The first chemical studies on Cannabis plant started at the beginning of the twentieth century when Frankel succeeded in isolating Cannabinol from the plant and the Australian chemist Max Czerkis managed to study the chemical properties of this substance in 1907. Later on, in 1940 another constituent was isolated and named Cannabidiol. However, both compounds failed to induce the same actions of Cannabis extract. This finding led to discovery of the active component tetrahydrocannabinol in 1960. Indeed, Cannabinol was not found in fresh Cannabis material and is believed to be a decomposition product of the active constituent tetrahydrocannabinol. Tetrahydrocannabinol was first decarboxylated to give tetrahydrocannabivarin that was converted by air oxidation to Cannabinol.

In 1964 Mechoulam and his associates succeeded in characterizing the active constituent of Cannabis as (-)-Δ⁹-transtetrahydrocannabinol (Δ⁹THC) and managed to synthesize it chemically. This synthetic material is known in Trade as Hashish oil. Δ⁹-THC is also known as Δ^{1 (2)}-THC (due to a different chemical numbering system considering it as a monoterpene derivative).

(-)- Trans Δ⁹-THC is resinous oil with pKa value of 10.6. It is very sensitive to heat and light and oxidizes rapidly to Cannabinol. It is adsorbed by glass or plastic surfaces. It can isomerize to Δ⁸-THC on exposure to air. It is very sensitive to acids since at PHs<4 it degrades to Cannabidiol and iso-cannabidiol due to ether cleavage. At pHs>4 but <7, it degrades to Δ⁸-THC and 9-hydroxyhexahydrocannabinol. The substance may be solubilized using ethanol, plasma, polyethylene glycol, sesame oil or olive oil.

Hashish active constituents can be easily extracted using petroleum ether, hexane or methanol and later purified using chromatography.

The percentage of the active constituent (-) trans- Δ⁹-THC in cannabis varies depending upon the plant tested and the origin of the plant. Generally, the percentage of Δ⁹-THC varies from 0.3-3.4% w/w in the flowers and 5-12% in the resin. The content of cannabinol and cannabidiol extracts is very low 0.15-0.17% w/w. Besides Δ⁹-THC, cannabinol and cannabidiol, hashish contains various other cannabinoids and non-cannabinoid materials. These substances include: Cannabiglendol, Cannabiterol, Cannabivarin, Cannabidiolic acid, Cannabigerol, 3, 5, 4-trihydroxybibenzyl Canabioxoic acid, Canabichromevarin and Cannabigerovarin and 3, 4-dihydroxy-5-methoxybibenzyl. Figure 1 shows the chemical structures of the most important cannabis or hashish constituents.

Figure 1: Major Cannabis Constituents

Simple Chemical Tests for Detection of Cannabinoids

1. Addition of 2% alcoholic KOH to cannabis extract gives a purple violet color.

2. To cannabis material add vanillin in ethanol containing acetaldehyde. A blue-purple color is produced. This is known as Duquenois-Levine Test. However, a positive test is given by some essential oils e.g. clove, spearmint, chenopodium, mentha arvensis and patchouli.