Alcohols, phenols, and ethers contain oxygen-containing functional groups that influence their chemical behavior. The presence of the hydroxyl (-OH) group in alcohols and phenols and the ether linkage (-O-) in ethers determines the types of reactions they undergo. These compounds participate in a variety of reactions such as bond cleavage, substitution, oxidation, and electrophilic substitution.
Alcohols
Alcohols undergo a variety of chemical reactions due to the presence of the hydroxyl (-OH) group. These reactions may involve cleavage of the O-H bond or the C-O bond and lead to the formation of several useful organic compounds.
1. Reaction with Active Metals
Alcohols react with active metals such as sodium, potassium, and magnesium to form alkoxides.
- During this reaction, the hydrogen atom of the hydroxyl group is replaced by the metal, and hydrogen gas is evolved.
- The evolution of hydrogen gas confirms the presence of the hydroxyl group in alcohols. This reaction also demonstrates the weak acidic nature of alcohols.
Example: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H2
2. Esterification
Alcohols react with carboxylic acids in the presence of concentrated sulfuric acid to form esters. This reaction is known as esterification.
- The ester formed, ethyl ethanoate, possesses a pleasant fruity smell.
- Esterification is widely used in the manufacture of perfumes, flavoring agents, and artificial fruit essences.
Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H2O
3. Dehydration
When alcohols are heated with concentrated sulfuric acid, they lose a molecule of water to form alkenes. This process is called dehydration. Dehydration is an important method for the industrial preparation of alkenes from alcohols.
Example: CH₃CH₂OH → CH₂=CH₂ + H₂O
4. Oxidation
Alcohols can be oxidized using oxidizing agents such as acidified potassium dichromate or potassium permanganate. The products formed depend on the type of alcohol.
- Primary alcohols are oxidized first to aldehydes and then to carboxylic acids.
- Secondary alcohols are oxidized to ketones, whereas tertiary alcohols are resistant to oxidation under ordinary conditions.
Example: CH₃CH₂OH → CH₃CHO → CH₃COOH
Phenols
Phenols contain a hydroxyl group directly attached to a benzene ring. Due to this unique structure, phenols exhibit reactions involving both the hydroxyl group and the aromatic ring.
1. Acidic Nature of Phenol
Phenol is more acidic than alcohols because the phenoxide ion formed after loss of a proton is stabilized by resonance.
- Therefore, phenol reacts with sodium hydroxide to form sodium phenoxide.
- However, phenol does not react with sodium bicarbonate because it is a weaker acid than carbonic acid.
Example: C₆H₅OH + NaOH → C₆H₅ONa + H₂O
2. Bromination
The hydroxyl group activates the benzene ring and directs incoming groups to the ortho and para positions. Therefore, phenol reacts readily with bromine water. A white precipitate of 2,4,6-tribromophenol is obtained. This reaction is often used as a test for phenol.
Example: C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr
3. Nitration
Phenol undergoes nitration more easily than benzene because the hydroxyl group increases the electron density of the ring.
- With dilute nitric acid, phenol produces a mixture of ortho-nitrophenol and para-nitrophenol.
- With concentrated nitric acid, phenol forms 2,4,6-trinitrophenol, commonly known as picric acid.
4. Kolbe's Reaction
When sodium phenoxide is treated with carbon dioxide under pressure followed by acidification, salicylic acid is formed.
- This reaction is known as Kolbe's reaction and is industrially important because salicylic acid is used in the manufacture of aspirin.
5. Reimer-Tiemann Reaction
Phenol reacts with chloroform in the presence of aqueous sodium hydroxide to produce salicylaldehyde. This reaction introduces an aldehyde group into the aromatic ring and is widely used in organic synthesis.
Ethers
Ethers are comparatively less reactive than alcohols and phenols because they do not contain an O-H bond. However, they undergo certain characteristic reactions under suitable conditions.
1. Cleavage by Hydrogen Halides
When ethers are heated with concentrated hydroiodic acid (HI) or hydrobromic acid (HBr), the C-O bond is cleaved to produce alcohols and alkyl halides.
- With excess HI, the alcohol formed further reacts to give another molecule of alkyl halide.
- This reaction is commonly used for the identification and analysis of ethers.
Example: C₂H₅OC₂H₅ + HI → C₂H₅I + C₂H₅OH
2. Electrophilic Substitution Reactions of Anisole
Anisole (methoxybenzene) contains a methoxy group (-OCH₃) attached to a benzene ring. The methoxy group activates the aromatic ring and directs incoming groups mainly to the ortho and para positions.
- Nitration: Anisole reacts with nitric acid to form ortho-nitroanisole and para-nitroanisole.
- Halogenation: Anisole reacts with bromine to produce ortho-bromoanisole and para-bromoanisole.
- Friedel-Crafts Reaction: Anisole undergoes Friedel-Crafts alkylation and acylation reactions more readily than benzene because the methoxy group increases the electron density of the ring.