Example 7.2

Example . (a) (b) Phenol, also known as carbolic acid, was first isolated in the early nineteenth century from coal tar. Nowadays, phenol is commercially produced synthetically. In the laboratory, phenols are prepared from benzene derivatives by any of the following methods: . . Preparation of Phenols The reaction of Grignard reagents with methanal produces a primary alcohol, with other aldehydes, secondary alcohols and with ketones, tertiary alcohols. (c) . From haloarenes Chlorobenzene is fused with NaOH at 623K and atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced (Unit , Class XII). . From benzenesulphonic acid Benzene is sulphonated with oleum and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol. . From diazonium salts A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (NaNO + HCl) at - K. Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids (Unit , Class XII). H O NH NaNO +HCl Aniline N Cl N + HCl + Benzene diazonium chloride Warm + – . From cumene Phenol is manufactured from the hydrocarbon, cumene. Cumene (isopropylbenzene) is oxidised in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. Acetone, a by-product of this reaction, is also obtained in large quantities by this method. Most of the worldwide production of phenol is from cumene. Alcohols and phenols consist of two parts, an alkyl/aryl group and a hydroxyl group. The properties of alcohols and phenols are chiefly due to the hydroxyl group. The nature of alkyl and aryl groups simply modify these properties. Boiling Points The boiling points of alcohols and phenols increase with increase in the number of carbon atoms (increase in van der Waals forces). In alcohols, the boiling points decrease with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area). The –OH group in alcohols and