Lecture notes Introduction to Phenols

Phenols are aromatic compounds in which a hydroxyl group is directly attached to a benzene ring, and this unique linkage makes them behave very differently from alcohols. They can be prepared by several methods such as from diazonium salts by hydrolysis, from benzene sulfonic acid by fusion with sodium hydroxide, by Dow’s process from chlorobenzene, and industrially by the cumene process. Phenols are weakly acidic because the phenoxide ion formed after loss of hydrogen is stabilized by resonance across the aromatic ring; this acidity increases in the presence of electron-withdrawing groups and decreases with electron-releasing groups. The –OH group activates the ring and directs electrophilic substitution to ortho and para positions, leading to important reactions like halogenation (forming tribromophenol), nitration (forming nitrophenols and picric acid), nitrosation, and sulfonation. Phenols have wide applications as disinfectants, in the manufacture of plastics, dyes, drugs, and resins, and they give characteristic qualitative tests such as the ferric chloride test (violet coloration), bromine water test (white precipitate), and Liebermann’s test. Overall, the preparation, acidity, reactions, derivatives, uses, and tests of phenols are all governed by the resonance interaction between the hydroxyl group and the benzene ring.