4-Hydroxybenzaldehyde derivatives

Vanillin is an organic compound with the molecular formula . It is a phenolic aldehyde. Its functional groups include aldehyde, hydroxyl, and ether. It is the primary component of the ethanolic extract of the vanilla bean. Synthetic vanillin is now used more often than natural vanilla extract as a flavoring in foods, beverages, and pharmaceuticals.

thumb|right|class=skin-invert-image|UV visible spectrum of quercetin, with lambda max at 369 nm

Ethylvanillin is the organic compound with the formula (C2H5O)(HO)C6H3CHO. This colorless solid consists of a benzene ring with hydroxyl, ethoxy, and formyl groups on the 4, 3, and 1 positions, respectively. It is a homologue of vanillin, differing on the 3 position.

4Hydroxy­benzaldehyde (parahydroxy­benzaldehyde) is an organic compound with the formula . Along with 2-hydroxybenzaldehyde and 3-hydroxybenzaldehyde, it is one of the three isomers of hydroxybenzaldehyde.

Syringaldehyde is an organic compound that occurs in trace amounts widely in nature. Some species of insects use syringaldehyde in their chemical communication systems. Scolytus multistriatus uses it as a signal to find a host tree during oviposition.

3,4-Dihydroxybenzaldehyde, also known as protocatechuic aldehyde, is a phenolic aldehyde, a compound released from cork stoppers into wine. It is an isomer of 2,4-dihydroxybenzaldehyde.