the group of atoms responsible for the characteristic reactions of a compound. The functional group is –OH for alcohols, –CHO for aldehydes, –COOH for carboxylic acids, etc., an atom, or a group of atoms that has similar chemical properties whenever it occurs in different compounds. It defines the characteristic physical and chemical properties of families of organic compounds. ;, orgaanilise ühendi molekuli osa (aatom või aatomirühm), mis määrab ühendi kuulumise mingisse kindlasse klassi ning millest sõltuvad tema keemilised omadused (nt hüdroksüülrühm)