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Phenol

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Phenol also known as carbolic acid, has the chemical formula $C_6H_5OH$. It is combustible, corrosive, toxic, white crystalline solid with a sweet tarry odor, commonly referred to as a 'hospital smell". Phenol is moderately soluble in water and commonly seen in aqueous solutions. Exposure to phenol through inhalation also leads to adverse health effects and possibly death. In halation causes severe irritation of the respiratory track with coughing, burns and breathing difficulty.

Phenols are aromatic hydrocarbons in which the hydroxyl group is directly linked to the carbon atoms of the benzene ring. If the hydroxyl is linked to the benzene ring by a carbon atom as part of a side chain, the compound is not a phenol but an aromatic alcohol.

Defination

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Phenol may be found as a translucent, clear or a light pink crystalline mass, a white powder or a clear liquid. It has a sweet, sharp odor. Phenol is used in many commercial available products including plastics, resins, fertilizers, paints, photographic developers and some medicines.

Phenol is absorbed easily into the body through the skin, lungs and stomach. The brain is very sensitive to phenol. Phenol can cause seizures and coma and may interfere with the brain control of regular breathing patterns. It can cause dangerous rhythm changes in the heart.

Naming 

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Phenols are aromatic hydroxy compounds in which the hydroxy group is directly attached to the aromatic ring. The simplest hydroxy derivative of benzene is a phenol. In the IUPAC system this name has been retained for phenol. The IUPAC rules for naming phenols are simply extensions of the rules used to name benzene derivatives with hydrocarbon or halogen substituents. The parent name is a phenol. The ring is numbering always begins with the hydroxyl group and proceeds in the direction that gives the lower number to the next carbon atom bearing a substituent.

The name phenol is derived from a combination of the terms phenyl and alcohol. The word phenol is used both as the name of a specific substance and as the family name for all hydroxy substituted aromatic compounds. For aromatic compounds phenol is used as the parent name rather than benzene.

Naming Phenols

Structure

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The functional group of a phenol is a hydroxyl group bonded to a benzene ring. Substituted phenols are named either as derivative of phenol or by common names.

Structure of Phenols

Phenols are widely distributed in nature. Phenols itself and the ismeric cresols are found in coal tar. Thymol and vanillin are important constituents of thyme and vanilla beans respectively. Urushiol is the main component of the irritating oil of poison ivy. It can cause severe contact dermatitis in sensitive individuals.

Phenol Reactions

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Reactions of the OH group

  • With base the phenol acts as an acid forming water and the phenoxide ion. With sodium hydroxide water and sodium phenoxide are formed.
NaOH + C6H5OH $\rightarrow$ C6H5O-Na+ + H2O
  • With sodium, phenol gives sodium phenoxide and hydrogen gas.
Na + C6H5OH $\rightarrow$ C6H5O-Na+ + $\frac{1}{2}$H2
Reactions of the benzene ring
  • It reacts more readily than benzene because of the increased electron density on the benzene ring. This means that the conditions needed for phenol to react are much less severe than for benzene. The reactants can be more dilute and the temperature will be lower.
  • Substitution occurs at the sites because of the increased electron density at some sites compared with the others.
  • Substitution often occurs at not just one of these sites but at all three because of the extra reactivity.

Acidity of Phenols

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Phenols are weak acids, with pKa values of approximately 10. Most phenols are insoluble in water, but they react with strong bases such as NaOH and KOH to form water-soluble salts.

Acidity of Phenols

Most phenols are such weak acids that they do not react with weak bases such as sodium bicarbonate that is they do not dissolve in aqueous sodium bicarbonate.

Phenol Ring

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Phenol is a solid made up of crystals. Its melting point is low (43oC), but higher than that of benzene. Hydrogen bonding accounts for the difference.

Phenol Ring

The oxygen atom in phenol has two lone pairs of electrons that are not involved in bonding. These are drawn towards the delocalised system around the benzene ring. This also helps to explain why phenol can lose an H+ ion.

Phenol Oxidation

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Phenol oxidation by H2O2 is widely used in the chemical industry for preparation of the dihydroxylated derivatives. The oxidation process takes place via the decomposition of H2O2, with formation of an unstable electrophilic intermediate which attacks the phenol nucleus to give a phenoxy ion.

Oxidation of phenols is shown below.

C6H5OH + 0.5O2 $\rightarrow$ C6H4(OH)2 (dihydric phenols)

C6H4(OH)2 + 0.5O2 $\overset{r_{2}}{\rightarrow}$ C6H4O2 (benzoquinones) + H2O

Xylenol

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Xylenol have been used for manufacture of carbolic soaps, disinfectants, wire enamels and fire retardant plasticizers. However, 2,4-xylenol, 2,6-xylenol and 3,5-xylenols have been used for organic chemical synthesis. 2,6-xylenol is a precursor for an engineering plastic polyphenylene oxide also known as polyphenylene ether.

Xylenol

Xylenol belongs to the aromatic alcohol type of Frothers. In appearance xylenol is a dark brown liquid similar to phenols with the same characteristic smell. Xylenol frother is a mixture of a higher molecular structure of phenol isomers.

Resorcinol

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Resorcinol is also known as 1,3-dihydroxybenzene or as benzene-1,3-diol or as m-dihydroxy benzene. A dihydric phenol. This material has a melting point 110oC and boiling point of 280oC and a relative density of 1.27 .Resorcinol as a dihydri phenol is a very interesting intermediate material for the production of thermo setting resins. Resorcinol or resorcinol formaldehyde pre-polymers can be used as accelerating compounds for curing phenolic resins. Resorcinol can otherwise be used as an intemediate material for azo and triphenylmethane and other dyes, pharmaceuticals, cosmetics, tanning agents, textile treating agents and antioxidants.

The only commercial process for the production of resorcinol is the alkali fusion of m-benzenedi sulfonic acid according to the following equation.

Resorcinol

Thymol

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Thymol is a phenol. It is available from the volatile oils of thymus vulgaris and trachspermum ammi. Chemically thymol is 2-isopropyl-5-methyl phenol. It occurs naturally in oil of thyme and can be synthesized as given below.

Thymol

Thymol is available as a colorless crystalline compound with a pungent odour. It has pungent taste. Thymol is insoluble in water but soluble in organic solvents like alcohol, solvent ether and chloroform.

Cresol

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It is a mixture of ortho, meta and para isomers of methyl phenol obtained from coal tar. It occurs as colorless or pale brownish yellow color liquid. It is micible with water.

Cresol

Cresol exihibits bactericidal activity. Cresol is used to prepare with soap solution (lysol) which is used as a disinfectant in hospitals and homes.
Phenol occurs as a colorless to pale pink crystalline solid.
  • Phenol is a good antiseptic and disinfectant against a wide range of microorganisms and viruses.
  • Liquid phenol is used in dentistry and as an analgesic in a 5% solution to desensitize the pulp of deciduous teeth.
  • It is slightly soluble in water but freely soluble in alcohol, methanol and salol.