Phenol are the organic compounds containing benzene ring bonded to a hydroxyl group. They are also known as carbolic acids. They are weak acids and generally form phenoxide ions by losing one positive hydrogen ion (H+) from hydroxyl group. In earlier days, phenol were primarily synthesized from coal tar. Nowadays, with advancements in technologies, certain new methods have come up for the preparation of phenols in laboratories. In laboratories, phenol is primarily synthesized from benzene derivatives. Some of the methods of preparation of phenols are explained below:
Preparation of phenols from haloarenes: Chlorobenzene is an example of haloarenes which is formed by mono substitution of benzene ring. When chlorobenzene is fused with sodium hydroxide at 623K and 320 atm sodium phenoxide is produced. Finally, sodium phenoxide on acidification gives phenols.
Preparation of phenols from benzene sulphonic acid: Benzenesulphonic acid can be obtained from benzene by reacting it with oleum. Benzenesulphonic acid thus formed is treated with molten sodium hydroxide at high temperature which leads to the formation of sodium phenoxide. Finally, sodium phenoxide on acidification gives phenols.
Preparation of phenols from diazonium salts: When an aromatic primary amine is treated with nitrous (NaNO2 + HCl) acid at 273 – 278 K, diazonium salts are obtained. These diazonium salts are highly reactive in nature. Upon warming with water, these diazonium salts finally hydrolyse to phenols. Phenols can also be obtained from diazonium salts by treating it with dilute acids.
Preparation of phenols from cumene: Cumene is an organic compound obtained by Friedel-Crafts alkylation of benzene with propylene. Upon oxidation of cumene (isopropylbenzene) in presence of air, cumene hydroperoxide is obtained. Upon further treatment of cumene hydroperoxide with dilute acid phenols are obtained. Acetone is also produced as one of the by-products of this reaction in large quantities. Hence, phenols prepared by these methods need purifications.
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