Phenols are aromatic compounds containing the one or more hydroxy groups bonded on benzene ring. The aromatic compound with one hydroxy group is commonly known as phenol and all other derivatives of phenol are called as phenolic compounds
Phenol was first isolated from coal tar in 1834 and found naturally in coal and decaying dead organic matter like rotting vegetables. In laboratory, phenol is prepared by prolonged sulphonation of benzene which produces a benzene sulphonic acid.Benzene sulphonic acid fused with caustic alkalis to form phenol. The molecular formula of phenol is C6H5OH with molecular mass 94.11 g mol−1.
Phenol is a white crystalline compound with melting point 182° C and boiling point 41° C. Phenol is also called as benzenol, phenylic acid, carbolic acid, hydroxybenzene or phenic acid. Phenol has a sweet odor and widely used as in low cost, versatile resins mainly thermoset resins like in construction, plywood adhesion and the motorcar industry.
The simplest derivative of benzene is phenol, which is also accepted by IUPAC system. If there is any additional group in a phenol molecule, the position of additional group shown either by ortho, meta, para prefix
or by the numbering system.
In IUPAC nomenclature, phenol is named as benzenol. If in a compound the hydroxyl group presents as a principal functional group, it must be considered as a derivative of phenol and can be named as a substituted phenol in which the hydroxy group numbered as one. For example
, the systematic name for thymol is 5-methyl-2-isopropylphenol.
In the case of disubstituted Phenols, molecule names by using either the appropriate numbers or the ortho (1,2), meta (1,3), and para (1,4) prefix. In the presence of other principal functional groups, phenols can be named with the hydroxyl group as a hydroxy substituent. For example, the vanillin is named as 4-hydroxy-3-methoxybenzaldehyde
Phenolic compounds are generally referred by their common names. For example, methyl phenols are also called as cresols
. Some other phenolic compounds with their common name are as follow.
Compare to alcohols, phenols are stronger acid as the pKa
value for phenol is 10 while for alcohols it’s close to 16. Because of acidic nature, phenols can turn blue litmus red and react with aqueous alkali to form phenate. Both reactions are not shown by alcohols. Compare to carboxylic acids, phenols are weaker acid.
Hence hydroxy group bonded to benzene ring is much more acidic than hydroxy group to alcohol. This is because of the resonance in phenol. Because of the resonance in phenol, the oxygen atom acquires a positive charge which weakens the oxygen-hydrogen bond and facilitates the release of a proton.
The deprotonation of phenol forms phenoxide ion or phenate which also exists as a resonance.
Hence both phenol and phenoxide ion are stabilized by resonance. Compare to phenol, phenoxide ion is more stable as the negative charge gets delocalized over benzene ring. But the resonating structure of phenol involves the separation of negative and positive charges. Therefore phenol has greater tendency to form phenate by releasing the proton. The deprotonation of alcohol forms alkoxide ion which is not stable due to negative inductive effect of alkyl group and alcohols become less acidic than phenol.
Effect of Substituent o Acidity of Phenol
As the acidic nature of phenol is due to the resonance stabilization of phenate compare to phenol. Therefore the presence of any substituent on aromatic ring which can stabilized the phenoxide ion will tend to increases the acidity of phenol. While any substituent which destabilized the phenate ion by increasing the negative charge will decrease the acidic nature of phenol.
In other words; the presence of electron withdrawing group on benzene ring in phenol increase the acidity of phenol and electron releasing group will decrease the acidity. For example; if there is a nitro group substituted on phenol, it will increase the acidic nature of phenol. Hence nitrophenol will be more acidic than phenol as nitro group imparts negative mesomeric effect and negative inductive effect, hence acts as electron withdrawing group.
The position of nitro group on phenol will affect the acidity of phenol. A nitro group at -ortho and para-position withdraws electrons from hydroxy group of phenol by stronger –M effect while nitro group
at -meta position withdraws electrons by weaker –I effect only, as meta position cannot involve in resonance with hydroxy group. Hence o- and p-nitrophenols are more acidic than m-nitrophenol. Similarly as the number of nitro groups increases on phenol, the acidic nature of phenol increases. The decreasing order of acidity of nitrophenols is as given below.
