Cannabidiol (CBD):  By-product of the Non-Enzymatic Decarboxylation of the Synthesis of the Product of ‘C-dominant’ variety of Cannabis Sativa.L (Linnaeus)

‘C-dominant’ variety of Cannabis Sativa.L produces two compounds:

Cannabigerolic Acid (CBGA)

Cannabidiolic Acid Synthase (CBDA-Synthase / CBDAS/CBDA-S)

During Post-Harvest Curing, these two products Synthesize:

Cannabidiolic Acid (CBDA)

When CBDA is heated to above 182.5C the Carboxyl Group is released through Non-Enzymatic Decarboxylation to Synthesize;

Cannabidiol (CBD)

Source: http://hempassociation.org/sop/