Steam has been used for a couple hundred years to harvest the mono and sesquiterpenes, but by the time we reach the diterpenes, the boiling points are too high, so solvents are commonly used.
Probably the most common solvents used are the simple unsaturated Alkanes Propane and Butane, with some Pentane, Hexane, and Heptane, as well as the Alkane alcohol Ethanol, and the isomer of Propanol, Isopropanol (2-Propanol, Isopropyl alcohol).
Add the simple Alkane Dimethyl Ether to that mixture and you have something for everyone. Extracting with Dimethyl Ether extraction has gained some popularity in recent years, but has yet to make any serious encroachment in the BHO market for reasons we’ll discuss later.
Here’s a diagram of some of those molecules showing what atoms they contain and their general arrangement.
Alkane Molecules
The simple unsaturated Alkanes are hydrocarbons in a simple straight chain, consisting of carbon atoms single bonded to each other and all other bonds filled with hydrogen atoms.
Since a carbon atom can maintain four bonds, up to three with other carbon atoms, simple alkanes always have twice as many hydrogen atoms as carbon atoms, plus two more for the hydrogen atoms on each end of the chain.
Pentane and above follow the Greek alphabet, and easiest way for my tired old brain to keep track of the names of the simple Alkanes before Pentane, is the pneumonic device Mary Eats Peanut Butter.
That stands for Methl, Ethyl, Propyl, and Butyl, or Methane, Ethane, Propane, and Butane.
Methane (CH4) is one carbon atom surrounded by four hydrogen atoms, Ethane (C2H6) is two carbon atoms and six hydrogen atoms, Propane is C3H8, and Butane is C4H10.
Following the Greek alphabet, C5H12 becomes Pentane, C6H14 becomes Hexane, and C7H16 becomes Heptane. It goes on, but for our purposes, anything heavier than Pentane, with a boiling point of around 36C/97F, can be problematic removing without changing the form of some of the terpenes like carboxylic acids.
Hexane’s boiling point is only 68C/154.4F at sea level and of course boiling points can be dropped using vacuum, but Hexane and Heptane can also be converted into Diones by our livers. 2.5 Hexane Dione is a known carcinogen, so Hexane is considered a Class II solvent by the FDA.
Methane boils at -161.5C/-258.7F and Ethane at -89C/128.2F, so are of little use as a liquid solvent, but Propane doesn’t boil until -42C/-43.6F and Butane at -1C/30.2F at atmospheric pressure, so can be maintained as a liquid at moderate pressure.
With its 36C/97F boiling point, Pentane is a liquid at atmospheric pressure, though it evaporates fast without tight containment.https://www.accessdata.fda.gov/scripts/fdcc/?set=SCOGS
Propane, Butane and Pentane are Class III solvents, with Butane also GRAS, or Generally Regarded As Safe.
https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=184.
https://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/UCM073403.pdf
There are also Alkane alcohols and ethers, which are the same with regard to the addition of one 0xygen molecule, depending on where it is in the chain. In addition, there are isomers of the simple alkane alcohols, such as Isopropyl isomer of simple alkane Propyl alcohol.
In the case of the simple alkane alcohols, the oxygen atom slips between the carbon and the hydrogen atoms on the end of the chain, and ethers are in the middle You will note that isomer Isopropyl, has the oxygen falling in the middle, between a carbon and an hydrogen atom.
All of these solvents do a good job of extracting our target molecules, but there are differences in what else they extract, and how easy they are to remove afterwards.
The three biggest differences in the solvents is their polarity, their boiling points, and how miscible they are in water.
We measure polarity using the solvents dielectric index, with water having a dielectric index about 80. Anything with an index below 15 is considered non polar and greater than 15 is considered polar. In the case of the alcohols and ether, it is the addition of the oxygen atom that raises its polarity.
| NO | SOLVENT | Dielectric Index | Boiling Point | Miscibility with Water | FDA Class |
| 1 | Propane | 1.6 | -42C/-43.6F | 0.0047% | 3 |
| 2 | Butane | 1.4 | -1C/30.2F | 0.0661% | 3 |
| 3 | Pentane | 1.8 | 36C/97F | 0.004% | 3 |
| 4 | Hexane | 2.0 | 69C/156.2F | 0.001% | 2 |
| 5 | Heptane | 1.9 | 96C/204.8F | 0.0003% | 3 |
| 6 | Methanol | 32.7 | 65C/149F | 100% | 2 |
| 7 | Ethanol | 24.5 | 79C/174.2F | 100% | 3 |
| 8 | Isopropanol | 17.9 | 82.6C/180.7F | 100% | 3 |
| 9 | Dimethyl Ether | 7.3 | -24C/-11.2F | 100% | NL (*3?) |
Dielectric Constants
* Not listed. Ethyl Ether Class 3.
Polarity influences what elements are extracted from the prospective that like attracts like, so the more polar molecules like chlorophyll and the anthrocyanin plant pigment/glucosides will attracted to the more polar solvents, and non polar plant waxes will be attracted to the more non polar solvents.
Miscibility with water is important from the stand point of the solvent picking up water and water solubles from the process, and from the stand point that chlorophyll can be transported as micelles in water, so increased water pickup potentially also means more chlorophyll, even though it is technically not water soluble.
Boiling point makes a big difference from the standpoint of being able to purge the residual solvent after extraction, without losing or changing the nature of the mono and sesquiterpenes. Some are in carboxylic acid form, and heat decarboxylates them into their base form.
