Compounds in which a hydroxy group -OH is directly attached to a benzene ring are called phenols. Phenol C6H5OH is the representative example. Phenols have properties similar to, but different from, alcohols.
Basic Knowledge
Properties of phenol:
① Weak acidity (Ka≈10-10; more acidic than alcohols but weaker than carbonic acid): C6H5OH + NaOH → C6H5ONa + H2O.
② Because it is weaker than carbonic acid, bubbling CO2 through sodium phenoxide liberates phenol: C6H5ONa + CO2 + H2O → C6H5OH + NaHCO3.
③ Iron(III) chloride reaction: phenol + FeCl3 → violet to blue-violet color (a detection reaction for phenolic -OH groups in general).
④ Halogenation: Adding bromine water produces 2,4,6-tribromophenol (white precipitate), substituted at all ortho and para positions, because of the strong activating ability of -OH.
⑤ Coupling reaction: diazonium salt + phenoxide → azo compound (dye).
Industrial production of phenol: ① Cumene process (modern main method): benzene + propylene → cumene → oxidation by air → cumene hydroperoxide → decomposition → phenol + acetone (both products are useful and atom efficiency is high). ② Alkaline fusion of benzenesulfonic acid. ③ Chlorobenzene + NaOH at high temperature (old method).
Main phenols: phenol, o-/m-/p-cresol (methylphenol), hydroquinone (1,4-dihydroxybenzene, photographic development), resorcinol (1,3-dihydroxybenzene), catechol (1,2-), picric acid (2,4,6-trinitrophenol, explosives and dyes), naphthol (naphthalene + OH), and salicylic acid (o-hydroxybenzoic acid, raw material for aspirin).
Phenol C6H5OH (weakly acidic, preservative and disinfectant, highly toxic)
Phenoxide ion (C6H5O-, stabilized by resonance)
Cumene process (benzene + propylene → phenol + acetone, high atom efficiency)
FeCl3 color reaction (blue-violet color with phenolic -OH; detection reaction)
Salicylic acid (o-HOC6H4COOH, raw material for aspirin and methyl salicylate)
Phenolic resin (Bakelite) (phenol + formaldehyde, the first synthetic resin)
Further Study (Principles and Applications)
Why phenol is more acidic than alcohols: The negative charge of the phenoxide ion C6H5O- is delocalized by resonance into the π system of the benzene ring and stabilized (the charge is distributed through resonance structures to the ortho and para positions). Alkoxides R-O- do not have this stabilization.
Applications of salicylic acid: Acetylation produces acetylsalicylic acid (aspirin) (the world's oldest antipyretic and analgesic drug, Bayer, 1899). Esterification produces methyl salicylate (a topical analgesic). The reaction sites differ: aspirin involves acetylation of -OH, whereas methyl salicylate involves methyl esterification of -COOH.
Phenolic resin (Bakelite): The first fully synthetic resin, synthesized by Baekeland in 1907. Phenol and formaldehyde form a three-dimensional network structure that is hard and an excellent electrical insulator. It became famous for telephone bodies, ashtrays, and early plastic products.
Toxicity: Phenol causes skin irritation and protein denaturation and was used as a disinfectant (the Lister method, the starting point of modern surgery), but because it is highly toxic, its use is now limited. Picric acid (2,4,6-trinitrophenol) was used as a yellow dye and explosive, but it is unstable and caused many explosions, so its use is now limited.
- Phenol = benzene ring + directly attached -OH.
- It is weakly acidic, reacts with NaOH, and is weaker than carbonic acid.
- It gives a violet color with FeCl3 as a detection reaction.
- Bromine water produces a white precipitate of 2,4,6-tribromophenol.
- The industrial method is the cumene process, which produces acetone as a by-product.
- Salicylic acid produces aspirin (anti-inflammatory analgesic) and methyl salicylate (topical analgesic).
- Phenolic resin was the first synthetic plastic.
Points of Caution (Common Confusions and Frequent Errors)
① Phenols and alcohols both have R-OH, but phenols are weakly acidic and react with NaOH, whereas alcohols do not. ② Acid strength: carboxylic acids (R-COOH)>carbonic acid (H2CO3)>phenol (C6H5OH)>water≫alcohols. With aqueous NaHCO3, only carboxylic acids liberate CO2; phenols do not react with NaHCO3. ③ Benzyl alcohol C6H5-CH2-OH has an -OH group not directly attached to the ring, so it is a normal alcohol (negative FeCl3 test; oxidation gives an aldehyde or acid).
Practice
- Explain the difference in acid strength between phenol and ethanol from the viewpoint of resonance structures.
- Write the equation for the reaction when bromine water is added to phenol.
- Describe the advantages of the cumene process for producing phenol from the viewpoints of atom efficiency and by-products.