Electron donating groups like amino (-NH2
), alkyl (-R) decreases the acidity of phenol. For example, cresol or methylphenols are less acidic compare to phenol due to positive inductive effect as well as hyperconjugation of methyl group. As both of these effects increases the electron density on hydroxy group and results in low acidity of phenols. Hyperconjugation
can operate only at ortho and para position while inductive effect operates at all the three positions. Therefore o-cresol is least acidic. The decreasing order of acidity is as follow.
Some of the substituted phenols with their pKa
values are as follow.
||Effect of substituent
||+M and -I
||+I and hyperconjucation
||-I and +M
||-M and -I
||-M and -I
||+M and -I
|| +I and hyperconjugation
||-I and +M
||-M and -I
Phenols can show three types of reactions.
- Involves the cleavage of oxygen-hydrogen bond and show the acidic nature of phenol.
- Involves the cleavage of carbon-oxygen bond.
- Miscellaneous reaction.
1. Reactions involve the cleavage of oxygen-hydrogen bond
Such reactions show the acidic nature of phenols and form salts.
(a) Reaction with Metal
: Phenol reacts with metals like sodium metal to form sodium phenate and releases hydrogen gas.
(b) Reaction with Acid Chloride and Anhydrides: Phenol reacts with acid chloride (RCOCl) and anhydride (RCOOCOR) to form esters. Reaction is catalyzed by pyridine or any acid like concentrated sulfuric acid.
C6H5OH + RCOCl $\to$C6H5 OCOR + HCl
C6H5 OH + RCOOCOR $\to$C6H5 OCOR + RCOOH
The reaction of phenol with benzoyl chloride (
COCl) in the presence of aqueous sodium hydroxide yields phenyl benzoate(
) and reaction is called as Schotten-Baumann reaction.
C6H5 OH + C6H5COCl $\to$ C6H5 OCO
C6H5 + HCl(c) Reaction with Grignard Reagent:
Phenol reacts with Grignard reagent (RMgX) to form alkane.
C6H5 OH + RMgX $\to$ RH + Mg(OC6H5 )X
2. Involves the cleavage of carbon-oxygen bond
(a) Reaction with Phosphorus Halide:
Phenols reacts with phosphorus halide like PCl5
to form chlorobenzene with triphenylphospahte((
C6H5 OH + PCl5 $\to$
C6H5 Cl + (
C6H5 O)3P=O(b) Reaction with Ammonia: Phenol reacts with ammonia at 573 K temperature and high pressure in the presence of ZnCl2 to form aniline (
C6H5 NH2) and water.
C6H5 OH + NH3 $\to$C6H5 NH2+ H2O(c) Reaction with Zinc Dust:
This reaction is used for the preparation of benzene as phenol forms benzene in the presence of zinc dust at high temperature.
C6H5OH + Zn $\to$ C6H6 + ZnO
3. Miscellaneous Reaction
Phenol shows some miscellaneous reaction sue to the presence of aromatic ring as well as hydroxy group. Some of these reactions are as follow.
(a) Electrophilic Substitution Reactions
Due to the presence of activating group (hydroxy group) in phenol, it undergoes electrophilic substitution reactions readily and forms polysubstitution products. The hydroxy group in phenol is –o and –p directing group, hence substitution takes place at both position. Some common electrophilic substitution reactions of phenol are as follow.(i) Bromination:
Phenol forms polyhalogen derivatives with chlorine or bromine water. For example; with bromine water, it forms 2,4,6-tribromophenol which is precipitated as white precipitate.
Phenol forms –o and –p-nitrophenol with dilute nitric acid, while with concentrated nitric acid, it forms picric acid (2,4,6-trinitrophenol).
(iii) Friedel Craft Alkylation:
Phenol undergoes alkylation with alkyl halide in the presence of anhydrous AlCl3
and forms alkyl substituted phenol.
C6H5OH + CH3Cl $\to$ C6H5OH(CH3) + HCl
Similarly Acylation is also possible with phenol to form –o and –p-hydroxyacetophenone.
