Alcohols - Reactions replacing OH group

  • Alcohols have an -OH group which can be replaced by different reagents.
  • These reactions help in synthesizing a wide range of compounds.
  • The reaction mechanism involves nucleophilic substitution or elimination reactions.
  • Let’s explore some of the reactions that replace the -OH group in alcohols.

1. Acidic dehydration

  • In the presence of a strong acid catalyst like concentrated sulfuric acid,
  • Alcohols undergo dehydration to form alkenes.
  • For example: CH3CH2OH (Ethanol) can be dehydrated to form ethene (CH2=CH2).

2. Oxidation to aldehydes

  • Primary alcohols can be oxidized to aldehydes.
  • This reaction is carried out using mild oxidizing agents like PCC (pyridinium chlorochromate).
  • For example: CH3CH2OH (Ethanol) can be oxidized to ethanal (CH3CHO).

3. Oxidation to carboxylic acids

  • Primary alcohols can be further oxidized to carboxylic acids.
  • Strong oxidizing agents like KMnO4 or CrO3 are used for this reaction.
  • For example: CH3CH2OH (Ethanol) can be oxidized to ethanoic acid (CH3COOH).

4. Dehydrogenation to ketones

  • Secondary alcohols undergo dehydrogenation to form ketones.
  • Oxidizing agents like CrO3 can be used for this reaction.
  • For example: CH3CHOHCH3 (2-Propanol) can be dehydrogenated to form propanone (CH3COCH3).

5. Formation of alkyl halides

  • Alcohols can react with hydrogen halides (HCl, HBr, HI) to form alkyl halides.
  • In this reaction, the -OH group is replaced by the halogen atom (Cl, Br, I).
  • For example: CH3CH2OH (Ethanol) can react with HCl to form ethyl chloride (CH3CH2Cl).

6. Reaction with thionyl chloride

  • Thionyl chloride (SOCl2) can be used to convert alcohols into alkyl chlorides.
  • This reaction is commonly known as the Swern oxidation.
  • For example: CH3CH2OH (Ethanol) can react with SOCl2 to form ethyl chloride (CH3CH2Cl).

7. Formation of ethers

  • Alcohols can undergo nucleophilic substitution reactions to form ethers.
  • This reaction is often carried out in the presence of acid catalysts like sulfuric acid.
  • For example: CH3CH2OH (Ethanol) can react with CH3CH2OH (Ethanol) to form diethyl ether (CH3CH2OCH2CH3).

8. Williamson ether synthesis

  • Williamson ether synthesis is a method to prepare ethers by reacting alkoxide ions with alkyl halides.
  • This reaction is often carried out in a polar solvent like dimethyl sulfoxide (DMSO).
  • For example: CH3O^- (methoxide ion) can react with CH3CH2Cl (ethyl chloride) to form CH3OCH2CH3 (diethyl ether).

9. Ether formation via dehydration

  • Alcohols can be dehydrated using concentrated sulfuric acid to form ethers.
  • This reaction is commonly known as the Williamson ether synthesis.
  • For example: CH3CH2OH (Ethanol) can be dehydrated to form ethyl ether (CH3CH2OCH2CH3).

