Short Answer
Heptane and n-heptane are not interchangeable labels. Heptane is the general name for the family of saturated hydrocarbons with the molecular formula C7H16, which covers nine structural isomers including straight-chain and branched arrangements. n-Heptane is one specific member of that family: the unbranched, straight-chain isomer CH3CH2CH2CH2CH2CH2CH3. When a specification says "heptane" it usually means a mixture of isomers, often called commercial heptane or heptane 60/40; when it says "n-heptane" it means a high-purity single isomer, typically 99 % minimum.
Property Comparison
| Property | n-Heptane | Branched C7H16 isomers |
|---|---|---|
| Carbon skeleton | Straight chain only | One or more branches |
| Molecular formula | C7H16 | C7H16 |
| Molecular weight | 100.20 | 100.20 |
| Boiling point | Approximately 98.4 C | Approximately 80 to 92 C depending on isomer |
| Density at 20 C | Approximately 0.684 g/cm3 | Lower than the straight-chain isomer |
| Octane number by definition | 0 (zero reference fuel) | Higher than zero, varying by isomer |
| Commercial purity | 99 % min for n-heptane grade | Mixed isomer grades, purity quoted on total C7 |
Distillation range is the standard way to distinguish the two in a certificate of analysis, and it is run under ASTM D86 or ASTM D1078. Density is checked under ASTM D1298 or ISO 12185 and flash point under ASTM D93 or ISO 2719.
Industrial Uses of Each Grade
n-Heptane is used where a defined, narrow-boiling non-polar solvent is required: laboratory extractions and chromatography, pharmaceutical processing, polymerisation reaction media, and degreasing of precision components. Its very low aromatic content and low toxicity compared with aromatic solvents make it attractive in applications where a benign hydrocarbon solvent is needed.
Mixed heptane, frequently supplied as a narrow-cut C7 fraction, is used in rubber cements and contact adhesives, in printing inks, in the denaturing of ethanol, and as a general industrial cleaning solvent. The mixture has a broader distillation range but is significantly cheaper per tonne.
Frequently Asked Questions
Q: Is heptane the same as n-heptane?
A: No. Heptane is the family name for all C7H16 isomers, while n-heptane is only the straight-chain isomer. A product labelled heptane is normally a mixture and will have a wider distillation range and a lower purity figure.
Q: Which is more expensive, n-heptane or mixed heptane?
A: n-Heptane. Producing a single isomer at 99 % minimum purity requires additional distillation and quality control compared with a mixed C7 cut, so the high-purity grade carries a premium.
Q: Which boiling point should I expect for n-heptane?
A: Approximately 98.4 C at atmospheric pressure, against roughly 80 to 92 C for the branched isomers. A narrow distillation range close to 98 C is good evidence of a genuine n-heptane grade.
Q: Why is n-heptane called a zero octane fuel?
A: By definition the octane scale sets n-heptane at 0 and iso-octane at 100, and test engines per ASTM D2699 and ASTM D2700 are calibrated against blends of the two primary reference fuels.
Q: How should heptane be stored?
A: In closed, properly earthed containers in a cool, ventilated, flameproof area, away from oxidising agents and ignition sources. Because the flash point is around -4 C, vapour control rather than liquid containment is the main safety concern.
Q: What tests appear on a typical certificate of analysis?
A: Distillation range, density, flash point, colour, water content by Karl Fischer under ASTM E203, and purity by gas chromatography, with sulfur and aromatic content reported for high-purity grades.