Decarboxylation is an important step in the processing of cannabis. It’s the removal of CO2 from acidic cannabinoids, such as CBDA or THCA, to convert them to their neutral forms, CBD and THC. It’s helpful to “decarb” cannabis prior to consumption because it changes the biological activity. THCA is not psychoactive, for example, but decarboxylated THC is. I’ve heard a few repeated anecdotes from fellow extraction chemists lately. They observe widely variable times and temperatures needed to decarb cannabis extract crude oil, so I wanted to unpack that here. It turns out, water plays a crucial role in the decarboxylation of cannabis. If you’re unfamiliar with decarboxylation, you can read about various in-home decarb techniques in our previous blog post here.
Acidic cannabinoids like THCA and CBDA are produced by the flower naturally. Cannabis flower is typically grown, trimmed, and ‘cured’ before hitting the shelf. Curing is the process of decarboxylating cannabis by hanging it in a humidity-controlled environment. Curing does not typically result in full decarboxylation, so you may notice both THCA and THC are reported on product labels. When the flower/extract is smoked/vaporized, the temperature gets hot enough to force decarboxylation to completion. In other words, lighting up cannabis flower converts THCA to THC, making it psychoactive. This actually degrades the cannabinoids and terpenes, making smoking the least potent form of consumption.
In the post processing of cannabis, we can stop at “cured” flower. Alternatively, we can make distillates, isolates, edibles, tinctures, etc. from extracted cured flower (or even uncured flower, in the case of live resin). With respect to cannabis extracts, decarboxylation must be done manually with heat. This can be done at different stages of crude extract purification, depending on the extraction method used (solventless, CO2, butane, ethanol, etc.). It’s typical to decarboxylate cannabis flower before a CO2 extraction, for example, but it may be one of the last things you do with an ethanol extract.
Decarboxylation mechanism without water
Let’s discuss decarboxylation now from an in-depth chemistry point of view. As I mentioned previously, decarboxylation is the elimination of CO2. Let’s see what that looks like exactly in a worked example with THCA:
The carboxylic acid group (-COOH highlighted in blue) is what makes the THCA acidic (Tetrahydro Cannabinoilic-Acid). The carboxylic acid group is also where the CO2 comes from in the decarboxylation of THCA to THC (Tetrahyrdo Cannabinol) pictured above.
Decarboxylation mechanism assisted by water
Carboxylic acid groups are considered “polar” and have a similar polarity to water. You may remember that “like dissolves like” from Freshman chemistry, if you subjected yourself to such torture. The water molecules being similar in polarity to the carboxylic acid group will gravitate towards -COOH like tiny magnets. This changes the exact mechanism of hydrogen transfer during decarboxylation. An example of this is pictured in the image below:
Here the letter R can represent the structure of any cannabinoid, and the affected transition state of the reaction is in brackets. Water assists in the movement of electrons and hydrogen atoms, allowing decarboxylation to take place with more ease. In this case, the hydrogen bonding of the water molecules assists in the hydrogen transfer from the carboxylic acid group to the cannabinoid.
In even nerdier chemistry speak (if that’s possible), it does so by driving down the activation energy of the process. This also enhances the speed or rate of reaction. In the decarboxylation of the simplest carboxylic acid, formic acid HCOOH, the activation energy of decarboxylation goes down with each additional water molecule. Chen et. al. calculated this in both the gas and aqueous (liquid) phases for up to 3 water molecules. They found that one molecule of water bound to the carboxylic acid group will speed things up. Three molecules of water bound to the carboxylic acid group will speed up decarboxylation even more.
Curing and extract decarboxylation will be affected by water
This could explain why humidity is so important in the “curing” of cannabis flower. It could also explain the wide array of times and temperatures folks have seen for decarboxylation of extract. The water content of crude cannabis extract will vary widely, depending on the relative humidity of the starting plant material. The water content will help drive the chemistry forward to a certain extent, likely until it hits a wall where water no longer helps.
I would love to hear your thoughts on decarboxylation time and temperature variability! Tell me about your experiences in the comments below.