B.Sc. 2nd Year Chemistry – Detailed and Exam-Oriented Notes
Phenols are aromatic organic compounds in which a hydroxyl group (–OH) is directly attached to an aromatic carbon atom of a benzene ring. The simplest member of this class is phenol, having the molecular formula C6H5OH.
Phenols are important in organic chemistry because their properties differ significantly from those of ordinary alcohols. The lone pair of electrons on the oxygen atom can interact with the aromatic ring, producing resonance and influencing both the chemical reactivity and acidity of phenols.
Phenolic compounds occur naturally in plants and are also widely used in pharmaceuticals, disinfectants, dyes, polymers and other industrial products.
Phenols containing one hydroxyl group are called monohydric phenols.
Example: Phenol, C6H5OH.
Phenols containing two hydroxyl groups attached to an aromatic ring are called dihydric phenols.
| Compound | Position of –OH groups | Common name |
|---|---|---|
| C6H4(OH)2 | 1,2- | Catechol |
| C6H4(OH)2 | 1,3- | Resorcinol |
| C6H4(OH)2 | 1,4- | Hydroquinone |
Phenols containing three hydroxyl groups are called trihydric phenols.
Example: Pyrogallol, C6H3(OH)3.
The parent compound is phenol when the hydroxyl group is the principal functional group attached to the benzene ring.
The carbon bearing the –OH group is assigned position 1 and the ring is numbered to give substituents the lowest possible locants.
| Common name | IUPAC name | Formula |
|---|---|---|
| Phenol | Phenol | C6H5OH |
| o-Cresol | 2-Methylphenol | CH3C6H4OH |
| m-Cresol | 3-Methylphenol | CH3C6H4OH |
| p-Cresol | 4-Methylphenol | CH3C6H4OH |
The oxygen atom in phenol is approximately sp3-hybridized, but one of its lone pairs can overlap with the π-electron system of the benzene ring.
This produces resonance between the oxygen atom and the aromatic ring. The resonance structures show increased electron density at the ortho and para positions of the ring.
This resonance has two important consequences:
Chlorobenzene can be converted into sodium phenoxide by treatment with sodium hydroxide at high temperature and pressure. Acidification of sodium phenoxide gives phenol.
Benzene sulphonic acid or its salt can be fused with sodium hydroxide to form sodium phenoxide, which gives phenol on acidification.
Aromatic diazonium salts undergo hydrolysis on warming with water to form phenol with liberation of nitrogen gas.
The cumene process is an important industrial method for producing phenol. Cumene is first oxidized to cumene hydroperoxide, which is subsequently cleaved under acidic conditions to give phenol and acetone.
Phenol is a weak acid and can donate a proton from its hydroxyl group.
The conjugate base, phenoxide ion, is stabilized by resonance because the negative charge can be delocalized into the aromatic ring.
This resonance stabilization makes phenol more acidic than ordinary aliphatic alcohols.
This reaction demonstrates that phenol is sufficiently acidic to react with a strong base such as sodium hydroxide.
| Property | Phenol | Alcohol |
|---|---|---|
| Functional arrangement | –OH directly attached to aromatic ring | –OH attached to saturated carbon |
| Acidity | Greater | Generally lower |
| Conjugate base | Phenoxide ion is resonance stabilized | Alkoxide ion is not similarly stabilized |
| Reaction with NaOH | Phenol reacts | Ordinary alcohols generally do not react appreciably |
| Electrophilic substitution | Highly reactive aromatic ring | Not applicable in the same manner |
Electron-withdrawing groups such as –NO2 stabilize the negative charge of the phenoxide ion and therefore increase the acidity of phenols.
Nitro-substituted phenols are consequently more acidic than phenol.
Electron-donating groups such as alkyl groups generally destabilize the negative charge of the phenoxide ion and tend to decrease acidity.
The hydroxyl group strongly activates the benzene ring toward electrophilic substitution. It directs incoming electrophiles mainly to the ortho and para positions.
Phenol reacts readily with bromine water at room temperature to form 2,4,6-tribromophenol as a white precipitate.
Phenol undergoes nitration readily. Depending on reaction conditions, mono- and poly-nitrated products can be obtained.
Under stronger nitrating conditions, further nitration can occur.
Phenol undergoes sulphonation with concentrated sulfuric acid. The product distribution depends on temperature.
