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Effects of substituents on the acidity of phenols: Existence of electron attracting group, (for example-NO2, -X, - NR, -CN, -CHO, -COOH) on the benzene ring increases the acidity of phenol since it enables the ring to draw much more electrons from the phenoxy oxygen and so releasing easily the proton.
Polyphenols have poor bioavailability, indicating that most of what are consumed are extensively metabolized and excreted. Gallic acid and isoflavones may show absorption of about 5%, with amounts of catechins (flavan-3-ols), flavanones, and quercetin glucosides even less. The least well-absorbed phenols are the proanthocyanidins, galloylated tea catechins, and anthocyanins.