Phenol

What is phenol?

Phenol is a colorless aromatic organic compound with the molecular formula C₆H₅OH. The structure of phenol contains one hydroxyl group directly attached to the benzene ring.

It is a crystalline solid which turns pink on exposure to air or light. It behaves like a weak acid due to losing one hydrogen ion to form a phenoxide anion. The acidity of phenol can be explained by the stability or resonance of phenoxide ions.

Phenol is moderately soluble in cold water but solubility increases in ethanol or ether solution due to the presence of a benzene ring. Phenol is used as an antiseptic and disinfectant. It is also used for the production of dyes, drugs, and phenol-formaldehyde resin or bakelite.

It can be extracted first from coal tar but today it can be extracted commercially by the cumene phenol process.

Structure of phenol

It is an organic compound with the molecular formula C₆H₅OH. Therefore, the structure of phenol contains a benzene ring which is directly attached to a hydroxyl group.

Properties of phenol

It is monohydric aromatic alcohol that is moderately soluble in cold water but readily soluble in ethanol or ether. It is a stronger acid than alcohol due to the resonance structure or formation of a phenoxide anion.

The infrared absorption region of the phenolic hydroxyl group is the same as that of the alcoholic hydroxyl group. They can be differentiated by their nucleus which has its own absorption spectrum.

Phenol
IUPAC name	benzenol
Other names	Phenic acid Carbolic acid Phenylic acid Hydroxybenzene
Chemical formula	C6H6O
Molar mass	94.113 g/mol
Appearance	colorless crystalline solid
Density	1.07 g/cm3
Melting point	40.5 °C
Boiling point	181.7 °C
Conjugate base	Phenoxide ion (C6H5O−)
Dipole moment	1.224 D
Solubility in water	8.3 g/100 mL
CAS Number	108-95-2

Boiling point

Like other alcohol, phenol molecules are associated through intermolecular hydrogen bonding which extends over a chain. Therefore, the boiling point of phenol is much higher than the expected value obtained from their molecular weight.

Acidity of phenol

The acidity of phenol is weaker than carboxylic acids but stronger than alcohols. Due to resonance, the oxygen atom of the hydroxyl group acquires a positive charge. Therefore, it can be facilitated the release of the proton.

The acidity of phenol is also explained by the stability of the phenoxide anion. The phenoxide ion has also resonance structures. It is more stabilized by resonance than the un-ionized phenol molecule due to the spreading of the negative charge.

Solubility of phenol in water

The solubility of phenol in water can be explained by hydrogen bonding with water molecules. Like other alcohol, it can also form hydrogen bonds with water molecules.

The solubility of phenol is much less than that of other alcohol due to the presence of the benzene ring. With the increasing hydroxyl group, the solubility of phenol in water increases.

Chemical reactions

Most of the reactions of phenol are connected with the benzene ring rather than the hydroxyl group.

• It gives a violet colour with ferric chloride.
• Phenol behaves as a weak acid and forms phenoxide salts with strong alkalies.
  C₆H₅OH + NaOH → C₆H₅ONa + H₂O
  Alkali phenoxide reacts with alkali halide to form the phenolic esters.
• It forms phenyl esters when warmed with an acid chloride or acid anhydride.
  C₆H₅OH + (CH₃CO)₂O → CH₃COOC₆H₅ + CH₃COOH
• Many of the reactions of phenol are carried out by electrophilic aromatic substitution in ortho and para positions. For example, the alkylation of phenol forms p-derivative with a small amount of o-derivative.
• It can be hydrogenated in presence of a nickel catalyst at 160 °C to form cyclohexanol.
  C₆H₅OH + 3H₂ → C₆H₁₁OH

Phenol formaldehyde resin

At low temperatures, in presence of dilute acid or alkali phenol condensed with formalin solution which contains 40 percent aqueous formaldehyde to form ortho and para-hydroxy benzyl alcohol. It is the basis of the preparation of phenol-formaldehyde resin or Bakelite.

Production Process

It can be first extracted from coal tar. It is the oldest method for the extraction of phenol. But such type of extraction is now insufficient to give the required amount of phenol for the industry. Therefore, we prepared it synthetically.

The cumene phenol process is the most important process for the production of phenol. It is carried out by the oxidation of cumene to its hydroperoxide. It is decomposed by acid to form phenol and acetone. Both the products have commercial importance.

It is also prepared by oxidation of toluene by air in the presence of a chemical catalyst such as manganous or cupric salts.
C₆H₅CH₃ → C₆H₅OH + CO₂

It is produced by the steam distillation of benzene diazonium sulfate solution.
C₆H₅N₂HSO₄ + H₂O → C₆H₅OH + N₂ + H₂SO₄

It may be prepared by the fusion of sodium benzyl sulphonate with sodium hydroxide. It is an early commercial route for the production of phenol.
C₆H₅SO₃Na + 2NaOH → C₆H₅ONa + Na₂SO₃ + H₂O

What is phenol used for?

• A major part of phenol can be used for the production of phenolic resins.
• It is the base material for the manufacture of nylon and other synthetic fibers.
• Phenolic resins can be prepared by the reactions of phenolic compounds with formaldehyde. The most famous example of phenolic resins is Bakelite.
• Phenol–chloroform extraction technique in molecular biology is used for the separation of nucleic acids such as DNA and RNA from proteins and lipids.
• It is also used for the production of different types of household products, antiseptics, and disinfectants.
• Phenol is the first antiseptic and disinfectant which we used in surgical instruments.
• Phenolic compounds such as salicylic acid, salol, aspirin, methyl salicylate, and phenacetin can be used for making different types of drugs in the pharmaceutical industry.
• Due to its antiseptic and disinfectant properties, phenol is used for the production of different types of cosmetics like carbolic soap, sunscreens, hair colorings, skin lightening, or toners. Such types of use can be banned by European Union and Canada due to their safety concerns.
• Phenol can be used for the production of explosive picric acid, indicator phenolphthalein, and solvent cyclohexanol.