Explain the following with an example.
(i)Kolbe’s reaction.
(ii)Reimer-Tiemann reaction.
(iii)Williamson ether synthesis.
(iv) Unsymmetrical ether.
Explain the following with an example.
(i)Kolbe’s reaction.
(ii)Reimer-Tiemann reaction.
(iii)Williamson ether synthesis.
(iv) Unsymmetrical ether.
Solution and Explanation
(i)Kolbe's reaction: When phenol is treated with sodium hydroxide, sodium phenoxide is produced. This sodium phenoxide when treated with carbon dioxide, followed by acidification, undergoes electrophilic substitution to give ortho-hydroxybenzoic acid as the main product. This reaction is known as Kolbe's reaction.
(ii)Reimer-Tiemann reaction: When phenol is treated with chloroform(CHCl3)in the presence of sodium hydroxide, a -CHO group is introduced at the ortho position of the benzene ring. This reaction is known as the Reimer-Tiemann reaction. The intermediate is hydrolyzed in the presence of alkalis to produce salicylaldehyde.
(iii)Williamson ether synthesis: Williamson ether synthesis is a laboratory method to prepare symmetrical and unsymmetrical ethers by allowing alkyl halides to react with sodium alkoxides. This reaction involves an SN2 attack of the alkoxide ion on the alkyl halide. Better results are obtained in the case of primary alkyl halides. If the alkyl halide is secondary or tertiary, then elimination competes over substitution.
(iv) Unsymmetrical ether: An unsymmetrical ether is an ether where two groups on the two sides of an oxygen atom differ (i.e., have an unequal number of carbon atoms). For example: ethyl methyl ether (CH3-O-CH2CH3)
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Notes on Alcohols, Phenols And Ethers
Concepts Used:
Preparation - Alcohols, Phenols and Ethers
Alcohols, phenols, and ethers are organic compounds that can be prepared by various methods.
Preparation of Alcohols:
- Direct hydration of alkenes: Alcohols can be prepared by the addition of water to an alkene in the presence of a strong acid catalyst.
- Reduction of carbonyl compounds: Alcohols can be prepared by the reduction of aldehydes, ketones, or carboxylic acids using reducing agents like NaBH4 or LiAlH4.
- Grignard reaction: Alcohols can be prepared by reacting Grignard reagents with carbonyl compounds.
- Hydroboration-oxidation: Alcohols can be prepared by the hydroboration of alkenes followed by oxidation with an oxidizing agent like H2O2.
Preparation of Phenols:
- Hydrolysis of diazonium salts: Phenols can be prepared by the hydrolysis of diazonium salts, which are formed by the reaction of aniline with nitrous acid.
- Oxidation of sulfonic acids: Phenols can be prepared by the oxidation of sulfonic acids using strong oxidizing agents like potassium permanganate or chromic acid.
Preparation of Ethers:
- Williamson synthesis: Ethers can be prepared by the reaction of an alkoxide ion with a primary alkyl halide or tosylate in the presence of a strong base like NaOH or KOH.
- Dehydration of alcohols: Ethers can be prepared by the dehydration of alcohols in the presence of a strong acid catalyst like H2SO4.
In summary, alcohols, phenols, and ethers can be prepared by a variety of methods, including hydration, reduction, Grignard reaction, hydroboration-oxidation, hydrolysis, oxidation, Williamson synthesis, and dehydration. The choice of the method depends on the availability of starting materials, the desired product, and the conditions of the reaction.