At lower temperatures, ortho substitution is favored, while at higher temperatures the para product becomes more significant.
The Kolbe-Schmitt reaction is an important reaction of sodium phenoxide with carbon dioxide. It is used for the preparation of salicylic acid.
Sodium phenoxide reacts with carbon dioxide under pressure and heat, followed by acidification.
The major product is salicylic acid, also known as 2-hydroxybenzoic acid.
The Reimer-Tiemann reaction introduces a formyl group (–CHO) into phenol, mainly at the ortho position, using chloroform and aqueous sodium hydroxide.
The reaction proceeds through the formation of a reactive carbene intermediate, commonly represented as dichlorocarbene, :CCl2.
The final hydrolysis and work-up produce the aldehyde group on the aromatic ring.
Phenyl esters can undergo rearrangement in the presence of a Lewis acid such as anhydrous aluminium chloride to produce hydroxyaryl ketones.
The reaction is known as the Fries rearrangement and is useful for introducing an acyl group into an aromatic ring.
Phenoxide ions can react with alkyl halides to form aromatic ethers. This is an example of the Williamson ether synthesis.
The reaction is particularly useful for preparing alkyl phenyl ethers, commonly called phenetole or anisole-type compounds depending on the alkyl group.
Phenols can undergo oxidation to produce quinones and other oxidized products depending on the structure and oxidizing conditions.
For example, oxidation of hydroquinone gives p-benzoquinone.
Quinones are important in biological electron-transfer processes and organic synthesis.
Phenol can be reduced by heating with zinc dust to produce benzene.
This reaction demonstrates that the hydroxyl group can be removed from phenol under strongly reducing conditions.
Many phenolic compounds form colored complexes with ferric ions. Phenol commonly gives a violet or purple coloration with neutral ferric chloride solution under suitable test conditions.
Catechol is benzene-1,2-diol. It contains two adjacent hydroxyl groups. It is used as an intermediate in chemical and pharmaceutical synthesis.
Resorcinol is benzene-1,3-diol. It is used in the production of resins, adhesives and some pharmaceutical preparations.
Hydroquinone is benzene-1,4-diol. It is readily oxidized to p-benzoquinone and has applications in photographic chemistry and other chemical processes.
Picric acid is 2,4,6-trinitrophenol. The presence of three strongly electron-withdrawing nitro groups makes it considerably more acidic than phenol.
| Substituent | General effect on acidity | Reason |
|---|---|---|
| –NO2 | Increases | Electron-withdrawing effect stabilizes phenoxide ion |
| –Cl | Generally increases | Strong inductive electron-withdrawing effect |
| –CH3 | Generally decreases | Electron-donating inductive effect |
| –OCH3 | Generally decreases through resonance donation | Electron donation destabilizes phenoxide relative to phenol |
The hydroxyl group is an activating and ortho/para-directing group in electrophilic aromatic substitution.
The oxygen atom donates electron density into the aromatic ring by resonance. As a result, the ortho and para positions become relatively electron-rich and are favored by electrophiles.
Para substitution may become significant when steric effects make ortho substitution less favorable.
| Feature | Phenol | Benzene |
|---|---|---|
| Substituent | –OH | None |
| Ring electron density | Increased by resonance donation | Normal aromatic electron density |
| Electrophilic substitution | More readily | Requires stronger conditions |
| Major orientation | Ortho and para | No directing substituent |
Phenolic compounds are widely distributed in plants and contribute to plant defense, pigmentation and antioxidant activity.
Many natural products contain phenolic functional groups. Examples include flavonoids, tannins and several plant-derived antioxidants.
Phenolic compounds are therefore important not only in industrial chemistry but also in biological and pharmaceutical chemistry.
Phenols are aromatic compounds in which the hydroxyl group is directly attached to an aromatic ring.
The simplest phenol is C6H5OH. The oxygen lone pair interacts with the aromatic π-system, making phenol relatively acidic and strongly activating the aromatic ring.
Important methods of preparation include the Dow process, hydrolysis of diazonium salts, conversion from benzene sulphonates and the industrial cumene process.
Important reactions include bromination, nitration, sulphonation, Kolbe-Schmitt reaction, Reimer-Tiemann reaction, Fries rearrangement, ether formation and oxidation.
Phenols and their derivatives have major applications in polymer manufacture, pharmaceuticals, dyes, disinfectants and organic synthesis.