(iv) Coupling Reaction:
In weakly alkaline medium, phenol reacts with benzenediazonium chloride to form colored substances which are called as azo dyes. The reaction takes place at 273-278 K temperature and also known as coupling reaction.
(b) Kolbe’s Reaction
When sodium phenoxide is heated with carbon dioxide at about 400 K under a pressure of 4-7 atmosphere followed by acidification gives 2-Hydroxy benzoic acid also known as salicylic acid with a little amount of 4-Hydroxybenzoic acid. The reaction is known as Kolbe’s reaction.
(c) Reimer-Tiemann reaction:
When phenol treated with chloroform in the presence of aqueous potassium hydroxide at 340 K temperature followed by hydrolysis yields 2-hydroxybenzaldehyde also called as salicylaldehyde and reaction called as Reimer-Tiemann reaction.
Resorcinol is an aromatic diol with two hydroxy groups at meta position and named as m-dihydroxybenzene or 1,3-benzenediol. Another common name used for resorcinol is resorcin. Resorcinol is a white solid which crystallised as colorless needles from benzene. It is easily soluble in water as well as in ether and alcohol but insoluble in chloroform and carbon disulphide.
- Resorcinol can show electrophilic substitution reactions as well as undergo nucleophilic substitution through the enone form.
- This diol is an essential component of an adhesive system which widely used in the tire manufacturing process and other fiber-reinforced rubber mechanical goods.
- Resorcinol-formaldehyde resins or Phenol-modified resorcinol-formaldehyde resins are used in manufacturing of many adhesives which are used for wood bonding applications demanding room
- temperature cure, waterproof characteristics and structural integrity.
- It acts as an important chemical intermediate during chemicals manufacturing like light screening agents which are used to protect plastics from exposure to ultraviolet light.
- Resorcinol is used to manufacture agricultural chemicals, pharmaceuticals, explosive primers, dyestuffs, flame retardants, fungicidal creams, lotions and antioxidants. It also used to improve the mechanical and chemical resistance of paper machine fabrics.
Another name for Thymol is 2-isopropyl-5-methylphenol (IPMP) with molecular formula C10
O (molar mass 134.24 g/mol). Thymol is in a colorless, translucent crystalline, also exits in flake form at room temperature. Thymol is found in Thyme oil and can be extracted from various plants like Trachyspermum ammi (Ajowan), Thymus vulgaris (chemotype thymol), Thymus (chemotype carvacrol), Thymus (chemotype thymol), Thymus serpyllum (Wild thyme or mother of thyme) and Thymus vulgaris (chemotype linalol).
Some other common names of Thymol are mint flavouring, 3-hydroxy-p-cymene, isopropyl cresol, 6-isopropyl m-cresol and 4-methyl-2-hydroxyisopropylbenzene. The melting point of Thymol is 49°C and boiling point is 233°C. It has very low vapour pressure around 0.04 mm Hg at 20°C temperature and flash point is 107°C. It is a stable molecule and less reactive strong oxidizing agents, organic materials and strong bases.
It is extremely soluble in alcohols and other organic solvents while slightly soluble in water. Due to the presence of phenolic group, It is also soluble in strongly alkaline aqueous solutions and gets deprotonated. As Thymol is a phenolic compound, it has shown antibacterial activity against bacterial strains like Aeromoans hydrophila and Staphylococcus aureus. Because of its antibacterial nature, it is widely used in mouthwash, toothpaste, breath freshener, body powder and household cleaners. Because of its degradable nature it is widely used as pesticide agent and offers a safe alternative compare to other more persistent chemical pesticides which can be dispersed in runoff and responsible for subsequent contamination. Some adverse effects of Thymol are listed below;
- Cardiac arrhythmias
- CNS depression
- Pulmonary edema
- Respiratory failure
- Myocardial damage
- Circulatory failure·
- Nausea and vomiting
Methylphenols are substituted phenols and also known as cresol. Cresols are mainly found in coal
tar. There are three possible structural isomers of cresol; o-cresol, p-cresol and m-cresol. Cresol can be considered as methyl derivative of phenol or hydroxy derivative of phenol. The IUPAC name of structural isomers of cresols is as follow;
- o-cresol: 2-methylphenol or 2-hydroxytoluene
- p-cresol: 4-methylphenol or 4-hydroxytoluene
- m-cresol: 3-methylphenol or 3-hydroxytoluene
Cresols show almost same chemical and physical properties. Hence it’s difficult to separate from their mixture which obtained from coal tar and known as cresylic acid. Some common properties of cresols are as follow.