10. Reaction with phosphorus halides

  • Alcohols can react with phosphorus halides (PCl3, PCl5) to form alkyl halides.
  • This reaction is known as the Hunsdiecker reaction.
  • For example: CH3CH2OH (Ethanol) can react with PCl5 to form ethyl chloride (CH3CH2Cl).
  1. Acidic dehydration
  • Alcohols can undergo acidic dehydration to form alkenes.
  • Strong acid catalysts like concentrated sulfuric acid or phosphoric acid are used.
  • The -OH group is replaced by a double bond.
  • For example, ethanol (CH3CH2OH) can be dehydrated to form ethene (CH2=CH2).
  • The reaction mechanism involves protonation of the -OH group followed by elimination of water.
  1. Oxidation to aldehydes
  • Primary alcohols can be oxidized to aldehydes.
  • This reaction is carried out using mild oxidizing agents like pyridinium chlorochromate (PCC).
  • For example, ethanol (CH3CH2OH) can be oxidized to ethanal (CH3CHO).
  • The reaction mechanism involves the conversion of the -OH group to a carbonyl group.
  1. Oxidation to carboxylic acids
  • Primary alcohols can be further oxidized to carboxylic acids.
  • Strong oxidizing agents like potassium permanganate (KMnO4) or chromium trioxide (CrO3) are used.
  • For example, ethanol (CH3CH2OH) can be oxidized to ethanoic acid (CH3COOH).
  • The reaction mechanism involves the formation of an aldehyde intermediate, followed by further oxidation to a carboxylic acid.
  1. Dehydrogenation to ketones
  • Secondary alcohols can undergo dehydrogenation to form ketones.
  • Oxidizing agents like chromium trioxide (CrO3) are commonly used.
  • For example, 2-propanol (CH3CHOHCH3) can be dehydrogenated to form propanone (CH3COCH3).
  • The reaction mechanism involves the loss of two hydrogen atoms from the alcohol molecule.
  1. Formation of alkyl halides
  • Alcohols can react with hydrogen halides (HCl, HBr, HI) to form alkyl halides.
  • The -OH group is replaced by a halogen atom (Cl, Br, I) during this reaction.
  • For example, ethanol (CH3CH2OH) can react with HCl to form ethyl chloride (CH3CH2Cl).
  • The reaction mechanism involves the nucleophilic attack of the halide ion on the carbon atom bonded to the -OH group.
  1. Reaction with thionyl chloride
  • Thionyl chloride (SOCl2) can be used to convert alcohols into alkyl chlorides.
  • This reaction is commonly known as the Swern oxidation.
  • For example, ethanol (CH3CH2OH) can react with SOCl2 to form ethyl chloride (CH3CH2Cl).
  • The reaction mechanism involves the formation of a sulfite intermediate, followed by nucleophilic substitution.
  1. Formation of ethers
  • Alcohols can undergo nucleophilic substitution reactions to form ethers.
  • This reaction is often carried out in the presence of acid catalysts like sulfuric acid.
  • For example, ethanol (CH3CH2OH) can react with ethanol (CH3CH2OH) to form diethyl ether (CH3CH2OCH2CH3).
  • The reaction mechanism involves the formation of a protonated alcohol intermediate, followed by nucleophilic attack of the alkoxide ion.
  1. Williamson ether synthesis
  • Williamson ether synthesis is a method to prepare ethers by reacting alkoxide ions with alkyl halides.
  • This reaction is often carried out in a polar solvent like dimethyl sulfoxide (DMSO).
  • For example, methoxide ion (CH3O-) can react with ethyl chloride (CH3CH2Cl) to form diethyl ether (CH3OCH2CH3).
  • The reaction mechanism involves the nucleophilic substitution of the halide ion by the alkoxide ion.
  1. Ether formation via dehydration
  • Alcohols can be dehydrated using concentrated sulfuric acid to form ethers.
  • This reaction is commonly known as the Williamson ether synthesis.
  • For example, ethanol (CH3CH2OH) can be dehydrated to form ethyl ether (CH3CH2OCH2CH3).
  • The reaction mechanism involves protonation of the alcohol molecule followed by nucleophilic substitution.
  1. Reaction with phosphorus halides
  • Alcohols can react with phosphorus halides (PCl3, PCl5) to form alkyl halides.
  • This reaction is known as the Hunsdiecker reaction.
  • For example, ethanol (CH3CH2OH) can react with PCl5 to form ethyl chloride (CH3CH2Cl).
  • The reaction mechanism involves the formation of an intermediate ester, followed by nucleophilic substitution.