Other factors to consider in selecting a solvent, are initial cost and recyclability. Some solvents like Ethanol are heavily taxed, adding to the initial cost, and a fine line of legality regarding redistilling it for recycle, and others like Dimethyl Ether are hard on elastomers in the gaskets, as well as valve and pump seals, limiting options.
Cost to Mother Earth brings into focus the cost of exploration, drilling, refining, and distribution of the simple Alkane solvents extracted from Crude oil.
Ethanol and Methanol can be made fermenting biomass, and in addition Methanol can be made from Natural Gas, or any ready supply of Methane.
Several ways to make Dimethyl Ether starting with Methane or Methanol, but it is manufactured, as is Isopropyl alcohol.
There is also the issue of toxicity, which brings us to Federal Drug Administration standards for the different solvents. Here is what the FDA has to say about the Class 2 and 3 solvents above.
Table 2. – Class 2 Solvents in Pharmaceutical Products
Solvents in Class 2 (Table 2) should be limited in pharmaceutical products because of their inherent toxicity. PDEs are given to the nearest 0.1 mg/day, and concentrations are given to the nearest 10 ppm. The stated values do not reflect the necessary analytical precision of determination. Precision should be determined as part of the validation of the method.
Table 3. –Class 3 Solvents in Pharmaceutical Products
Should Be Limited by Good Manufacturing Practices (GMP) or Other Quality-Based Requirements
Solvents in Class 3 (Table 3) may be regarded as less toxic and of lower risk to human health.
Class 3 includes no solvent known as a human health hazard at levels normally accepted in pharmaceuticals. However, there are no long-term toxicity or carcinogenicity studies for many of the solvents in Class 3. Available data indicate that they are less toxic in acute or short -term studies and negative in genotoxicity studies.
It is considered that amounts of these residual solvents of 50 mg per day or less (corresponding to 5,000 ppm or 0.5 percent under Option 1) would be acceptable without justification. Higher amounts may also be acceptable provided they are realistic in relation to manufacturing capability and good manufacturing practice (GMP ).
There is also the issue of FDA GRAS solvents, or Generally Regarded As Safe and of the solvents listed, only three are listed as GRAS for food stuff by the FDA.
| NO | GRAS Solvent | GRAS NO | Class |
| 1 | Butane | 184-1165 | 3 |
| 2 | Ethanol | 184-1293 | 3 |
| 3 | Propane | 184-1655 | 3 |
GRAS solvents
https://www.fda.gov/food/ingredientspackaginglabeling/foodadditivesingredients/ucm091048.htm
So that leaves us looking for a solvent that preferentially extracts what we do want and leaves as much of what we don’t want behind, is easy to remove without damaging fragile carboxylic acids or boiling/evaporating off the monoterpenes, and is minimally toxic.
As it so happens, the three extraction solvents that we regularly use are Butane, Ethanol, and Propane. We use some Pentane and Hexane in alchemy, but not typically for extraction, and don’t use Heptane because it too hard to purge to acceptable levels afterwards, without affecting the end product.
All three are easily recycled, so as to minimize insult to our planet, and Ethanol is readily made through fermentation, with the CO2 byproduct saleable for use in SCFE CO2 extraction, which is covered under that heading.
Butane and Propane are both nonpolar and Ethanol is polar, so extraction techniques are different. Ethanol, water, chlorophyll, and anthocyanin plant pigments/glucosides have a love affair with each other, while Butane and Propane extract very little chlorophyll unless they are full of water, and have limited capacity for water, but readily extract the non-polar plant waxes.
How we deal with those issues, depends on the end product that we desire, and we will handle them under BHO and QWET extraction, but as a general statement, we freeze the plant material and solvent to subzero temperatures and limit contact time.
The goodness we seek is located in the trichomes on the leaves and flowers, not from the insides of the plant, and freezing both ties up the free water and locks in the water solubles in the plant interior.
It also slows down the dissolution process, but slows down extraction of the longer C-30 size molecules more than the C-10 through C-22 target molecules.
With regard to solvent quality, they aren’t created equal, so let’s talk about selection.
Ethanol is perhaps easier, in that 95.6% azeotropic 190 proof Everclear/Clear Srings or 200 proof reagent grades will both work and are useable right off the shelf.
Higher water content mixtures work somewhat, but extract more water solubles and then we still have to purge the extra water, so I advise against it.
LPG extraction has a few more variables, including those related to constituents within the LPG longer chained and heavier than Propane or Butane. The level of those contaminants can vary considerably, depending on the source/grade and are commonly referred to as Mystery Oil.
During recovery, these heavier constituents are concentrated in the product recovered, and can’t be readily removed at that point, so best to remove Mystery Oil before hand.
As it can be removed by fractional-distillation and filtration, lesser grades of LPG can be used in an emergency, but we recommend and personally use a minimum of 99.5% Instrument Grade , and fractionally distill it anyway. 99.9% Research Grade is also available, but at notably higher price.
The 99.5% Praxair Instrument grade we’ve tested, has typically measured 99.9% purity with regard to molecules heavier than C-4, and after fractionally distilling and filtration using a VICI Metronics molecular sieve filter, it measured >99.99% purity afterwards.
We always fractionally distill new LPG regardless of quality certifications and always remove some, even if it is just a film on the inside of the trap.