Notes On Structure Of Functional Groups And Preparation Of Alcohols And Phenols - CBSE Class 12 Chemistry
Structure of hydroxyl group in alcohols: The oxygen is sp3 hybridised. It forms a sigma bond, with a sp3 hybrid orbital of carbon. Phenol, which is a planar molecule, also has a hydroxyl group, but the oxygen forms a sigma bond with a sp2 hybridised carbon. Methoxy methane is typical ether, with a bridging oxygen group. The four electron pairs around the oxygen— two bond pairs and two lone pairs—have an approximately tetrahedral arrangement. Preparation of alcohols: From alkenes: By acid-catalysed hydration: An alkene reacts with water in acidic conditions to form an alcohol. If the alkene is unsymmetrical — then two possible alcohol products could form. To determine which alcohol will be the major product of the reaction Markovnikov's rule is used. The hydrogen will add to the carbon with more hydrogen atoms bonded to it. The hydroxyl group adds to the carbon with fewer hydrogen atoms bonded to it. . Mechanism of acid-catalysed hydration of alkenes: By the hydroboration-oxidation of alkenes: This can be summarised as a two-step process The addition will takes place according to anti-Markovnikov hydration of the alkene. In contrast to acid-catalysed hydration, hydroboration-oxidation results in the hydroxyl group attaching to the carbon with more hydrogen substituents. By the reduction of aldehydes and ketones: This involves the addition of hydrogen gas in the presence of a suitable metal catalyst, such as platinum, palladium or nickel &reducing agents such as sodium borohydride or lithium aluminium hydride. Reduction of carboxylic acids using lithium aluminium hydride: Carboxylic acids can be reduced to primary alcohols. The carboxylic acid is converted into an ester, which is then reduced to alcohol By using Grignard reagent: Grignard reagents can be reacted with aldehydes and ketones to make alcohols in a two-step process. Step-1: Involves nucleophilic addition of the Grignard reagent to the carbonyl group to form an alkoxymagnesium halide adduct. Step-2: Adduct is hydrolysed to form an alcohol. Preparation of Phenol or carbolic acid: It can be synthetically prepared from benzene derivatives From haloarenes: Chlorobenzene is reacted with sodium hydroxide to form sodium phenoxide. Then, the sodium phenoxide is acidified to form phenol. From benzene-sulphonic acid: Benzene reacts with hot concentrated sulphuric acid form benzene-sulphonic acid. the benzene-sulphonic acid that results is treated with molten sodium hydroxide and subsequently acidified to form phenol. From diazonium salts: An aromatic primary amine is reacted with nitrous acid (from NaNO2 and HCl) to form a diazonium salt. Then, the diazonium salt is hydrolysed with warm water or dilute acid to phenol. From cumene, or isopropyl benzene: Cumene is oxidised in air to form cumene hydroperoxide. This is then treated with dilute acid to form a phenol.

#### Summary

Structure of hydroxyl group in alcohols: The oxygen is sp3 hybridised. It forms a sigma bond, with a sp3 hybrid orbital of carbon. Phenol, which is a planar molecule, also has a hydroxyl group, but the oxygen forms a sigma bond with a sp2 hybridised carbon. Methoxy methane is typical ether, with a bridging oxygen group. The four electron pairs around the oxygen— two bond pairs and two lone pairs—have an approximately tetrahedral arrangement. Preparation of alcohols: From alkenes: By acid-catalysed hydration: An alkene reacts with water in acidic conditions to form an alcohol. If the alkene is unsymmetrical — then two possible alcohol products could form. To determine which alcohol will be the major product of the reaction Markovnikov's rule is used. The hydrogen will add to the carbon with more hydrogen atoms bonded to it. The hydroxyl group adds to the carbon with fewer hydrogen atoms bonded to it. . Mechanism of acid-catalysed hydration of alkenes: By the hydroboration-oxidation of alkenes: This can be summarised as a two-step process The addition will takes place according to anti-Markovnikov hydration of the alkene. In contrast to acid-catalysed hydration, hydroboration-oxidation results in the hydroxyl group attaching to the carbon with more hydrogen substituents. By the reduction of aldehydes and ketones: This involves the addition of hydrogen gas in the presence of a suitable metal catalyst, such as platinum, palladium or nickel &reducing agents such as sodium borohydride or lithium aluminium hydride. Reduction of carboxylic acids using lithium aluminium hydride: Carboxylic acids can be reduced to primary alcohols. The carboxylic acid is converted into an ester, which is then reduced to alcohol By using Grignard reagent: Grignard reagents can be reacted with aldehydes and ketones to make alcohols in a two-step process. Step-1: Involves nucleophilic addition of the Grignard reagent to the carbonyl group to form an alkoxymagnesium halide adduct. Step-2: Adduct is hydrolysed to form an alcohol. Preparation of Phenol or carbolic acid: It can be synthetically prepared from benzene derivatives From haloarenes: Chlorobenzene is reacted with sodium hydroxide to form sodium phenoxide. Then, the sodium phenoxide is acidified to form phenol. From benzene-sulphonic acid: Benzene reacts with hot concentrated sulphuric acid form benzene-sulphonic acid. the benzene-sulphonic acid that results is treated with molten sodium hydroxide and subsequently acidified to form phenol. From diazonium salts: An aromatic primary amine is reacted with nitrous acid (from NaNO2 and HCl) to form a diazonium salt. Then, the diazonium salt is hydrolysed with warm water or dilute acid to phenol. From cumene, or isopropyl benzene: Cumene is oxidised in air to form cumene hydroperoxide. This is then treated with dilute acid to form a phenol.

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