|Solubility/100ml of pure water
|| 86oC c.c
The cresols act as precursor in the manufacture of disinfectants and synthetic resins. They are highly corrosive in nature and can injure eye, skin, mouth and gastrointestinal tract. Some common adverse effects of cresols are nausea, diarrhea, hypotension, vomiting, neurological changes, myocardial failure, liver and renal toxicity, pulmonary edema, hemolysis and methemoglobinemia.
Esterification of phenol in the presence of acetic anhydride yields phenyl acetate. It can also produced by decarboxylation of aspirin or by reaction of phenol with acetic acid.
Phenyl acetate is clear colourless liquid with a sweetish solvent odor. The boiling point of phenyl acetate is 196 °C, density 1.073 g/mL at 25 °C. It is a quite stable, combustible compound and incompatible with strong acids and base as well as with strong reducing agents and strong oxidizing agents. Phenyl acetate is slightly soluble in water and mainly used as a laboratory reagent in the production of some organic chemicals. Inhalation or contact with phenyl acetate may irritate or burn skin and eyes. Phenyl acetate is corrosive in nature and vapors of phenyl acetate may cause dizziness or suffocation.
Aromatic compounds with two methyl group and one hydroxyl group are known as Xylenol or dimethylphenol. Xylenol is a trivial name made up of the combination of Xylene and phenol. There are six positional isomers of Xylenol;
- 2, 3 - dimethylphenol
- 2, 4 - dimethylphenol
- 2, 5 - dimethylphenol
- 2, 6 - dimethylphenol
- 3, 4 - dimethylphenol
- 3, 5 - dimethylphenol
These isomers show quite similar chemical and physical properties. Their melting point and boiling point is very close to each other.
|Melting point (k)
|Boiling point (k)
Xylenols are used to manufacture for a number of pesticides and other com pounds like antioxidants. One off the derivatiove of Xylenol; Xylenol orange is a redox indicator and 2,6-Xylenol acts as monomer for an engineering resin like poly(p-phenylene oxide).
Phenolic compound like ethylphenol can show three position isomers; 2-ethylphenol, 4-ethylphenol and 3-ethylphenol. Out of them, 4-ethylphenol is more common and also abbreviated as 4-EP. Ethylphenol can be prepared by Friedel-craft alkylation of phenol. Biochemically Ethylphenol produced by p-coumaric acid. p-coumaric acid first converts to 4-vinylphenol which further converts in 4-ethylphenol in the presence of enzyme cinnamate decarboxylase.
Ethylphenols are naturally-occurring phenolic compounds like para-ethylphenol is produced at low concentrations by yeast in beer and wine. The mixture of all isomers of Ethylphenol is called as cresylic acids. Ethylphenol mainly involves in the manufacturing of resins and plastics for circuit boards, can coatings and laminates. Manufacture of phosphate esters used as fire resistant hydraulic fluids in aircraft and lubricants in power plants. They act as reactive solvent and used to apply to magnet wire for transformers and electrical motors.
There are several tests for phenol like
- Phenols are widely used as an antiseptic and disinfectant in several mouthwash, deodorant, lotion and soap.
- They also used in manufacturing of drugs like aspirin, salol, phenacetin, salicylic acid etc.
- Phenol used in the manufacturing of resins like bakalite which used in manufacturing of electrical goods like plugs and switches etc.
- The nitro derivative of phenol like 2, 4, 6-trinitrophenol (picric acid) used to make explosives and dyeing wool and silk.
- Phenols also used in the preparation of other chemicals like; azo dyes, phenolphthalein and Cyclohexane.
- Phenol solutions turn blue litmus red.
- Phenols react with ferric chloride solution to give blue, violet or green coloration due to the formation of ferric phenoxide.
- Phenols react with diazonium salts in weak alkaline medium to form yellow or orange colored azo dyes.
- Phenol forms white precipitate with bromine water