Why is n-heptane a great choice for botanical extraction?

By Jillian Jastrzembski

Botanical extraction is an essential research tool for the study of bioactive compounds in plants. Plants, especially those used in traditional medicines, are considered a pharmacopeia of bioactive compounds.

Solvent choice is perhaps the most important parameter in extraction, and luckily, you have a lot of solvent choices. Some are ubiquitous in everyday life, like water, acetone, and some alcohols. Some are staples in almost every chemistry lab, like acetonitrile, dichloromethane, and hexanes.

N-heptane is becoming more common as it is often used to replace n-hexane for sustainability reasons. When is n-heptane a good choice for botanical extraction, and is it really “greener” than n-hexane? In this article, we’ll discuss these questions, and give some examples of how n-heptane has recently been used for botanical extraction in the scientific research community.

What is n-heptane?

N-heptane, or normal heptane, is an alkane (or straight carbon chain) consisting of seven carbons. The prefix hept- always indicates the number seven. N-heptane is not the same as “heptane,” because heptane contains a mixture of branches seven-carbon isomers. There are several chemicals that have the same formula, but the atoms are arranged in a different configuration. Heptane is suitable for fuel use or industrial uses that do not require a single pure isomer.

N-heptane specifically refers to only the unbranched, straight-carbon chain in its pure form. Therefore, it is a better choice for high-precision applications such as analytical experiments performed in the lab.

The polarity index of n-heptane is very low, since it has no functional groups. Compared to water, the most polar solvent, which has a polarity index of 10.2, n-heptane is on the opposite end of the spectrum with a polarity index of 0.1.

Following the principal of “like dissolves like,” that means we should choose n-heptane when we want to extract non-polar analytes.

Is n-heptane really a green solvent?

N-heptane is not a far cry from n-hexane. N-hexane is essentially the same, just with one extra carbon. They both have a polarity index of 0.1, and can easily be substituted one for the other.

Because of this, n-heptane is sometimes being used to replace hexane as a “greener” solvent choice. However, it is important to remember that “green” is a relative term. While it may be preferable to n-hexane, n-heptane is still by no means bio-based. N-heptane shows a wide range in greenness ratings. This could be due to the lack of data on this solvent. We happen to know a lot more about the environmental issues with n-hexane. However, just because we lack data on n-heptane doesn’t mean that it is automatically a green choice.

Some case studies

Botanical extraction is an essential research tool for the discovery of bioactive compounds. Here are three examples from the literature that used n-hexane for botanical extraction of plant compounds.

1. A 2022 study looked at the antiparasitic activity of extracts from 10 different tree barks, based on their use in traditional medicine. The researchers experimented with extraction solvents of different polarities, ranging from the least polar (n-heptane), to the most polar (methanol/water). They found that the n-heptane extract of one of the barks exhibited the highest anti-parasitic activity of all the extracts. Using multiple polarities is important when the analyte is unknown. Once the anti-parasitic fraction had been selected, the researchers could identify the bioactive compounds of that fraction. This research is useful for development of anti-parasitic medications, as well as to optimize the preparation techniques for traditional medicine.
2. In a 2020 study, researchers reported a maceration/extraction technique for natural production isolation. They used a combination of five solvents to complete a tri-phasic extraction. The five solvents had a wide range of polarities, with water being the most polar and n-heptane being the least polar. This wide range of solvents allowed for extraction of a many different analytes, which could be partitioned into three phases based on their polarity.
3. Another study from 2020 investigated the best solvent system for simultaneous extraction of CBD and elimination of pesticides. They investigated many different solvent systems, and found that the ones that did not contain either n-heptane of n-hexane were to polar to achieve the desired results (particularly since CBD is a hydrophobic compound).

Conclusion

N-heptane is a popular solvent for botanical extraction of bioactive compounds, and is often used to replace n-hexane since it is thought to be the greener solvent choice. N-heptane refers to the pure straight-chain form of the 7-carbon alkane, without any impurities from other branched isomers. Therefore, it is preferred for high-precision analytical or laboratory experiments. Since n-heptane is a nonpolar compound, it is used when the analyte is also non-polar. It is sometimes combined in mixtures with slightly more polar compounds in order to extract a wider range of bioactive compounds.

References

Byrne, Fergal P., et al. “Tools and techniques for solvent selection: green solvent selection guides.” Sustainable Chemical Processes 4 (2016): 1-24.

Darme, Pierre et al. “Investigation of Antiparasitic Activity of 10 European Tree Bark Extracts on Toxoplasma gondii and Bioguided Identification of Triterpenes in Alnus glutinosa Barks.” Antimicrobial agents and chemotherapy vol. 66,1 (2022): e0109821. doi:10.1128/AAC.01098-21

Gori, Anthonin et al. “Development of an innovative maceration technique to optimize extraction and phase partition of natural products.” Fitoterapia vol. 148 (2021): 104798. doi:10.1016/j.fitote.2020.104798

Luca, Simon Vlad, et al. “Approach for simultaneous cannabidiol isolation and pesticide removal from hemp extracts with liquid-liquid chromatography.” Industrial Crops and Products 155 (2020): 112726.

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