Phenol

Preparation

1. From haloarenes
Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced.(SNAr)

2. From benzenesulphonic 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. (SNAr)

3. 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. (S_NAr)

4. 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.

5. From Coal Tar:

A number of phenols are present in coal tar, from which they may be separated by extraction with alkali. Acidification releases the phenols. Phenol itself, and o-, m-, and p-methylphenols (o-, m-, and p-cresols) may be obtained in this way.

Reactions of phenols

1. Acidity of phenols
(i) Reaction with metals: phenols react with active metals such as sodium, potassium and aluminium to yield phenoxides and hydrogen.

In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides.

The above reactions show that phenols are acidic in nature.

(ii) 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 reaction of phenol with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water.

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.

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.

COMPOUND	FORMULA	PKA
o-Nitrophenol	o-O2N-C6H4-OH	7.2
m-Nitrophenol	o-O2N-C6H4-OH	8.3
Phenol	C6H5-oH	10.0
o-Cresol	o-CH3-C6H4- OH	10.2
m-Cresol	m-CH3C6H4-OH	10.1
p-Cresol	p-CH3-C6H4-OH	10.2
Ethanol	C2H5OH	15.9
pKa Values of Phenols and Ethanol

2. Esterification
Phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.

The reaction with carboxylic acid and acid anhydride is carried out in the presence of a small amount of concentrated sulphuric acid. The reaction is reversible, and therefore, water is removed as soon as it is formed.

The reaction with acid chloride is carried out in the presence of a base (pyridine) so as to neutralise HCl which is formed during the reaction. It shifts the equilibrium to the right hand side.

The introduction of acetyl (CH₃CO) group in alcohols or phenols is known as acetylation. Acetylation of salicylic acid produces aspirin.

Electrophilic aromatic substitution

In phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions. The –OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to the resonance effect caused by –OH group. The resonance structures are shown under acidity of phenols.
Common electrophilic aromatic substitution reactions taking place in phenol are as follows:

(3) Nitration:

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.

Nowadays picric acid is prepared by treating phenol first with concentrated sulphuric acid which converts it to phenol-2,4-disulphonic acid, and then with concentrated nitric acid to get 2,4,6-trinitrophenol.

(4) Halogenation:

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.

5. Kolbe’s reaction
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.

6. Reimer-Tiemann reaction
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

7. Phenol with zinc dust
Phenol is converted to benzene on heating with zinc dust.

8. Oxidation
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.

Coupling reactions

Phenol is dissolved in sodium hydroxide solution to give a solution of sodium phenoxide.The solution is cooled in ice, and cold benzenediazonium chloride solution is added. There is a reaction between the diazonium ion and the phenoxide ion and a yellow-orange solution or precipitate is formed. The product is one of the simplest of what are known as azo compounds..

The naphthalen-2-ol is dissolved in sodium hydroxide solution to produce an ion just like the phenol one. This solution is cooled and mixed with the benzenediazonium chloride solution. An intense orange-red precipitate is formed - another azo compound.

The Fries rearrangement

The Fries rearrangement, is a rearrangement reaction of a phenolic ester to a hydroxy aryl ketone by catalysis of Lewis acids.It involves migration of an acyl group of phenol ester to the aryl ring. The reaction is ortho and para selective and one of the two products can be favoured by changing reaction conditions, such as temperature and solvent.

The reaction progress is not dependent on solvent or substrate. A widely accepted mechanism involves a carbocation intermediate.

Phenol-formaldehyde resins

Phenol-formaldehyde resins , trade name Bakelite are inexpensive, heat-resistant, and waterproof, though somewhat brittle. The polymerization of phenol with formaldehyde involves electrophilic aromatic substitution at the ortho and para positions of phenol (probably somewhat randomly), followed by cross-linking of the polymeric chains.

Learning Chart of Phenol