Their common feature is the presence of at least one hydroxyl-substituted aromatic ring system. Most phenolic compounds belong to the flavonoids . Lignin , the primary substance of wood, is most frequent. The following table outlines the most important classes. The starting product of the biosynthesis of most phenolic compounds is shikimate .
Phenols are acidic due to the dissociability of their -OH group.
They are rather reactive compounds and as long as no steric inhibition due to additional side chains occurs do they form hydrogen bonds. Consequently have many flavonoids intramolecular bonds. Another important feature is their ability to form chelate complexes with metals. The darkening of cut or dying plant parts is caused by this reaction. They have usually an inhibiting effect on plant growth.