21. Formation of esters

  • Alcohols can react with carboxylic acids to form esters.
  • This reaction is known as esterification.
  • An acid catalyst like concentrated sulfuric acid is typically used.
  • For example, ethanol (CH3CH2OH) can react with acetic acid (CH3COOH) to form ethyl acetate (CH3COOCH2CH3).
  • The reaction mechanism involves the nucleophilic attack of the -OH group on the carbonyl carbon of the carboxylic acid.

22. Reaction with acid halides

  • Alcohols can react with acid halides to form esters.
  • This reaction is similar to esterification but uses acid halides instead of carboxylic acids.
  • For example, ethanol (CH3CH2OH) can react with acetyl chloride (CH3COCl) to form ethyl acetate (CH3COOCH2CH3).
  • The reaction mechanism involves the nucleophilic substitution of the halide ion by the alkoxide ion.

23. Formation of carbonates

  • Primary and secondary alcohols can react with carbon dioxide to form carbonates.
  • This reaction is known as carbonate formation.
  • For example, ethanol (CH3CH2OH) can react with carbon dioxide (CO2) to form ethyl carbonate (CH3CH2OCO2CH2CH3).
  • The reaction mechanism involves the nucleophilic attack of the -OH group on the carbonyl carbon of carbon dioxide.

24. Reduction to alkanes

  • Alcohols can be reduced to alkanes using reducing agents like lithium aluminum hydride (LiAlH4).
  • This reaction is known as alcohol reduction.
  • For example, ethanol (CH3CH2OH) can be reduced to ethane (CH3CH3).
  • The reaction mechanism involves the hydride ion (H-) attacking the carbon atom bonded to the -OH group.

25. Dehydration to alkenes using phosphoric acid

  • Alcohols can undergo dehydration to form alkenes using phosphoric acid (H3PO4).
  • This reaction is similar to acidic dehydration but uses phosphoric acid as the catalyst.
  • For example, ethanol (CH3CH2OH) can be dehydrated to form ethene (CH2=CH2).
  • The reaction mechanism involves protonation of the -OH group followed by elimination of water.

26. Smiles rearrangement

  • Alcohols can undergo rearrangement reactions to form different compounds.
  • Smiles rearrangement is a common example of this reaction.
  • For example, 2-pentanol can undergo Smiles rearrangement to form 3-pentanol.
  • The reaction mechanism involves the migration of a hydrogen atom from one carbon atom to an adjacent carbon atom.

27. Reaction with peroxides

  • Alcohols can react with peroxides to form alkoxyl radicals.
  • This reaction is known as peroxyacid formation.
  • For example, ethanol (CH3CH2OH) can react with hydrogen peroxide (H2O2) to form ethoxyl radical (CH3CH2O·).
  • The reaction mechanism involves the homolytic cleavage of the peroxide bond.

28. Reaction with acyl chlorides

  • Alcohols can react with acyl chlorides to form esters.
  • This reaction is similar to esterification but uses acyl chlorides instead of carboxylic acids.
  • For example, ethanol (CH3CH2OH) can react with acetyl chloride (CH3COCl) to form ethyl acetate (CH3COOCH2CH3).
  • The reaction mechanism involves the nucleophilic substitution of the chlorine atom by the alkoxide ion.

29. Reaction with sodium

  • Alcohols can react with sodium metal (Na) to form alkoxides and hydrogen gas.
  • This reaction is known as the reaction with sodium.
  • For example, ethanol (CH3CH2OH) can react with sodium (Na) to form sodium ethoxide (CH3CH2ONa) and hydrogen gas (H2).
  • The reaction mechanism involves the displacement of the -OH group by the alkoxide ion.

30. Reaction with metal hydrides

  • Alcohols can react with metal hydrides like lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4).
  • This reaction is known as reduction using metal hydrides.
  • For example, ethanol (CH3CH2OH) can be reduced to ethane (CH3CH3) using lithium aluminum hydride (LiAlH4).
  • The reaction mechanism involves the attack of the hydride ion (H-) on the carbon atom bonded to the -OH group.