HEPTANE (C7H16): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The heptane is an organic compound whose chemical formula is C ₇ H ₁₆ and comprises nine structural isomers, of which the best known is linear. It is a hydrocarbon, specifically an alkane or paraffin, which is found in most organic chemistry laboratories, be they teaching or research.

Unlike other paraffinic solvents, heptane has lower volatility, which makes it relatively safer to use; as long as there is no heat source surrounding your vapors and you are working inside an extractor hood. Its flammability aside, it is a compound inert enough to serve as a medium for organic reactions.

N-Heptane molecule represented by a ball-and-stick model. Source: Ben Mills and Jynto

The upper image shows the structure of n -heptane, the linear isomer of all heptanes. Because it is the most common and commercially valuable isomer, as well as the easiest to synthesize, it tends to be understood that the term 'heptane' refers exclusively to n-heptane; unless otherwise stated.

However, in the bottles of this liquid compound it is specified that it contains n -heptane. They have to be uncovered inside an extractor hood and measurements taken carefully.

It is an excellent solvent for fats and oils, which is why it is frequently used during the extractions of plant essences or other natural products.

Structure

n-heptane and its intermolecular interactions

As can be seen in the first image, the n -heptane molecule is linear, and due to the chemical hybridization of its carbon atoms, the chain assumes a zigzag shape. This molecule is dynamic in that its CC bonds can rotate, causing the chain to bend slightly at different angles. This contributes to their intermolecular interactions.

The n-heptane is a nonpolar, hydrophobic molecule, and therefore its interactions are based on the dispersive forces of London; These are those that depend on the molecular mass of the compound and its contact area. Two molecules of n -heptane approach each other in such a way that their chains "wedge" on top of each other.

These interactions are effective enough to keep n-heptane molecules cohesive in a liquid boiling at 98ºC.

Isomers

The nine isomers of heptane. Source: Steffen 962

At first it was said that the formula C ₇ H ₁₆ represented a total of nine structural isomers, n-heptane being the most relevant (1). The other eight isomers are shown in the image above. Note at a glance that some are more branched than others. From left to right, starting from the top, we have:

(2): 2-methylhexane

(3): 3-methylhexane, which consists of a pair of enantiomers (a and b)

(4): 2,2-dimethylpentane, also known as neoheptane

(5): 2,3-dimethylpentane, again with a pair of enantiomers

(6): 2,4-dimethylpentane

(7): 3,3-dimethylpentane

(8): 3-ethylpentane

(9): 2,2,3-trimethylbutane.

Each of these isomers have independent properties and applications of n -heptane, reserved mostly for the fields of organic synthesis.

Properties of heptane

Physical appearance

Colorless liquid with a gasoline-like odor.

Molar mass

100.205 g / mol

Melting point

-90.549 ºC, becoming a molecular crystal.

Boiling point

98.38 ° C.

Vapor pressure

52.60 atm at 20 ° C. Note how high its vapor pressure is, despite being less volatile than other paraffinic solvents, such as hexane and pentane.

Density

0.6795 g / cm ³. On the other hand, heptane vapors are 3.45 times denser than air, which means that its vapors will linger in spaces where some of its liquid spills.

Water solubility

Since heptane is a hydrophobic compound, it can barely dissolve in water to produce a solution with a concentration of 0.0003% at a temperature of 20ºC.

Solubility in other solvents

Heptane is miscible with carbon tetrachloride, ethanol, acetone, light petroleum, and chloroform.

Refractive index (

1.3855.

Viscosity

0.389 mPa s

Heat capacity

224.64 J / K mol

ignition point

-4 ºC

Autoignition temperature

223 ºC

Surface tension

19.66 mN / m at 25 ºC

Heat of combustion

4817 kJ / mol.

Reactivity

Heptane vapors when close to a heat source (a flame), react exothermically and vigorously with the oxygen in the air:

C ₇ H ₁₆ + 11O ₂ => 7CO ₂ + 8H ₂ O

However, outside of the combustion reaction, heptane is a fairly stable liquid. Its lack of reactivity is due to the fact that its CH bonds are difficult to break, so it is not susceptible to substitution. Likewise, it is not very sensitive to strong oxidizing agents, as long as there is no fire nearby.

The greatest danger of heptane is its high volatility and flammability, so there is a risk of fire if it is spilled in hot places.

Applications

Solvent and reaction medium

Heptane is an excellent solvent for dissolving oils and fats. Source: Pxhere.

The hydrophobic character of heptane makes it an excellent solvent for dissolving oils and fats. In this aspect it has been used as a degreaser. However, its greatest application lies in being used as an extracting solvent, since it dissolves the lipid components, as well as the other organic compounds of a sample.

For example, if you wanted to extract all the components of ground coffee, it would be macerated in heptane instead of water. This method and its variations have been implemented with all kinds of seeds, thanks to which plant essences and other natural products have been obtained.

Heptane, which is naturally colorless, will turn to the color of the extracted oil. Then, it is rotated to finally have a volume of the oil that is as pure as possible.

On the other hand, the low reactivity of heptane also allows it to be an option when considering a reaction medium to carry out a synthesis. Being a good solvent for organic compounds, it ensures that the reagents remain in solution and interact with each other properly while reacting.

Precipitating agent

In petroleum chemistry it is common practice to precipitate asphaltenes from a crude sample by adding heptane. This method allows studying the stability of different crude oils and determining how susceptible their asphaltenes content is to precipitating and causing a series of problems for the oil industry.

Octane

Heptane has been used as a fuel because of the large amount of heat it gives off when it burns. However, as far as car engines are concerned, it would be detrimental to their performance if used in pure form. Because it burns very explosively, it serves to define 0 on the gasoline octane scale.

Gasoline contains a high percentage of heptane and other hydrocarbons to bring the octane number up to known values ​​(91, 95, 87, 89, etc.).

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. (10th edition.). Wiley Plus.
4. Wikipedia. (2020). Heptane. Recovered from: en.wikipedia.org
5. National Center for Biotechnology Information. (2020). Heptane PubChem Database. CID = 8900. Recovered from: pubchem.ncbi.nlm.nih.gov
6. Elsevier BV (2020). Heptans. ScienceDirect. Recovered from: sciencedirect.com
7. Bell Chem Corp. (September 7, 2018). Industrial Uses of Heptane. Recovered from: bellchem.com
8. Andrea Kropp. (2020). Heptane: Structure, Uses & Formula. Study. Recovered from: study.com