The rising popularity of D8 (delta-8-tetrahydrocannabinol) is unmatched, so much so that it even earned its own subreddit in 2020. Just recently, the US Hemp Round Table muddled the conversation by urging cautious optimism for D8; the DEA might soon be cracking down on this “legal-psychoactive” hemp compound.
To grasp how and why D8 is a legal grey area, it is important to have a clear understanding about its chemical make-up and how it compares to that of THC (delta-9-tetrahydrocannabinol); the major psychoactive compound in cannabis.
D8 and THC have a similar chemical structure. Both compounds are made up of carbon, oxygen, and hydrogen atoms. Precisely, they both have 21 carbons, 31 hydrogens, and 2 oxygen atoms.  It can also be said that Delta-9-THC (C₂₁H₃₀O₂) has many isomers and D8 is one of them.
The major difference between delta-8-THC and delta-9-THC is the placement of one double bond. In delta 8, the double bond is located at the eight carbon chain while in delta-9, the double bond is located at the ninth carbon chain.  That makes, delta-8 and delta-9 precisely.
However, this slight difference contributes to significant differences in the properties of the two compounds.
The chemical structure of THC allows it to have a high affinity for CB1 (G-coupled) receptors of the endocannabinoid system which are primarily located in the brain and central nervous system.  At this receptor, THC is a strong agonist due to the double bond it has on the ninth carbon chain.
Because of the difference in the location of one of the carbon-carbon double bonds, D8 has half the potency of THC. D8 binds weakly to the CB1 receptors.
However, this difference makes D8 more stable than THC. It also has a prolonged shelf life and is more resistant to oxidation. THC on the other hand is quickly transformed to cannabinol when exposed to higher temperatures. 
By binding to the G-protein coupled CB1 receptor, both compounds are able to induce intense happiness and euphoria. Both compounds are also lipophilic and viscous at room temperature.
It seems like cannabidiol (CBD) is not the only cannabinoid that is trying to oust THC from its pedestal, Delta-8 is equally in the race for the top seat.
- National Center for Biotechnology Information (2021). PubChem Compound Summary for CID 2978, delta9-Tetrahydrocannabinol. Retrieved March 27, 2021 from https://pubchem.ncbi.nlm.nih.gov/compound/delta9-Tetrahydrocannabinol.
- National Center for Biotechnology Information (2021). PubChem Compound Summary for CID 2977, delta8-THC. Retrieved March 27, 2021 from https://pubchem.ncbi.nlm.nih.gov/compound/delta8-THC.
- Reggio P. H. (2010). Endocannabinoid binding to the cannabinoid receptors: what is known and what remains unknown. Current medicinal chemistry, 17(14), 1468–1486.
- Repka, M. A., ElSohly, M. A., Munjal, M., & Ross, S. A. (2006). Temperature stability and bioadhesive properties of delta9-tetrahydrocannabinol incorporated hydroxypropylcellulose polymer matrix systems. Drug development and industrial pharmacy, 32(1), 21–32.