Phenols are the compounds formed when a hydrogen atom in an aromatic ring, is replaced by a hydroxyl group (-OH group).
Classification of phenols
On the basis of number of hydroxyl groups (−OH) present, phenols can be divided into the following categories:
• Monohydric phenols:
They contain only one OH group.
• Dihydric phenols:
Such phenols contains two OH groups.
• Trihydric alcohols:
This type of phenol contains three OH groups.
Nomenclature of Phenols
Common naming system:
• The simplest hydroxy derivative of benzene is a phenol. Phenol is the common name as well as an accepted IUPAC name of the compound.
• In the case of substituted phenol compounds, the terms ortho (1,2- disubstituted), meta (1,3-disubstituted) and para (1,4-disubstituted) are often used as prefixes in the common names.
IUPAC naming system:
IUPAC names for some important phenolic compounds are given below
Structure of phenol
In phenols, the –OH group is attached to sp2 hybridised carbon of an aromatic ring. The C–O bond length (136 pm) in phenol is slightly less than that in methanol. This is due to (i) partial double bond character caused by the conjugation of unshared electron pair of oxygen with the aromatic ring and (ii) sp2 hybridised state of carbon to which oxygen is attached. The C−O−H bond angle in alcohols is slightly less than the tetrahedral angle (109°-28′) due to the repulsion between two lone pairs of electrons present on oxygen.
Physical properties of Phenols
• Phenols are colourless liquids or crystalline solids but become coloured due to slow oxidation with air.
• Due to the presence of strong intermolecular hydrogen bonding, phenols have a higher boiling point than the corresponding hydrocarbon or aryl halides.
• Due to their ability to form hydrogen bonds with water, phenols are moderately soluble in H2O.
• The phenols are acidic in nature and stronger acids than alcohols. This is due to the fact that the sp2 hybridised carbon of phenol to which −OH is attached, is highly electronegative which causes a decrease in electron density on oxygen. This Increases the polarity of O−H bond and results in an increase in ionisatlon of phenols than that of alcohols.
Moreover, the phenoxide ion so produced is stabilised by the delocalization of charge in phenol.
Note: The presence of electron withdrawing group like NO2 group, increases the acidic strength whereas the electron donating groups like an alkyl group decreases the acidic strength. Therefore, the acidic strength order is
Preparation of Phenols
Phenol, also known as carbolic acid, was first isolated in the early nineteenth century from coal tar. Nowadays, phenol is commercially produced synthetically. In the laboratory, phenols are prepared from benzene derivatives by any of the following methods:
Phenol from haloarenes
Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced
Phenol from benzene sulphonic acid
Benzene is sulphonated with oleum and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol.
Phenol from diazonium salts
A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (NaNO2 + HCl) at 273-278 K. Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids.
Phenol from cumene
Phenol is manufactured from the hydrocarbon, cumene. Cumene (isopropylbenzene) is oxidised in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. Acetone, a by-product of this reaction, is also obtained in large quantities by this method.
Acidity of phenols
The reactions of phenol with metals (e.g., sodium, aluminium) and sodium hydroxide indicate its acidic nature. The hydroxyl group, in phenol is directly attached to the sp2 hybridised carbon of benzene ring which acts as an electron withdrawing group. Due to this, the charge distribution in phenol molecule, as depicted in its resonance structures, causes the oxygen of –OH group to be positive.
The ionisation of an alcohol and a phenol takes place as follows:
Due to the higher electronegativity of sp2 hybridised carbon of phenol to which –OH is attached, electron density decreases on oxygen. This increases the polarity of O–H bond and results in an increase in ionisation of phenols than that of alcohols. Now let us examine the stabilities of alkoxide and phenoxide ions. In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised.
The delocalisation of negative charge (structures I-V) makes phenoxide ion more stable and favours the ionisation of phenol. Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion.
In substituted phenols, the presence of electron withdrawing groups such as nitro group, enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalisation of negative charge in phenoxide ion when substituent is at ortho or para position. On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength. Cresols, for example, are less acidic than phenol.
Esterification of phenol
Electrophilic aromatic substitution
The presence of OH group on benzene increases the electron density on the benzene ring making it more susceptible to attack by an electrophile. The reactions involving benzene ring are electropnilic substitution reaction. The presence of OH group makes the orthoand para carbon of benzene more electron rich than meta position. The OH group is called o ‒, p ‒ directing group.
Nitration of phenol
With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitrophenols.
The ortho and para isomers can be separated by steam distillation. o-Nitrophenol is steam volatile due to intramolecular hydrogen bonding while p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules.
With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol. The product is commonly known as picric acid. The yield of the reaction product is poor.
Halogenation of phenol
On treating phenol with bromine, different reaction products are formed under different experimental conditions.
(a) When the reaction is carried out in solvents of low polarity such as CHCl3 or CS2 and at low temperature, monobromophenols are formed.
The usual halogenation of benzene takes place in the presence of a Lewis acid, such as FeBr3 , which polarises the halogen molecule. In case of phenol, the polarisation of bromine molecule takes place even in the absence of Lewis acid. It is due to the highly activating effect of –OH group attached to the benzene ring.
(b) When phenol is treated with bromine water, 2,4,6-tribromophenol is formed as white precipitate.
Phenoxide ion generated by treating phenol with sodium hydroxide is even more reactive than phenol towards electrophilic aromatic substitution. Hence, it undergoes electrophilic substitution with carbon dioxide, a weak electrophile. Ortho hydroxybenzoic acid is formed as the main reaction product.
On treating phenol with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position of benzene ring. This reaction is known as Reimer - Tiemann reaction.
The intermediate substituted benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde.
Reaction of phenol with zinc dust
Phenol is converted to benzene on heating with zinc dust.
Oxidation of phenol
Oxidation of phenol with chromic acid produces a conjugated diketone known as benzoquinone. In the presence of air, phenols are slowly oxidised to dark coloured mixtures containing quinones.