbon thermodynamics, phase equilibria, combustion, molecular transport and nonpolar solution chemistry.

Chemical and physical properties

n-Heptane is a clear, colorless and volatile liquid under ambient conditions. Experimental data report a melting point of approximately −90.6 °C and a normal boiling point of approximately 98.4 °C.

The density of n-heptane is approximately 0.68 g/cm³ near room temperature, making it substantially less dense than water. Its refractive index is approximately 1.386 at 25 °C, while its surface tension is approximately 19.7 mN/m at the same temperature.

n-Heptane is essentially insoluble in water. Experimental measurements report an aqueous solubility of only approximately 3.4 mg/L at 25 °C. In contrast, it is miscible or highly soluble in many nonpolar and moderately polar organic solvents, including diethyl ether, benzene, chloroform and other hydrocarbons.

The compound has a very low dielectric constant, approximately 1.92 at 20 °C. Consequently, n-heptane is poorly suited for stabilizing separated ionic species but is an effective medium for dissolving many hydrophobic and nonpolar organic compounds.

n-Heptane is relatively volatile, with a vapor pressure of approximately 46 mmHg at 25 °C. Its heat of vaporization is approximately 36.6 kJ/mol at 25 °C. These properties have made it a frequently investigated reference hydrocarbon in experimental and theoretical studies of vapor–liquid equilibria and thermodynamic properties.

Applications

n-Heptane is widely used as a nonpolar solvent in organic chemistry. It is suitable for dissolving hydrocarbons, oils, waxes and other hydrophobic organic substances and can be employed as a reaction medium when a chemically simple, weakly coordinating and nonpolar environment is required.

In synthetic chemistry, n-heptane is frequently used as a solvent or co-solvent in reactions involving moisture-sensitive organometallic compounds and other reagents for which strongly coordinating solvents are undesirable. Its relatively low boiling point also facilitates removal from reaction mixtures by evaporation or distillation.

n-Heptane is extensively employed in separation and purification procedures. It can be used as a mobile-phase component in normal-phase chromatography, where its low polarity makes it useful for controlling the elution of organic compounds on polar stationary phases. Mixtures of n-heptane with more polar solvents allow the overall elution strength of chromatographic systems to be adjusted.

In materials and surface chemistry, n-heptane is used as a nonpolar probe liquid and as a solvent for hydrophobic compounds. Its well-characterized surface tension and dispersive intermolecular interactions make it useful in investigations of wetting, adsorption and surface-energy phenomena.

n-Heptane also has an important role as a reference hydrocarbon in combustion and fuel research. In the conventional octane-number scale, n-heptane represents the low knock-resistance reference component and is assigned an octane number of zero, while 2,2,4-trimethylpentane (isooctane) represents the high-reference component. Consequently, n-heptane combustion and oxidation mechanisms have been extensively studied in experimental and computational research.

Because its thermodynamic and transport properties are well characterized, n-heptane is additionally used as a model compound in studies of hydrocarbon mixtures, vapor–liquid equilibria, diffusion, viscosity, density, thermal properties and molecular simulation.

Scientific references

National Institute of Standards and Technology (NIST). NIST Chemistry WebBook: Heptane, CAS 142-82-5. Experimental thermodynamic, phase-change, fluid, spectroscopic and molecular data for n-heptane.

National Center for Biotechnology Information (NCBI), PubChem. Heptane, CID 8900. Chemical identity and physicochemical data including molecular formula, molecular mass, structure, melting point, boiling point, density, vapor pressure, refractive index and aqueous solubility.

Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. A Comprehensive Modeling Study of n-Heptane Oxidation. Combustion and Flame, 1998, 114(1–2), 149–177. DOI: 10.1016/S0010-2180(97)00282-4.

Ciezki, H. K.; Adomeit, G. Shock-tube investigation of self-ignition of n-heptane-air mixtures under engine relevant conditions. Combustion and Flame, 1993, 93(4), 421–433. DOI: 10.1016/0010-2180(93)90142-P.

Miller, M. M.; Wasik, S. P.; Huang, G. L.; Shiu, W. Y.; Mackay, D. Relationships between octanol-water partition coefficient and aqueous solubility. Environmental Science & Technology, 1985, 19(6), 522–529. DOI: 10.1021/es00136a007.

Safety

Signal Word: Danger

GHS Hazard Statements:
H225: Highly flammable liquid and vapor
H304: May be fatal if swallowed and enters airways
H336: May cause drowsiness or dizziness
H411: Toxic to aquatic life with long lasting effects

Precautionary Statements:
P210: Keep away from heat, hot surfaces, sparks, open flames, and other ignition sources. No smoking.
P240: Ground and bond container and receiving equipment.
P241: Use explosion-proof electrical/ventilating/lighting equipment.
P261: Avoid breathing vapors.
P271: Use only outdoors or in a well-ventilated area.
P273: Avoid release to the environment.
P301+P310: IF SWALLOWED: Immediately call a POISON CENTER or doctor.
P331: Do NOT induce vomiting.
P304+P340: IF INHALED: Remove person to fresh air and keep comfortable for breathing.
P403+P235: Store in a well-ventilated place. Keep cool.
P501: Dispose of contents/container in accordance with local/regional/national regulations.

First Aid Measures

Eye Contact: Rinse with plenty of water for several minutes. Seek medical attention if irritation occurs.
Skin Contact: Wash with soap and water. Remove contaminated clothing. Seek medical attention if irritation develops.
Inhalation: Move to fresh air and keep at rest. Seek medical attention if symptoms persist.
Ingestion: Do not induce vomiting. Rinse mouth with water and seek immediate medical help.

Handling and Storage

Storage Conditions: Store in a tightly closed container in a cool, dry, and well-ventilated place. Keep away from heat and sources of ignition.
Handling Precautions: Use spark-proof tools and explosion-proof equipment. Avoid inhalation of vapors. Use personal protective equipment.